The first dose of the Pfizer/BioNTech vaccination is 85 percent effective against coronavirus infection between two and four weeks after inoculation, according to a study published in the Lancet medical journal.
The survey was carried out on healthcare workers at the largest hospital in Israel, which on December 19 launched a mass vaccination campaign regarded as the world’s fastest.
Israeli studies have found the vaccine developed by US pharma giant Pfizer and its German partner BioNTech to be 95 percent effective one week after a second jab, while the Lancet report focused on more than 9,000 medical staff at Sheba hospital near Tel Aviv.
Some 7,000 of them received the first dose and the rest were not inoculated.
From the group, 170 were diagnosed with COVID-19 after tests carried out only on those showing symptoms or who had been in contact with coronavirus carriers.
Fifty-two percent of them were found to have not been vaccinated.
Comparing the two groups, the Sheba study calculated that the vaccine was 47 percent effective between one and 14 days after inoculation, rising to 85 percent after 15 to 28 days.
“What we see is a really high effectiveness already right after two weeks, between two weeks to four weeks after vaccine, already high effectiveness of 85 percent reduction of symptomatic infection,” Gili Regev-Yochay, co-author of the study, told a small group of journalists.
He said that despite the vaccine being “amazingly effective”, scientists are still studying whether fully vaccinated people can transmit the virus to others.
“That is the big, big, question. We are working on it. This is not on this paper and I hope we will have some good news soon,” said Regev-Yochay.
Israel has delivered a shot of the Pfizer/BioNTech vaccine to 4.23 million residents, or 47 percent of its nine million population, 2.85 million of whom have received the recommended full course of two jabs, latest health ministry figures show.
Separately on Friday, Pfizer and BioNTech said their vaccine can stand warmer temperatures than initially thought, potentially simplifying the jab’s complex cold-chain logistics.
The companies said they have asked the US Food and Drug Administration to allow for the vaccine to be stored for up to two weeks at minus 25 to minus 15 degrees Celsius (minus 13 to five degrees Fahrenheit)—temperatures commonly found in pharmaceutical freezers and refrigerators.
Under the existing guidelines, the Pfizer/BioNTech jab needs to be stored at a frigid minus 80 to minus 60 C until five days before use, a delicate process that requires special ultra-cold containers for shipping and dry ice for storage.
All the vaccines are either based solely on the viral Spike (S)-protein, which is administered by various methods including expression from non-replicating adenoviruses and nucleic acid vectors or as recombinant proteins, or are inactivated viruses that include the S-protein together with all other structural viral proteins (Table 1).
The vaccines are all based on S-proteins containing D614, which was the dominant strain when they were designed. A variant virus with G614 in its S-protein has since emerged to dominance globally because of its greater transmissibility (36–38). However, this D614G change does not affect sensitivity to neutralization by sera from infected or vaccinated people, or to neutralizing MAbs (monoclonal antibodies), and therefore is not problematic for vaccine efficacy (36–38).
Some of the more recent papers reviewed below include neutralization data using the G614 virus. All the Adenovirus, mRNA and DNA vaccine candidates listed in Table 1 involve full-length S-proteins; variants with truncations of the transmembrane region and/or the cytoplasmic tail were tested as comparators in two macaque studies (4, 6).
The recombinant proteins from Novavax and Clover Biopharm. are based on full-length S-proteins (9, 10, 12, 21). Many S-protein constructs incorporate two proline substitutions in the S2 region (K986P and V987P) that stabilize the expressed trimer in the pre-fusion structure that is considered to be optimally immunogenic for the induction of NAbs, while minimizing non-NAb responses (39).
The same method was used to stabilize the respiratory syncytial virus F-protein and improve its immunogenicity (40, 41). In one macaque study of Ad26 virus variants, the NAb response to the two-proline mutant S-protein was stronger than to other constructs that contained or lacked stabilizing changes, truncations or alternative leader sequences (4).
A mouse immunogenicity experiment that also compared Ad26 virus variants led to a similar conclusion (42). Comparative experiments in mice led to the inclusion of the same double proline change (plus a furin cleavage site knockout) in the Sanofi Pasteur mRNA vaccine (13).
Recombinant S-protein immunogens with the same combination of mutations provided the strongest protection against virus challenge in a mouse model, when compared with proteins that lacked these stabilizing changes (43). In a macaque comparison of DNA vaccines expressing various S-protein constructs, the authors reported that a soluble S-protein that contained the two proline substitutions together with a cleavage site knock-out and a trimerization domain (S.dTM.PP) was better than the corresponding truncated S-protein (S.dTM) at conferring protection from SARS-CoV-2 challenge (6).
Experimentation and precedent therefore support the use of stabilizing changes that maintain the SARS-CoV-2 S-protein trimer in its pre-fusion conformation. However, the S-protein expressed in the AstraZeneca/Oxford University ChAdOx1 nCoV-19 vaccine is a wild type sequence that does not include any stabilizing changes (3, 16, 17).
The Sputnik V rAd5 and rAd26 adenovirus vectors express a full-length S-protein, but the published report does not mention whether stabilizing mutations were added (28). Similarly, it was not stated whether the S-protein in the CanSinoBio Ad5-nCoV vaccine was stabilized (23, 24). Whether the known or likely absence of stabilizing changes impacts the performance of these various adenovirus vaccines is not known.
The Pfizer/BioNTech BNT162b1 vaccine was based on the S-protein’s receptor-binding domain (RBD), but its development was terminated after Phase 1/2 trials, in favor of the BNT162b2 construct that expresses the complete, stabilized S-protein (Table 4) (11, 25–27). All the mRNA vaccines are encapsulated within liposomes of unpublished composition, which accounts for their thermal fragility and need for storage and shipment in freezers at various temperatures.
It is of considerable scientific and public interest to know the immunogenicity of the leading vaccines in absolute and, to the extent possible, comparative terms. Here, we have reviewed antibody and T-cell immune response data derived from published studies of vaccines that were tested in non-human primates (NHPs) and then progressed into human Phase 1/2 trials, or that are in human trials without a prior NHP experiment (Tables 2-4).
We have also evaluated macaque vaccine-challenge experiments, including how they were performed, as the outcomes are relevant to understanding the protective potential of SARS-CoV-2 vaccines (Table 3). The NHP experiments are described in references (1–13), the human trials in references (14–32).
Immunogenicity of vaccine candidates in humans
Key antibody and T-cell response data summarized below for individual trials are presented in Table 4. As with the NHP studies, the primary papers and reviews should be consulted for additional details of the human trials, which are variously described as Phase 1, Phase 2 or combined Phase 1/2a trials (14–32). Vaccine safety assessments were a key component of these trials; in all cases, reported side effects and adverse events were considered to be minor or moderate; the primary papers contain the details, which we have not attempted to summarize.
A grade 3 serious adverse event (SAE) due to a neurological complication happened in the AstraZeneca/Oxford vaccine Phase 3 trial in the UK, leading to a now-concluded pause while the case was investigated. A placebo recipient in the Brazilian arm of the Phase 3 trial of this vaccine reportedly died of COVID-19. An SAE, also triggering a temporary clinical hold, occurred in the Janssen Phase 3 trial, although no details have been reported.
Details of the Phase 2b/3 efficacy trials for vaccines reaching that stage can be found at: (https://clinicaltrials.gov/ct2/results?recrs=&cond=Covid19&term=vaccine&cntry=&state=&city=&dist=). The preliminary outcomes of the Pfizer/BioNTech, Moderna, Gamalaya Center and AstraZeneca Phase 3 trials, as judged by information in press releases, are summarized in a separate section later in this article.
The initial human trials have predominantly involved young or middle-aged, healthy adults (see primary papers for details). Some information is becoming available on age-dependent decreases in immunogenicity. In the CanSino Ad5-nCoV vaccine, participants aged older than 55 responded with weaker antibody responses than their younger counterparts. However, that outcome could reflect either the aging process or time-dependent increases in exposure to other Ad5 viruses that compromise expression of the immunogen from by the vector (24).
An ~2-3-fold reduction in antibody responses was seen in older adults (aged 65-85) compared to younger ones (aged 18-55) in a Pfizer/BioNTech mRNA vaccine trial (27). Moderna has now reported similar findings for their mRNA vaccine; in a small-scale (40 volunteer) extension to their original Phase 1 trial, antibody and NAb responses were comparable in volunteers aged 56-60 and over 71 and similar to what was reported for those in the 18-55 age range (19, 20). NAb responses were slightly lower in volunteers aged over 60 than in ones in the 18-59 range, during the BBIBP-CorV inactivated Phase 1 trial.
The ratio was ~2-fold, but varied with the time point and vaccine dose, and group sizes were small (15). The Sinovac inactivated virus vaccine trial only involved volunteers under 60, but an analysis of the 18-29 vs. 50-59 age groups suggested that NAb responses were ~2-fold higher in the younger people. Overall, there was a modest trend toward weaker immunogenicity with age (14).
A limited amount of preliminary data on the Janssen Ad26.COV2 vaccine also indicates that immunogenicity in volunteers aged over 65 is only modestly reduced (18). When the ChAdOx1 nCoV-19 vaccine two-dose regimen was tested in volunteers aged 18-55, 56-69 or over 70, there was little or no age-dependent reduction in immunogenicity, judged by the same suite of antibody and T-cell assays used in the earlier trial (16, 17). In a macaque study, the immunogenicity of the Janssen Ad26.COV2 vaccine was comparable in adult and aged animals (5).
Taken together, the studies summarized above are encouraging for the efficacy of the various vaccines in older adults being comparable to what is seen in the Phase 3 trials that are being mostly conducted in younger people.
There has also been an under-representation of minority groups in the USA and European trials, so again information on how immunogenicity might vary in different populations is lacking. These various lacunae will need to be filled in Phase 3 trials, given that COVID-19 is more severe in older people and in African-American and Latinx populations.
Inactivated virus vaccines
The Sinopharm/WIBP inactivated virus vaccine was delivered in Alum adjuvant. It was first tested in 96 volunteers in a Phase 1 trial and then in 224 more people in a Phase 2 study (22). The study cohorts were based on healthy individuals aged from 18-59. The Phase 1 trial was dose-ranging (2.5, 5, 10 μg of viral protein) and involved i.m. injections on days 0, 28 and 56, while in Phase 2 only the 5 μg dose was tested in two sub-studies that involved immunizations on days 0 and 14 or on days 0 and 21.
Immune responses were measured by ELISA using inactivated virus as the detecting antigen, which does not allow a comparison with other vaccines, and by a RV neutralization assay. For sera collected 14 days after the final dose, the NAb titers (GM ID50 values) in the Phase 1 trial were 316, 206 and 297 in the low, medium and high dose groups respectively. Allowing for the titer ranges among participants, the three doses induced similar antibody responses.
In the Phase 2 trial, the corresponding NAb titer values were 121 and 247 for the 0, 14 day and 0, 21 day groups, respectively. Anti-virus ELISA endpoint GM titers were also similar among the different test groups in the two trials, and were ~200-300 in Phase 1 and ~90-200 in Phase 2 (Table 4). There were no T-cell data in the paper. Phase 3 trials are now in progress in South America, although the vaccine dose and delivery regimen (i.e., the number and spacing of doses) was not specified in the report on the Phase 1 and 2 trials (22).
Sinovac’s PiCoVacc inactivated vaccine was renamed CoronaVac prior to Phase 1 and 2 human trials (Table 1, 3) (1, 14). The production process used to make the vaccine was stated to be changed between the Phase 1 and 2 trials to yield a product with an ~2-fold higher S-protein content, which the authors suggest improved its immunogenicity (14). However, only data from the Phase 1 trial have been reported to date, the Phase 2 results still pending.
For Phase 1, the Alum-adjuvanted vaccine (or a placebo) was given to 600 adults aged 18-59 years in a two-dose regimen on days 0 and 14 or days 0 and 28. For each of the two regimens, two vaccine doses, 3 μg and 6 μg, were tested in 120 volunteers, compared to a placebo group of 60. The safety profile was unexceptional. Antibody responses were measured on days 28 and 42 in an anti-RBD ELISA and NAbs in an RV assay with a CPE readout (the cutoff for neither assay was reported). Anti-RBD GM titers were ~1000 in all the four vaccine groups on day 28. The NAb GMT values for all the groups at all the time points were generally in the 32-64 range (Table 4). The age-dependency of the antibody responses was noted above. No T-cell data were reported (14).
The BBIBP-CorV Alum-adjuvanted inactivated virus vaccine also advanced from macaque studies into Phase 1 and 2 human trials (15). The Phase 1 trial involved 192 volunteers aged 18-59 and 60 or older, who received vaccine doses of 2, 4 and 8 μg on days 0 and 28. Only volunteers in the younger age-range participated in the Phase 2 trial, in which 448 people received a single vaccine dose of 8 μg or two 4 μg doses given first on day 0 and then on day 14, 21 or 28. Taken together, the trials involved multiple small sub-groups, which limits the statistical power of any comparisons.
The paper should be consulted for details of how the different dosing regimens performed. The vaccine was generally safe, with only minor adverse events reported. Immunogenicity was assessed only in an RV NAb assay. The resulting NAb titers were modestly dose-dependent, were much stronger after two doses than one, and were slightly lower in the younger than older age groups.
In Phase 1, the 8 μg on day 0 and 28 regimen gave a GM titer of 228.7; in Phase 2, the highest NAb titers were seen in the group given an 8 μg dose on day 0 and 21, for which the GM titer was 282.7 (Table 4). No data on ELISA anti-virus or anti-S-protein titers, or on T-cell responses, were reported. Phase 3 trials are underway, using a two-dose regimen, but no details of the doses and scheduling chosen were provided (15).
A fourth Alum-adjuvanted, inactivated virus vaccine, from IMB/CAMS/PUMC, has also entered human trials, although no preclinical data were reported (29). This vaccine virus, KMS-1, was also produced in Vero cells but, unlike the other three, it was inactivated first with formaldehyde before beta-propionolactone treatment. In the Phase 1 trial, the IMB/CAMS/PUMC vaccine was given twice to 192 people aged 18-59 on days 0 and either 14 or 28, at doses of 50, 100 or 150 EU (the stated unit of antigen content).
There were no significant adverse events and sera from selected volunteers did not trigger antibody-dependent enhancement in vitro. Immunogenicity was assessed at several time points using an RV NAb assay, via various ELISAs including anti-S-protein and anti-virion, and by IFN-gamma ELISPOT. The paper should be consulted for data on the 9 individual sub-groups in the trial, but in general the GMT NAb titers (CPE with unspecified cutoff) were all <100 and often <50. Anti-S-protein GM titers (with no cut-off specified) were 2000-4000 for the two highest doses of immunogen (Table 4).
Only limited data were presented for the day 0, 28 group after the second dose. Thus, on day 90, the GMT NAb titer was <10 and the anti-S-protein titer was ~500, which indicate a time-dependent reduction in the initial antibody levels. IFN-gamma ELISPOT assays using S peptides gave AM values of 30-250 per 106 cells for the two vaccine doses, indicative of a generally weak T-cell response (29). In a Phase 2 study, 742 adults (aged 18-59) were given the higher dose of the inactivated vaccine on days 0 and 14 (30). The binding Ab and NAb titers reported were similar to those seen in the Phase 1 trial, but T-cell responses were not analyzed (Table 4).
Adenovirus vector vaccines
In a Phase 1 trial, the immunogenicity of the CanSino Ad5-nCoV vaccine candidate was found to be dose-dependent (23). Doses of 5 × 1010, 1 × 1011 or 1.5 × 1011 virus particles were given once to three different sub-groups. In the highest dose group, the anti-S-protein and anti-RBD GM titers on day 28 were 596.4 and 1445.8, respectively (in references 12 and 13, the cut-offs for titer determinations are not specified; we refer to them as “titers”).
The GM NAb titers were 34.0 and 45.6 in RV and PV assays, respectively, and were strongly correlated with anti-S and -RBD titers. A Phase 2 trial was then conducted on 508 participants, of which 126 received a placebo (24). The protocol again involved a single administration of the Ad5 virus, which was tested at doses of 1 × 1010 or 1.5 × 1010 in sub-groups.
The anti-RBD GM titers on day-28 was 656.5 range, which is a ~2-fold lower than in the Phase 1 trial. NAb titers in the RV and PV assays were 19.5 and 61.4, respectively, and hence similar to the Phase 1 trial data. T cell responses were measured by ELISPOT on samples taken before vaccination and then on days 14 and 28.
Freshly drawn PBMC were incubated with S-protein peptide pools for >12 hours, with the data expressed as SFC/105 cells after subtraction of background values derived from unstimulated control cells. (Note that the data in Table 4 have been adjusted to SFC/106 cells to facilitate comparison to other datasets). There was no mention of a positive control method, nor of the number of replicates.
An ELISPOT result was stated to be positive if the number of IFN-gamma secreting T cells responding to the S-protein peptides was increased 2 times above baseline post vaccination. TNF-alpha, IL-2 and IFN-gamma responses to the vaccine were also assessed by CFC. T cell responses peaked at day 14 post vaccine, and ranged from 200 SFC/106 cells in the low dose group to 580 SFC/106 cells in the high dose group. In CFC assays, both CD4+ and CD8+ T cells were found to be responsive (24).
AstraZeneca’s ChAdOx1 nCoV-19 recombinant virus vaccine (also known as AZD1222) was tested in a randomized Phase 1/2 trial involving 543 people; another 544 participants were given a meningococcal control vaccine (16). The original protocol involved a single dose of 5 × 1010 virus particles, which is twice the amount given to macaques (Tables 2 and 4).
However, a decision was taken during the trial to give ten participants a second dose of ChAdOx1 on day 28 in a non-randomized boosting protocol. It is assumed that the decision was taken because of the limited immunogenicity of the single-dose regimen (a modest boosting effect of a second dose was seen in the NHP study, see above; 3).
Anti-S-protein binding was measured at single dilutions and converted to “ELISA Units”, an approach that complicates comparisons with anti-S-protein responses to other vaccine candidates in humans and that represents an unexplained change from how the macaque sera were analyzed by titration in ELISA (3). By day 14 and 28, the responses in most of the participants were in the 100-1000 Units range (medians 102.7 and 157.1, respectively), with little change by day 56 in the sub-group that was assayed at that time point (median 119).
After the second vaccine dose in the prime-boost protocol, a ~5-fold increase in median anti-S-protein ELISA Units was measured 14 days later (median 997.5), and the levels were largely maintained by day 56 (median 639.2). NAbs were measured 14 days after the booster immunization using one PV and three different RV assays. NAb data from the PV assay and from the only RV assay that reported ID50 values are given in Table 4.
The primary paper should be consulted for other aspects of the neutralization data generated in various assays (16). Overall, the apparently modest NAb responses to the single-dose vaccine were increased a few-fold by the day 28 boosting immunization, at least in the short term (until day 42). The median titers for the prime-boost group on day 42 were 372-450.9 (Table 4). ELISPOT assays were performed on freshly isolated PBMC at days 0, 7, 14, 28 and 56, and at day 35 for the participants who received 2 doses.
Pooled peptides were used as antigens, and data were excluded if the assay background response rate was deemed to be too high. The measured responses peaked at day 14 at a value of 856 SFC/106 PBMC in the prime group and 1642.3 SFC/106 PBMC in the prime-boost group (i.e., after one dose in either group). The results for other time points are given in Table 4. Of note is that ~10% of recipients of this vaccine appear to generate no measurable T cell response after the first dose. Furthermore, the booster dose given to ten trial participants did not further increase their T-cell responses (16).
The initial UK-based Phase 1/2 clinical trial was extended to encompass 20 different sub-groups with variables that included the number of vaccine doses (one or two) given to volunteers in three different age groups: 18-55 (n = 100), 56-69 (n = 120) and 70+ (n = 200) (17). Control group members were given a Meningitis vaccine. Since there were 560 volunteers in total, the number of people in each individual sub-group was necessarily small, which limits the statistical power of any comparisons between the sub-groups.
Nonetheless, the data pattern supported the use of two doses in the subsequent Phase 3 trial. The immunogenicity of the two-dose regimen was comparable among the different age groups, judged by the same suite of antibody and T-cell assays used in the earlier trial (16, 17). However, the confidence intervals on the various data sets are quite wide, which may have prevented the detection of modest differences. It was also noted that reactogenicity diminished with age (17).
This vaccine has now advanced into Phase 3 trials in several international locations, including Brazil and South Africa. These trials were initiated as a single-dose regimen but were later changed to incorporate the second, boosting dose. A two dose Phase 3 trial started in the USA at the beginning of September, 2020.
The initial Phase 3 trial of the Janssen Ad26.COV2 adenovirus vector involves a one-dose regimen. Some of the human immunogenicity data on which this scheme was reportedly based have been described in an ‘interim report’ of a Phase 1/2a trial (18).
The vaccine was tested at two doses (5 × 1010 and 1 × 1011 viral particles) that were given i.m. either once (day 0 only) or twice (days 0 and 56) to healthy adults aged 18-55 (n = 402) or >65 (n =394). Safety data were generally unexceptional, although two SAEs were reported and deemed, after investigation, to be either not vaccine-related or not problematic (a high fever that was resolved).
Antibody immunogenicity was measured by S-protein ELISA, with data reported as Units/ml and an RV NAb assay with an IC50 endpoint. T-cell responses to the S-protein were measured by ICS, and cytokine release profiles were used to gauge Th1 versus Th2 bias. The paper should be consulted for data on the multiple individual sub-groups. Here, we will summarize what was reported for the initial Phase 3 trial regimen, a single dose of 5 × 1010 virus particles (Table 4).
Moreover, although the two-dose groups are mentioned in the paper, no data were presented for the antibody and T-cell responses to the second dose. And only a small subset of some one-dose groups were included in several immunogenicity analyses. For the Phase 3 regimen, the GMT anti-S-protein ELISA values on day 29 were 528 and 507 Units for 15 of the younger and older volunteers, respectively.
The corresponding NAb GMT IC50 titers were 214 and 196, although some of the samples were said to need re-assaying and additional data from a PV NAb assay are reportedly pending. The weak T-cell response data show the expected Th1 bias. The ICS percentages of CD4+ and CD8+ T-cells expressing IFN-gamma and IL-2 at day 15 for the Phase 3 regimen were 0.08% and 0.07%, respectively for the younger adults, and 0.36% and 0.05% for the older group (with large confidence intervals).
The T-cell response rate varied depending on the sub-group and assay, and was anything from 33% to 100%, although given the low number of samples in many cases the meaning of these data are not clear (18). Although the Phase 3 trial of the one-dose regimen is still ongoing, Janssen has initiated a second efficacy trial, in this case involving two doses of 5 × 1010 virus particles given at weeks 0 and 8. As noted above, this regimen evoked stronger and more sustained antibody responses than a single dose when evaluated in rhesus macaques (5).
Several weeks after the Russian Government approved the widespread use an adenovirus-vector based vaccine, Gam-COVID-Vac, a report appeared on how it had performed in Phase 1/2 trials (28). The vaccine involves the sequential delivery of rAd5 and rAd26 vectors that each express a full-length S-protein, which are given intramuscularly at 1 × 1011 particles per dose.
Two sub-trials, each involving 38 volunteers aged 18-60, compared frozen/thawed (Gam-COVID-Vac), or lyophilized/reconstituted (Gam-COVID-Vac-Lyo) vaccine formulations, which performed similarly (Gam-COVID-Vac was chosen for widespread use on convenience grounds). In the combination trial (n = 20), the first dose was rAd26 on day 0 followed by rAd5 on day 21.
Smaller sub-groups (n = 9) received only rAd5 or only rAd26. Phase 2 trials began a mere 5 days after Phase 1 ended, based on a successful interim safety assessment. Safety studies over 42 days (maximum) revealed nothing other than the generally mild reactions reported in other Ad-vector studies.
Immunogenicity assessments involved determining endpoint titers in anti-RBD and antiS1 IgG ELISAs and in an RV NAb assay, at weekly intervals. T-cell response data were derived from an INF-gamma ELISPOT and CD4+ and CD8+ T-cell proliferation assays. Anti-rAd5 antibodies were also measured to assess the possible influence of pre-existing immunity. Anti-RBD endpoint titers peaked at ~2000 by day 21 in the single-vaccine groups, and were boosted to 10,000-15,000 in the combination vaccine groups by day 42.
On day 42, the anti-S1 GM endpoint titer in the Phase 2 trial combination group was 53,006. NAb CPE67 titers at on day 28 in the single vaccine groups were in the range 5-10, but rose to GM values of 45.95-49.25 by day 42 in the combination groups. The authors themselves note that these titers are lower than were seen in the AstraZeneca/Oxford ChAdOx1 and mRNA vaccine trials, a difference to which the different measurements of NAb titers (ID50 versus CPE67) may contribute (see Table 4).
Cell-mediated immunity was measured via a T-cell proliferation assay that is rarely used in the trials of the other vaccines reviewed here. Proliferative responses were detected in all participants but seem weak in magnitude and were not boosted by the second dose.
On day 28, after the boosting immunization, median T-cell proliferation values were 2·5% vs. 1.3% for CD4+ and 1·3% vs. 1.1% for CD8+ cells in the groups receiving the frozen and lyophilized formulations, respectively. INF-gamma ELISPOT data were presented only as fold-increase from baseline values, so cannot be compared with other studies. However, there was again no boosting effect of the second immunization. A Phase 3 trial of the Gam-COVID-Vac combination vaccine began on August 26, and is planned to involve 40,000 volunteers of various ages and risk groups (28).
mRNA vaccines
The Phase 1 trial of the Moderna mRNA-1273 vaccine involved 45 volunteers in three dosing groups who were given 25, 100 or 250 μg of the immunogen by the i.m. route on days 1 and 29 (19). Antibody immunogenicity was dose-dependent and much stronger after the second dose than the first. Anti-S-protein GM endpoint titers in the 100 μg group on day 57 (28 days after the second dose) were 782,000, while the corresponding anti-RBD endpoint titers were ~30,000.
Most of the NAb data were derived from a PV assay; on day 43, the GM ID50 titer for the 100 μg group was 344. An RV assay was also used on a subset of day 43 samples. The resulting ID80 titers were 654 for the 100 μg group. Note that these are not ID50 values, which would be higher numbers. No detailed data on the longevity of the antibody responses were reported, but inspection of the graphs suggest that the antibody titers on a downward trend at the day 57 time point compared to days 36 and 43. T cell responses were measured only by CFC, and no data on their magnitude was reported.
For both vaccine dose groups, the peptide pools activated specific Th1 responses from <0.3% of the CD4+ T cells, and no Th2 responses were detectable. CD8+ T cell activity was, at most, minimal (19). In an extension of the trial that involved older adults (56-70 and over 71), the magnitudes of the anti-S-protein, anti-RBD and NAb responses to the two-dose regimens (25 μg or 100 μg) were similar to those reported for the 18-55 age groups (20) (Table 4). The 100 μg, two dose regimen was chosen for the Phase 3 studies that began in the USA during August 2020.
The Pfizer/BioNtech consortium has conducted three Phase 1 trials of lipid nanoparticle-encapsulated mRNAs that eventually led to the selection of the clinical candidate for now ongoing Phase 2/3 studies (25–27). In the first trial, the BNT162b1 mRNA expressing a soluble, trimerized version of the RBD was given at two doses (10 μg and 30 μg) on days 1 and 21 to groups of 12 participants, and once at 100 μg on day 1 to a third group of 12.
There were also 9 placebo recipients (25). Immunogenicity was assessed by anti-RBD (25, 26) or anti-S1 (27) binding Abs on days 7, 21, 28 and 35, although the data were reported in a non-traditional format that does not allow for cross-study comparison (25–27). All recipients in the two lower dose groups developed anti-RBD antibodies by day 21 that were boosted ~10-20-fold by the second immunization when measured on day 28 and unchanged by the end of the study on day 35. The 30 μg group was more immunogenic than 10 μg by ~3-fold.
The pattern of the NAb data was similar, although fewer time points were studied. In all three groups, the NAb responses to the initial immunization were low, but were boosted by the second dose. On day 28, the GM ID50 values in an RV assay for the 10 and 30 μg groups were 168 and 267, respectively (25).
A second Phase 1 trial, conducted in Germany, also explored dosing regimens (26). Multiple doses of BNT162b1 mRNA, in the range 1-50 μg, were tested, as were single doses and a prime-boost protocol involving two doses on days 0 and 21. Overall, and as expected, the immunogenicity data were comparable to what was seen in the first trial. Higher immunogen doses and the prime-boost format were associated with stronger responses, as expected.
The anti-RBD ELISA data were again presented in a non-traditional format. After the second dose, NAb ID50 GM titers in the higher dose groups were 578 in a RV assay and ~3100 in a PV assay. In an additional analysis, selected sera were tested in the PV-NAb assay against RBD and S-protein sequence variants (including the D614G change); no significant sensitivity differences were observed. T-cell responses were measured by a modified ELISPOT in which either CD4+ or CD8+ T cells were depleted from the effector population, or by CFC.
An unpublished ‘normalization’ method was applied to enable direct comparison of spot counts/strength of response to anti-CD3 stimulation between individuals. Because PBMC were separated into either CD4+ or CD8+ subpopulations in the ELISPOT assay, no direct comparison can be made with ELISPOT data on the other vaccines reviewed here due to differences in methodology. CD4+ and CD8+ T-cell responses were analyzed immediately before vaccination and then on day 29, i.e., 7 days after the booster immunization.
The magnitudes of both T-cell responses were dose-dependent. At the highest dose, the majority of participants had T-cell responses >1500 SFC/106 cells. The magnitudes of the CD4+ and CD8+ responses were comparable. Approximately equal proportions of the CD4+ responders fell into groups with <500, 501-1500 and >1500 SFC/106 cells. Cytokine secretion profiles showed that CD4+ T cells producing only IL-2 were the most abundant subset, while IL-4 release was minimal. This pattern is of potential concern, as CD4+ cells secreting IL-2 can polarize CD4+ T cells toward the Th2 phenotype that may be associated with VAERD (34-30=6). The responding CD8+ T cells mostly produced IFN-gamma (26).
The third Pfizer/BioNTech Phase 1 trial compared the BNT162b1 RBD-based construct with BNT162b2, an mRNA expressing a full-length, membrane-anchored S-protein (27). The two constructs were comparably immunogenic but BNT162b2 was associated with lower reactogenicity levels. Accordingly, BNT162b2 at a 30 μg dose was selected to progress into Phase 2/3 trials.
The Phase 1 trial had two principal sub-components, involving adults aged 18-55 and ones aged 65-85. For each of the two mRNAs, 3 or 4 different doses (10, 20, 30 μg and in one case 100 μg of mRNA, plus placebo) were tested in a two-immunization protocol (days 0, 21), so the 195 participants were split among 13 different groups in all. Here, we list the immunogenicity data only for the clinical candidate (BNT162b2, 30 μg), on day 28. NAb ID50 GM titers in a RV assay were 361 and 149 for the 18-55 and 65-85 age groups respectively, while the corresponding GM antibody endpoints to the S1 protein in a Luminex assay were 9136 and 7985 (see also Table 4).
Thus, for the clinical candidate, the NAb titers for the older group were 41% of those in their younger counterparts. Visual inspection of other antibody data sets suggests that the age-related reduction is generally ~2-3 fold, a decline that is perhaps meaningful but not catastrophic. No longer term antibody data and no information on T-cell responses were presented (27). The BNT162b2 vaccine candidate is now in Phase 3 trials in the USA and Europe, which involve a 2- dose regimen.
The Curevac CVnCoV mRNA vaccine was tested in 231 German adults aged 18-60, who were given different vaccine doses (2-12 μg) twice, on days 0 and 28 (32). Moderate, dose-dependent side effects were reported. Anti-S protein and anti-RBD median endpoint titers on day 43 were moderately dose-dependent and highest for the 12-μg group (5463 and 1007, respectively). The median CPE50 NAb titer measured in an RV assay at this time was 113. No T-cell response data were presented. The 12-μg dose was chosen for a Phase 2b/3 trial (32).
Recombinant protein vaccines
The first report on how a recombinant S-protein performs in humans described the Novavax NVX-CoV2373 vaccine candidate (21). The immunogen is an insect cell-derived soluble S-protein. When mixed with detergent, 5 or 6 S-proteins become attached non-covalently via their bases to the resulting micelles (63). This component of the immunogen was co-administered with Matrix-M adjuvant.
Two formulations (5 μg and 25 μg of S-protein) were tested in 106 people with or without adjuvant, in a two-dose regimen on days 0 and 21. In the absence of adjuvant, antibody responses were, as expected, very weak, while the 5- and 25-μg doses performed comparably when the adjuvant was present. Anti-S-protein ELISA data were presented as Units, which again prevents cross-study comparison.
The highest values recorded, on day 35, were 63,160. NAbs were measured in an RV assay and reported as ID>99 values. Here, the peak GM values were 3906 and 3305 for the 5 and 25 μg groups, respectively, on day 35. As noted above when discussing the corresponding macaque experiment, when NAb data are presented as ID>99 or CPE100 values the reported numbers are likely to be several-fold lower than the more commonly used ID50 values.
CFC was used to measure CD4+ T-cell responses at days 0 and 28, but in only 4 participants per group. There were no responses in the placebo or protein with no-adjuvant recipients, but CD4+ T cell signals could be measured in the adjuvanted protein groups at day 28, with two protein doses inducing similar but moderate responses. Both Th1 and Th2 cytokines were released although the Th1 signals were more consistent, particularly at the lower protein dose (21). A Phase 3 trial of the Novavax vaccine began in the UK in September 2020, and its U.S. counterpart is scheduled to commence in December 2020.
The CoVLP vaccine from Medicago Inc. and McGill University is based on a stabilized S-protein engrafted to an influenza HA TM/CT region. These constructs, expressed in the plant cells, self-assemble into virus-like particles (VLPs) (31). In a Phase 1 study, the VLPs were administered without adjuvant or in either AS03 (GSK) or CpG 1018 (Dynavax) adjuvants, on days 0 and 21, at doses of 3.75, 7.5 or 15 μg.
The safety profile was unexceptional. Antibody responses were assessed by S-protein ELISA and in RV and PV NAb assays. The responses in the no-adjuvant group were, as expected, weak. The three dosing groups behaved fairly similarly; the lowest-dose responses were at least as high as the others.
The AS03 adjuvant consistently out-performed CpG 1018 by 10-50 fold in various groups at different times, which is a useful result with more general implications. On day 42, GM EC50 anti-S titers in the AS03 groups were ~300,000; NAb GM ID50 values were ~2200 in the PV assay and ~630 in the RV assay (responses after the first dose only were very much weaker). T-cell responses were assessed by IFN-gamma and IL-4 ELISPOT assays and were again strongest in the AS03 groups (the differential vs CpG 1018 was greater for IFN-gamma than IL-4). The highest IFN-gamma signal (AS03, day 42) was ~ 500 SFU per million cells, and ~400 for IL-4. The lowest dose (3.75 μg) group with AS03 will proceed into additional clinical trials involving a two immunizations regime (31).
Taken together, and with caveats about comparing data from different studies, two features of the binding-antibody and NAb data stand out (Table 4). The seemingly strongest responses were induced by the NVX-CoV2373 adjuvanted recombinant S-protein. This judgement takes into account the presentation of ID>99 values, rather than the more commonly used ID50 titers (21).
The relationship between ID>99 and ID50 values depends on the shape of the titration curve, but ID>99 are by necessity lower, often >10-fold (Fig. 3). The superior immunogenicity of the recombinant S-protein mirrors its performance in macaques (Table 2) (9, 10). The second conclusion we can draw is that binding antibody and NAb responses to the single dose adenovirus vector vaccines are quite weak, although a second dose does improve their performance, as judged by the limited dataset available (Table 4) (16, 18, 23, 24). The T-cell response data are too limited, and the protocols used too variable, for us to draw any conclusions about relative immunogenicity.
Vaccine efficacy, as reported in press releases
The press releases referred to below are all archived on the websites of the relevant institutions, and should be consulted for additional details or the precise language used. The first indications of vaccine efficacy came from a Pfizer/BioNTech release issued on 11/9/20. At that point, the companies had accumulated data from 94 COVID-19 cases during the BNT162b2 Phase 3 trial in which 38,955 volunteers had been fully vaccinated (7 days after the second dose).
Although there was no breakdown of the 94 cases by vaccine versus placebo, the efficacy level was stated to be “above 90%”. Two linked releases from the same group on 11/18/20 and 11/20/20 provided additional information based on over 41,135 fully vaccinated trial participants. By then, the number of cases had reached 170 that were now broken down into 162 placebo recipients versus 8 given the vaccine, leading to an efficacy estimate of 95%. High-level efficacy was reported for all demographic groups, including adults over 65. In addition, it was announced that 10 severe COVID-19 cases had been documented, of which 9 were in the placebo group, an early although inconclusive indication that preventing severe disease might be possible.
Within about a day of the initial Pfizer/BioNTech release, one was issued by the Russian Direct Investment Fund and The Gamalaya National Center concerning their Sputnik V vaccine (11/11/20). It reported that 16,000 trial participants had received both vaccine doses; with 20 confirmed symptomatic cases and a calculated efficacy of 92%.
No vaccine versus placebo breakdown was provided. It was also stated that the number of COVID-19 cases identified in 10,000 more vaccine recipients who were not enrolled in clinical trials confirmed that vaccine efficacy was over 90%. On 11/24/20, a second press announcement on Sputnik V reported vaccine efficacy of 91.4% based on 39 COVID-19 cases among 18,794 fully vaccinated volunteers. Of these cases, 8 occurred in 14,095 vaccinated volunteers, while 31 were in 4,699 placebo recipients. The press release also mentioned that vaccine efficacy was over 95% when an analysis was performed at a later time point (21 days after the second dose, as opposed to 7 days), but no additional details were given.
Moderna issued its first press release on mRNA-1273 efficacy on 11/16/20. In the COVE trial of over 30,000 volunteers, 95 symptomatic cases had been documented, of which 90 were in the placebo group. Vaccine efficacy was reported to be 94.5%. An additional analysis showed that all 11 cases of severe COVID-19 were in the placebo group. Consistent efficacy was seen in all demographic and age groups, although no details were reported.
A further announcement on 11/30/20 reported that 196 symptomatic cases had now accrued in the trial, with 185 of them being in the placebo group. Vaccine efficacy was accordingly stated to be 94.1%. Furthermore, all 30 cases of severe COVID-19, including one death, were in the placebo group, which strengthens the evidence that this vaccine also prevents serious disease. This second press release again stated that efficacy was consistent across age, race, ethnicity and gender demographics.
A press release was issued by AstraZeneca on 11/23/20, reporting on the company’s AZD1222 (ChAdOx1) vaccine. The information it contained was based on two trials in the UK and Brazil, and could be considered quite confusing (https://www.wired.com/story/the-astrazeneca-covid-vaccine-data-isnt-up-to-snuff/). Some clarifications emerged in subsequent media reporting and oral statements.
The summary below reflects what is known at the time of writing. In total, 131 COVID-19 cases occurred in the trials, but their distribution was not broken down either by trial or vaccine vs. placebo. One dosing regimen involving 2,741 UK volunteers was stated to confer 90% efficacy. It involved a half dose of vaccine followed by a full dose at least one month later, a regimen that was the result of an apparent error in dose calculations.
In contrast, the trials involving the originally intended protocol of two full doses yielded an efficacy of 62% from 8,895 vaccinated volunteers. Combining all the trials, which may or may not be appropriate for regulatory approval, led to a stated “average efficacy” of 70%. It was also reported that no hospitalizations or severe cases of COVID-19 occurred among vaccine recipients, which is presumably a reference only to the unstated number who became infected. Additional data from an ongoing phase 3 trial in the USA, and from a new one that may be initiated in the UK, may eventually clarify where this vaccine stands on the efficacy spectrum.
reference link: https://advances.sciencemag.org/content/early/2021/02/19/sciadv.abe8065.1
Journal information: The Lancet
