A British SARS-CoV-2 human challenge study led by Imperial College London and the commercial clinical-research organization hVIVO Services Ltd-UK that was also supported by researchers from various other hospitals and universities in the United Kingdom showed that young healthy individuals who were deliberately infected with the SARS-CoV-2 virus only displayed mild symptoms.
In order to establish a novel SARS-CoV-2 human challenge model, 36 volunteers aged 18-29 years without evidence of previous infection or vaccination were inoculated with 10 TCID50 of a wild-type virus (SARS-CoV-2/human/GBR/484861/2020) intranasally.
Mild-to-moderate symptoms were reported by 16 (89%) infected individuals, beginning 2-4 days post-inoculation. Anosmia/dysosmia developed more gradually in 12 (67%) participants. No quantitative correlation was noted between viral load and symptoms, with high viral load even in asymptomatic infection, followed by the development of serum spike-specific and neutralizing antibodies.
The study findings were published on a preprint server: Research Square and are being peer reviewed for publication into the journal: Nature Reports.
We here report the virological and clinical results from the first SARS-CoV-2 human challenge study. With a low inoculum dose of 10 TCID50, robust viral replication was observed in 53% of seronegative participants. After an incubation period of <2 days, VLs escalated rapidly, peaking at high levels and continuing for over a week.
Anosmia/dysosmia was common, occurred later than other symptoms and resolved without treatment in most participants within 90 days.
In those with residual smell disturbance, their sense of smell steadily improved during the follow-up period, consistent with the good long-term prognosis seen in community cases13. There was no evidence of pulmonary disease in infected participants based on clinical and radiological assessments.
The natural infectious dose of SARS-CoV-2 is unknown but based on in vitro and preclinical models, the virus is understood to be highly infectious14–16 and well-adapted to rapid and high-titre replication in human respiratory mucosa17. Early in the pandemic, a WHO Advisory Group published expert consensus guidelines recommending a starting dose of 102 TCID5010.
Here, based on in vitro data of high viral replication in primary human airway epithelial cells, we started with a tenfold lower dose of 10 TCID50 (equivalent to 55 FFU) and found it sufficient to meet the 50-70% target infection rate. With prospective household contact studies having similarly shown high secondary attack rates of ~38%12, this suggests that the model can recapitulate higher exposure than naturally-acquired infection events.
In contrast, experimental infections of non-human primates have used 1,000-10,000 times more virus, with intratracheal or combined upper/lower airway administration, which results in markedly different kinetics to those observed during human infection18,19. In human challenge studies with other respiratory viruses such as influenza viruses and RSV, inoculum doses are typically also much higher at 104-106 TCID50 since all volunteers have been exposed multiple times throughout life to those viruses, with pre-existing immunity reducing susceptibility and resulting in substantially lower peak viral loads at 103-104 copies/mL by PCR20,21. Thus, neither animal models nor human data from other viral infections were helpful in estimating the optimal SARS-CoV-2 inoculum dose.
Although some studies have measured the response to SARS-CoV-2 infection longitudinally in humans22–24, none can capture host features at the time of virus exposure, the early events prior to symptom onset, or the detailed course of infection that can be shown by experimental challenge.
Whilst the incubation period from the estimated time of natural exposure to perceived symptom onset has previously been estimated as ~5 days25,26, this best aligns with peak symptoms and is longer than the true incubation period. With close questioning, symptoms were found to be associated with viral shedding within 2 to 4 days of inoculation but did not peak until day 4-5.
Thus, virus was first detected (first in the throat, then the nose) ~2 days before peak symptoms and increased steeply to achieve a sustained peak, in many cases before peak symptoms were reached, consistent with modelling data indicating that up to 44% of transmissions occur before symptoms are noted6.
Anosmia was a later symptom, potentially explained by the proposed mechanism whereby only ACE2- and TMPRSS2-expressing supporting cells rather than neurones themselves are directly infected, leading to delayed secondary olfactory dysfunction27.
Pre-emptive remdesivir was administered to the first 6 infected participants as risk mitigation during early model development as trial data had suggested efficacy in shortening time to recovery in hospitalised patients28. However, no statistically significant effect on viral load or symptoms was detectable in this small cohort.
Field data have questioned the effectiveness of remdesivir in the hospitalised patient setting29 but antiviral treatment is commonly more effective early in the course of infection. This study was not designed nor powered to assess the efficacy of early treatment with remdesivir so this remains to be tested, but such prospective human challenge studies would be well placed to answer the question of antiviral efficacy, with treatment commenced at different times relative to virus exposure.
A key unresolved question for public health has been whether transmission is less likely to occur during asymptomatic/mild infection compared to more severe disease. Some studies have shown a correlation between disease severity and extent of viral shedding30,31, but others have not32.
Overall, peak VLs reported in natural infection (~105-108 copies/mL) are lower than those observed in this study6,33−36 However, these are invariably sampled at the time of case ascertainment and, where longitudinal samples have been taken, these indicate that patients are already in the downward phase of the VL curve24.
It is therefore likely that most samples miss the peak of viral shedding. With virus present at significantly higher titres in the nose than the throat, these data provide clear evidence that emphasises the critical importance of wearing face coverings over the nose as well as mouth.
Furthermore, our data clearly show that SARS-CoV-2 viral shedding occurs at high levels irrespective of symptom severity, thus explaining the high transmissibility of this infection and emphasising that symptom severity cannot be considered a surrogate for transmission risk in this disease.
This remains relevant with the widespread transmission of the Delta and Omicron variants, where antigenic divergence along with waning vaccine-induced immunity lead to VL during breakthrough infection at comparably high levels to those in seronegative individuals12,37.
Despite the relatively small sample size, limited variation between infected study participants and longitudinal analysis permits several conclusions of public health importance. Detailed viral kinetics show that some individuals still shed culturable virus at 12 days post-inoculation (i.e. up to 10 days after symptom onset) and, on average, viable virus was still detectable 10 days post-inoculation (up to 8 days after symptom onset).
These data therefore support the isolation periods of 10 days post-symptom onset advocated in many guidelines to minimise onward transmission38.
High levels of asymptomatic/pauci-symptomatic viral load also highlight the potential positive impact of routine asymptomatic testing programmes that attempt to diagnose infection in the community so that infection control measures such as self-isolation can be implemented to interrupt transmission.
In several jurisdictions, these rely on rapid antigen tests, with recent re-analysis of cross-sectional LFA validation data having suggested that sensitivity for infectious virus may be higher than previously estimated at ~80%39. Reassuringly, longitudinal LFA data following SARS-CoV-2 challenge also strongly predicted culturable virus aside from the very earliest time-points where sensitivity was lower.
In addition, LFA was highly reliable in predicting the disappearance of viable virus and therefore also underpin “test to release” strategies, which are increasingly being used to shorten the period of self-isolation. While positive LFA results were occasionally seen with negative FFA results (causing a reduction in specificity in relation to the viable virus assay), there were no false positives when comparing LFA to qPCR, implying the relatively lower sensitivity of viral culture rather than false positivity of LFA.
Although some uncertainty remains in directly extrapolating these data to the community where self-swabbing and more concentrated samples may alter sensitivity, these results support their continued use for identifying those most likely to be infectious. Our modelling also suggests that this strategy remains effective even if imperfectly implemented, with routine testing as little as every 7 days able to interrupt more than half the virus still to be shed by an individual, if acted upon.
Although these first-in-human data do not preclude rare adverse events that can only be detected in larger-scale studies, our results indicate that human challenge with SARS-CoV-2 is consistent with natural infection in healthy young adults, having caused no serious unexpected consequences and therefore supporting further development and expansion.
This first report focuses on safety, tolerability and virological responses, but the uniquely controlled nature of the model will also enable robust identification of host factors present at the time of inoculation and associated with protection in those individuals who resisted infection. Analysis of local and systemic immune markers (including potentially cross-reactive antibodies, T cells and soluble mediators) from this SARS-CoV-2 human challenge study that may explain these differences in susceptibility are therefore ongoing.
In addition, with the feasibility of this approach having been demonstrated using a prototypic wild-type strain, further challenge studies are now underway in which previously infected and vaccinated volunteers will be challenged with escalating inoculum doses and/or viral variants to investigate the interplay between virus and host factors that influence clinical outcome.
Together, these studies will thus optimise the platform for rapid evaluation of vaccines, antivirals and diagnostics by generating efficacy data early during clinical development and avoiding the uncertainties of studies that require ongoing community transmission.