However, those vaccinations alone are of no help against the widely circulating Omicron variant, show a new study by researchers at Yale and the Dominican Republic. The results are published in the journal Nature Medicine.
An analysis of blood serum from 101 individuals from the Dominican Republic showed that Omicron infection produced no neutralizing antibodies among those who received the standard two-shot regimen of the Sinovac vaccine.
But when researchers compared these samples with blood serum samples stored at Yale, they found that even those who had received two Sinovac shots and a booster had antibody levels that were only about the same as those who’d received two shots of the mRNA vaccines but no booster shot. In other studies, the two-shot mRNA regimen without a booster has been shown to offer only limited protection against Omicron.
The findings will likely complicate global efforts to combat the Omicron strain, which has supplanted the more dangerous but less transmissible Delta strain as the most dominant circulating virus in much of the world.
An additional booster shot – and possibly two – are clearly needed in areas of the globe where the Sinovac shot has been chief source of vaccination, said Akiko Iwasaki, the Waldemar Von Zedtwitz Professor of Immunobiology and senior author of the paper.
Omicron has proven particularly problematic to combat because it possesses 36 mutations on the spike proteins on its surface, which the virus uses to enter cells, researchers say. Existing mRNA vaccines are designed to trigger antibody response when spike proteins are recognized.
“But we need antibodies to prevent infection and slow transmission of the virus,” she said.
Carolina Lucas and Valter Silva Monteiro, both from the Yale School of Medicine, are co-lead authors of the paper. Eddy Perez-Then, of the Health Ministry of the Dominican Republic, and Marija Miric, of Two Oceans Health in Santo Domingo, are co-lead authors.
A study by two Hong Kong universities released this week found that three doses of the Sinovac COVID-19 vaccine (CoronaVac) failed to provide sufficient antibodies to fend off Omicron.
The study, which was jointly conducted by the University of Hong Kong and the Chinese University of Hong Kong, examined the efficacy of a third dose of the CoronaVac and Pfzier-BioNTech (BNT) vaccines against the Omicron variant. On Thursday (Dec. 23), the research teams announced that the study showed that a third dose given to participants who had received two previous doses of CoronaVac, “does not provide adequate levels of protective antibody,” while a third dose of BNT did.
The findings of the study revealed that two doses of either CoronaVac or BNT provide “very poor virus killing (neutralizing) antibody responses to Omicron.” A third dose of CoronaVac failed to provide adequate protective antibody levels.
However, a third dose of BNT in participants who had either received two previous doses of BNT or CoronaVac did result in protective antibodies against Omicron. Based on these findings, Malik Peiris, chair professor of virology at the School of Public Health of the University of Hong Kong, recommended that people who have either received two doses of BNT or CoronaVac, get a third dose of BNT about six months after their previous shot to “achieve optimal protection against (the) Omicron variant.”
The results are similar to a study released by the University of Hong Kong (HKU) on Dec. 13 that found those who received two doses of the Sinovac COVID-19 vaccine (CoronaVac) had not developed sufficient antibodies to fend off Omicron. Of the 25 who received two doses of BNT, only five developed a neutralizing ability against the two Omicron variants, while of 25 inoculated with two doses of CoronaVac, none produced a detectable antibody against Omicron.
Sinovac tried to counter that study by claiming that a third dose of CoronaVac was 94% effective against the Omicron variant. However, the firm did not provide details on the antibody levels produced by a third shot.
The apparent ineffectiveness CoronaVac has against the Omicron variant could have implications for China’s attempts to prevent Omicron from spreading internally. The lack of effectiveness against Omicron, which is 70 times more transmissible than the Delta variant, could also impact many developing countries with 2.3 billion doses of the Chinese vaccine produced and shipped worldwide.
More information: Eddy Pérez-Then et al, Neutralizing antibodies against the SARS-CoV-2 Delta and Omicron variants following heterologous CoronaVac plus BNT162b2 booster vaccination, Nature Medicine (2022). DOI: 10.1038/s41591-022-01705-6