Yale Researcher: Sinovac Recipients May Need Two Boosters

A study by Yale University and the Dominican Republic shows that even with a Pfizer booster after two Sinovac doses, protection against Omicron is only slightly higher than with two mRNA doses alone, hence two boosters after two Sinovac jabs may be needed.

KUALA LUMPUR, Feb 10 – A study by Yale University and the Dominican Republic’s Health Ministry suggested that people double-vaccinated with Sinovac may need two Covid-19 vaccine boosters to protect against Omicron.  

This is because a Pfizer-BioNTech booster after a Sinovac primer produced neutralising antibodies against Omicron that were only 1.4 times higher than with a two-dose mRNA vaccine. Two Sinovac vaccine doses alone showed no protection against the highly contagious coronavirus variant.

“In terms of public health, CoronaVac 2x is insufficient to neutralise Omicron. Even with CoronaVac 2x plus Pfizer booster, NAb (neutralising antibodies) is only 1.4x higher than 2x mRNA alone. 

“Thus, CoronaVac recipients may need 2 additional booster doses to reach levels needed against Omicron,” tweeted Prof Akiko Iwasaki, one of the study’s authors from the United States’ Yale University. CoronaVac is the name of the inactivated Covid-19 vaccine produced by China’s Sinovac Biotech.

Two Sinovac vaccine doses plus a Pfizer booster provide neutralising antibody levels against the Omicron coronavirus variant that are only 1.4 times higher than with two mRNA vaccine doses alone. DR refers to participants from the Dominican Republic, whereas Yale refers to participants from Yale University in the United States. Graphic from the paper titled “Immunogenicity of heterologous BNT162b2 booster in fully vaccinated individuals with CoronaVac against SARS-CoV-2 variants Delta and Omicron: the Dominican Republic Experience”, published December 29, 2021.

The study, which has not yet been peer-reviewed and was published last December 29, looked at the effectiveness of a Pfizer booster on individuals in the Dominican Republic who were double-vaccinated with Sinovac against the ancestral Covid-19 virus, and the Delta and Omicron variants. It did not compare Pfizer with Sinovac boosters for Sinovac recipients. 

A separate study by the University of Hong Kong and Chinese University of Hong Kong reportedly found that three Sinovac vaccine doses do not produce adequate levels of neutralising antibodies against Omicron. Researchers said a Pfizer booster instead for Sinovac recipients would “achieve optimal protection” against Omicron.

In line with previous research, the Yale-Dominican Republic study found that two vaccine doses of either Pfizer or Sinovac were less effective against Omicron compared to other variants. 

For two Pfizer doses, neutralising antibody levels against Omicron were 11.9 times lower compared to the ancestral virus.   

Notably, among participants double-jabbed with Sinovac, there was no detectable neutralisation against Omicron.

A Pfizer booster elicited detectable levels of neutralisation against Omicron among Sinovac recipients, but their antibody levels were 7.3 times lower against Omicron compared to the ancestral virus.

A single Pfizer booster for Sinovac recipients increased their neutralising antibody levels against the ancestral virus and Delta variant to similar protection from two mRNA vaccine doses.

Surprisingly, previous Covid-19 infection did not increase neutralisation against Omicron for Sinovac recipients who received a Pfizer booster.

Iwasaki pointed out that prior infection in those double-vaccinated with Pfizer, on the other hand, provided advantages in cross-protection against the ancestral virus and variants of concern, including Omicron.

Iwasaki, who is a professor of immunobiology and molecular, cellular and developmental biology, explained that prior infection only synergies with the mNRA vaccine to elevate broadly neutralising antibodies, but not with the inactivated vaccine.

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