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Pfizer’s vaccine is made using mRNA technology. Photo: Reuters

Covid-19 vaccines: Sinovac recipients may need two booster shots, says scientist

  • A preprint study suggests adding a dose of Pfizer after two shots of the Chinese vaccine boosts protection but was not enough to stop Omicron infection
  • Sinovac is widely used in China and developing countries
Two doses of the Sinovac Covid-19 vaccine boosted with one shot of Pfizer will induce higher levels of protective antibodies than two Pfizer doses, but this combination is not sufficient to stop Omicron infection, according to a preprint study.

The findings suggest countries that have used the Pfizer vaccine as a booster dose after Sinovac’s CoronaVac, widely used in China and developing countries, may still need to offer a second booster dose, according to one author of the paper, which has not been peer-reviewed.

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The researchers from Yale University analysed plasma samples from 101 participants in the Dominican Republic who received a booster dose of Pfizer’s vaccine more than four weeks after being given a second dose of Sinovac, which is made using inactivated, or dead, matter from the coronavirus to stimulate an immune response.

The results were then compared to samples from 37 health care workers from the Yale-New Haven Hospital who received two doses of Pfizer, made using cutting-edge mRNA technology.

They found that receiving two doses of CoronaVac followed by a Pfizer booster dose elevated the levels of neutralising antibodies 10.1 times against the original Sars-CoV-2 virus and 6.3 times against the Delta variant 28 days after the booster shot.

However, antibody neutralising activity for this combination against the Omicron variant was 6.3 times lower than against the original virus, and 2.7 lower than against the Delta variant.

There was no neutralising activity observed for participants who only received two doses of CoronaVac. Although the level of antibodies was 1.4 times higher than those who had received two doses of the Pfizer mRNA vaccine, this was not enough to protect against Omicron infection.

Both groups of study subjects included participants who had previous infections.

The study found that previous infection among the Pfizer vaccine recipients provided advantages in antibody protection against variants, including Omicron, but no such advantages were observed in Sinovac recipients who had received a Pfizer booster shot.

“The significantly reduced neutralisation titres against Omicron suggest a greater risk of vaccine breakthrough infections,” the researchers wrote, adding that clinical follow-up is needed to assess the risk of serious disease.

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The authors said the study, published in medRxiv.org on Wednesday, could be a reference point for countries that offered two doses of Sinovac and other inactivated Covid-19 vaccines.

“In terms of public health, two doses of CoronaVac is insufficient to neutralise Omicron. Even with two doses of CoronaVac plus Pfizer booster, neutralising antibodies is only 1.4 times higher than 2 doses of mRNA alone,” Akiko Iwasaki, a professor of immunology at Yale University who led the study, wrote Twitter on Thursday. “Thus, CoronaVac recipients may need 2 additional booster doses to reach levels needed against Omicron.”

The study came weeks after the World Health Organization advised countries using inactivated vaccines to use mRNA vaccines or vectored vaccines for subsequent doses.

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There are different layers of immunity and when antibody protection fails, and T-cells, a type of white blood cell that attacks infected cells, can form another line of defence.

Separate studies from South Africa and the Netherlands have found that T-cells were still holding up against Omicron after receiving mRNA or vectored vaccines and this may be one reason why the recent waves of Omicron have yet to result in proportionately higher levels of hospitalisation. There is no such data on inactivated vaccines so far.

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