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Why Sinovac Recipients May Need TWO Pfizer Booster Shots!

A Yale University study just showed that Sinovac recipients may need TWO Pfizer booster doses to receive adequate protection against the Omicron variant!

Here is what you need to know…


Yale : Sinovac Recipients May Need Two Pfizer Booster Doses!

When it rains, it pours… and right now, the Sinovac CoronaVac vaccine is getting drenched!

The RECoVaM and Hong Kong studies recently showed that the Sinovac vaccine was poor at fighting off COVID-19 variants, even with a third, booster dose.

And now, a new Yale University study has raised the possibility that even a Pfizer booster dose cannot make up for its deficits.

In fact, it suggests that a second Pfizer booster dose may be necessary to adequately protect Sinovac recipients against the Omicron variant.

Let me take you through the key findings of this Yale University study…


Why Sinovac Recipients May Need Two Pfizer Booster Doses!

The Yale University study of the Dominican Republic’s experience in using a Pfizer booster dose for people already fully-vaccinated with two doses of the Sinovac CoronaVac vaccine is titled, “Immunogenicity of heterologous BNT162b2 booster in fully vaccinated individuals with CoronaVac against SARS-CoV-2 variants Delta and Omicron: the Dominican Republic Experience“.

You can read the abstract here, and download the study in PDF here. But please avoid reading the Reuters news article, which mischaracterised the study.

With help from one of the study’s authors, Professor Akiko Iwasaki, here is what you need to know…

Takeaway #1 : Heterologous Vaccination Greatly Increases Antibody Levels

Sinovac recipients who took a Pfizer booster dose received a massive boost in antibody levels.

The violin plot is logarithmic, which means the antibody levels of those who received the Pfizer booster dose are exponentially better than those who only received the Sinovac vaccine.

This shows that heterologous vaccination (mixing of vaccines) offers a big boost in antibody levels.

However, it is also obvious that the Pfizer booster dose is not able to offer the same antibody levels as someone who received two doses of the Pfizer vaccine.

Takeaway #2 : Protection Equivalent To 2x Pfizer

A single Pfizer booster dose greatly elevated the levels of neutralising antibodies (NAb) against the ancestral (original COVID-19) and the Delta variant.

In other words, that Pfizer booster dose is able to deliver massively improved levels of neutralising antibodies, and bring protection level up to par with two Pfizer vaccine doses, against the ancestral and Delta variants.

Takeaway #3 : Omicron Has Greatest Vaccine Escape Ability

The Omicron variant has the greatest ability to “escape from neutralisation”, compared to the other COVID-19 variants.

This comparison shows the Plaque Reduction Half-Maximal Neutralising Titre (PRNT50) results of people who were fully vaccinated with two doses of the mRNA vaccine :

Takeaway #4 : 2-Dose Sinovac Is Useless Against Omicron

The Yale study found that there was NO detectable neutralisation against the Omicron variant, in people who were fully-vaccinated with two doses of the Sinovac vaccine.

Takeaway #5 : Pfizer Booster Boosted Protection For Sinovac Recipients

After receiving the Pfizer booster dose, Sinovac recipients had markedly higher neutralising antibody levels against the Ancestral, Delta and Omicro variants.

Professor Iwasaki stresses that even though the Pfizer booster dose offers better neutralisation, that protection was “reduced by 7.3x against Omicron compared to the ancestral virus“.

In other words – the protection offered by the single Pfizer booster dose may not offer adequate protection against the Omicron variant.

Takeaway #6 : Infection Boosts Protection From mRNA Vaccine

Here’s an interesting finding from the Yale study – people who were fully-vaccinated with mRNA vaccines gained additional protection after COVID-19 infection.

This “cross protection” greatly increased the amount of neutralising antibodies against all variants, including the Omicron variant.

Takeaway #7 : Cross Protection NOT Seen With Sinovac Vaccine

Unfortunately, this cross-protection benefit from a COVID-19 infection is NOT seen with Sinovac recipients, even when they receive a Pfizer booster dose.

According to Professor Iwasaki, “Immunologically, this difference is interesting. Prior infection only synergies with the mRNA vax to elevate broadly neutralizing Ab but not with inactivated vax. This may relate to the persistent GC (Germinal Centre) responses in mRNA vax“.

In other words, the synergistic boost in antibody levels from a COVID-19 infection is only seen with mRNA vaccines like the Pfizer COMIRNATY, probably due to persistent B cell response; but not seen with inactivated virus vaccines like the Sinovac CoronaVac.

Takeaway #8 : Sinovac Recipients May Require TWO Booster Doses

The Yale study confirms what the earlier Hong Kong University study revealed – that two doses of the Sinovac CoronaVac vaccine is useless against the Omicron variant.

Even though the Pfizer booster dose massively increases the antibody levels of fully-vaccinated Sinovac recipients, their neutralisation ability against Omicron is only increased by 40% (1.4x), compared to people vaccinated with two doses of the mRNA vaccines.

Hence, Professor Iwasaki concludes that “CoronaVac recipients may need 2 additional booster doses to reach levels needed against Omicron“.


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Dr. Adrian Wong has been writing about tech and science since 1997, even publishing a book with Prentice Hall called Breaking Through The BIOS Barrier (ISBN 978-0131455368) while in medical school.

He continues to devote countless hours every day writing about tech, medicine and science, in his pursuit of facts in a post-truth world.


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