【Comirnaty and CoronaVac; Article by Siddharth Sridhar】

Two COVID-19 vaccines are currently being used in Hong Kong – CoronaVac (an inactivated vaccine manufactured by Sinovac) and Comirnaty BNT162b2 (an mRNA vaccine manufactured by BioNTech). Antibody responses to these vaccines have been evaluated in separate phase I/II clinical trials finding that most vaccinated individuals develop antibodies after either vaccine (Walsh EE et al, NEJM; Zhang Y et al, Lancet Infect Dis). Antibodies are an important part of the immune response that protect us against infections, so this is a good thing. However, the amount and strength of the antibody response produced by these two vaccines was difficult to compare directly due to differences in methods used to measure antibodies in the company clinical trials.

Now that both vaccines have been launched in Hong Kong, local researchers could directly compare antibody responses produced by CoronaVac and Comirnaty BNT162b2 for the first time. They did this by applying the same antibody-detection methods to the blood of people receiving either jab and comparing the responses objectively. Three studies have now been published on this (Zee JST et al, HKMJ; Lim W et al, Lancet Microbe; Mok C et al, SSRN). Two studies were conducted among healthcare workers and one in the general population. All studies had consistent results: majority of participants developed antibodies after either vaccine, but individuals tended to have higher antibody responses after two doses of Comirnaty BNT162b2 compared to two doses of CoronaVac (even when groups were age-matched as in Mok C et al, SSRN). Importantly, this difference also applies to ‘neutralizing antibodies’ that are capable of preventing the virus from infecting cells.

So what does this mean for vaccine efficacy?

We know that there is a rough correlation between vaccine efficacy and amount of neutralizing antibodies produced (Khoury DS et al, Nat Med). Higher the neutralizing antibodies, better the protection against having symptoms following a COVID-19 infection. This neatly accounts for the differences in protection against symptomatic COVID-19 observed in the phase III clinical trials of CoronaVac and Comirnaty BNT162b2 (Palacios R et al, SSRN; Polack FP et al, NEJM). However, antibodies are not everything. Apart from antibodies, we also have T-cells that are particularly effective in protecting us against severe COVID-19. T-cell responses are induced by both these vaccines (Mok C et al, SSRN). This is why the protection against more severe forms of COVID-19 for both vaccines are roughly equivalent in Chile & Uruguay where these two vaccines are available at the same time.

So, in summary, the Hong Kong antibody data shows why Comirnaty BNT162b2 is superior to CoronaVac against preventing symptomatic COVID-19, but T-cell responses ensure that both vaccines remain effective against preventing severe complications of COVID-19. Comirnaty BNT162b2’s efficacy against symptomatic Delta variant infection has dropped slightly, but its protection against severe Delta remains very high (https://www.gov.uk/.../vaccines-highly-effective-against...). We are awaiting data on CoronaVac vs. Delta variant infection.

Protect yourself & get vaccinated.

And a big thank you to the people who participated in the Hong Kong studies!

This article is orginally published on Siddharth Sridhar's facebook