（立場編按：港大微生物學系臨床助理教授薛達 (Siddharth Sridhar) 於 Facebook 撰文，簡單解釋香港最近所做的 3 個關於科興疫苗與 BioNTech 疫苗「復必泰」對武漢肺炎 (COVID-19) 有效力的研究意義。雖然科興疫苗產生的中和抗體明顯低於「復必泰」，解釋到兩者對付感染有症狀 COVID-19 ，但薛達指出，中和抗體並非一切，免疫系統中有 T 細胞，可對抗重症發展，而現時數據顯示兩種疫苗均產生相若 T 細胞數量，因此兩者對抗重症 COVID-19 的效用相似。）

【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