Experts have rooted for heterologous boosting or mixing of vaccines for the precautionary dose following the surge of BA.2.75, a variant resulting from another major mutation of the Sars-CoV-2 virus. The protein sub-unit based Corbevax has been cleared as the third booster shot for both Covishield and Covaxin. Will this work?
1) Clinical trial data of Corbevax has shown a “significant enhancement” in immune response and an excellent safety profile required for an effective booster shot for COVID-19. Other countries have also experimented with mixed vaccines. Does this mean mixing vaccines is the way forward in pandemic management?
Studies have shown an increase in immune-response. We measure immune-response on two counts; one is cellular immunity and the other is the count of busting or neutralising antibodies. For regulatory purposes, one looks at the effectiveness of antibodies. And in various studies on double-vaccinated people where their antibodies had maxed out anywhere between four and six months, the top-up dose of Corbevax indeed increased immunity levels. Studies with the Astrozeneca-Novovax combination in the UK had proven results. That’s why the Government has cleared Corbevax as a booster shot that can go with the vaccines you have taken so far.
Having said that, does a mixed format guarantee more protection? That’s difficult to say because the problem with our vaccine companies is that they never compare data or results between themselves. Then it becomes very difficult to compare immunogenicity and decide whether three shots of the same kind or a combination work better. Of course, we had conducted a comparative study at CMC Vellore of Covishield and Covaxin and found that Covishield as the third booster shot worked far better on Covaxin in terms of amping up antibody counts than Covaxin itself.
What can be said of Corbevax now is that its protein sub-unit base works on both the inactivated virus (Covaxin) and adenovirus (Covishield) and has generated a good immune response. At this stage, we do not have any guarantees that it is a better vaccine till we have comparative data analysis. We also have to track its individual efficacy. If within the existing vaccine set-up, we can set up a mechanism for tracking the recipient response to vaccines, old or new, then we have a solid data bank for analysis.
2) Does enhanced immunity mean that mixed vaccines can protect us from variants and sub-variants?
We cannot say so because all three vaccines have been developed on the basis of the ancestral strain. What is true of existing vaccines is true of Corbevax, it ups your antibodies and offers protection against the severity of infection. All vaccines are arming you to fight the infection better. Of course, it is important for us to continue our efforts to modify our vaccines so that they can provide longer-lasting protection and tackle variants. That doesn’t mean our existing tools aren’t working at all. Variants always drift and acquire new mutations. BA.2.75 is derived from BA.2. If it continues to expand as a percentage of all strains found, it is more capable of spread and immune escape than current variants.
3) Are you saying that sporadic surges will continue?
We may not say so officially but the virus has been endemic since the end of last year. It’s not going to go away, it’s mutating, generating various sub-variants and weakening but we may get repeated bouts, just like any other flu.
4) What does reducing the gap between doses from nine months to six months do?
Now this is not an evidence-informed decision. We have had no study to evaluate if there was a weakness in the nine-month gap. A science-based policy recommendation on doses and gapping should depend on the population’s experience with infection and vaccination, the availability of local data on immunogenicity and effectiveness of the vaccines concerned.
5) Will mixed vaccine use ensure more vaccine equity?
It may work in countries that are still lagging behind targets but thankfully in India, the infection-induced immunity and the excellent vaccination coverage have meant that we do not have to deal with this question.
6) So far, Corbevax has been used in India for children aged 12-14 years. What has been the protective shield efficacy, particularly against re-infection among this group?
I will say this again, we are losing a golden chance to research and develop vaccines if we do not track the effectivity curve in the real world and in real time. We need to track the progress to understand better. We have good data in silos but we need to link them together.