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The third COVID vaccine dose: what do we know so far?

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Dr. M.S.S. Mukharjee, Senior Interventional Cardiologist-Medicover Hospitals and Director-Pulse Heart Center & Pulse Heart Charitable Trust shares his views on booster dose of COVID vaccine

The invincibility of COVID vaccines in terms of prevention of symptomatic infections was widely believed initially, but tested passionately by the new COVID genetic variants, spearheaded by the delta variant. Now that it is clear that symptomatic infections may occur after full course of vaccination – though hospitalizations and deaths are markedly reduced – people are wondering if a third dose vaccine (a second booster) may help bolster the immunity against this abominable foe.

The idea of giving booster doses has not started with COVID. Many countries give annual Influenza vaccinations. Most COVID vaccines are given as two doses. New evidence is cropping up regarding the third dose of a COVID vaccine. The published literature has two studies, which claim that a third dose may be beneficial. The first is the Oxford study, which has tried the AstraZeneca vaccine as a third dose. The viral vector vaccines are not the first choices for a third dose, as there is an issue of immunity against the viral vectors. The body develops antibodies not only against the COVID antigen, but also against the viral vector carrying it into the human body. But this study demonstrated good increment in the antibody level following the third dose. So the government of UK, while following the advise from the Joint Committee on Vaccination and Immunisation (JCVI), is planning to give the booster vaccine to vulnerable population from the September this year.

The second study that investigated the booster vaccine is a Swedish study, which tried Pfizer vaccine after AstraZeneca vaccine. The study concluded that Pfizer vaccine could be given as a booster following both doses of AstraZeneca vaccine.

A waning immunity after a previous infection, slow reduction in antibody levels after vaccination and emergence of new COVID variants at an alarming rate are the appealing reasons for recommending a vaccine booster in India as well. However, presently India will do well not to advise a third vaccine dose to its citizen because of several reasons. First, we are still struggling to provide first and second dose vaccinations all the citizens because of difficulties in manufacturing and maintenance of supply chain. Second, there is not sufficient evidence for the benefit of the booster dose. Third reason is that though the privileged take the booster dose, if the masses are not vaccinated, new variants keep cropping up and the security offered by the previously taken vaccines will be challenged.

A fourth reason is a question of common sense. If we are taking a booster to evade the threat posed by the new variants, why should we take the vaccine dose, which has a reduced efficacy against the newer variants? Please remember that the present vaccines were designed when the original strain (D614G) was in circulation. They have reduced efficacy against Alpha, Beta, Gamma and the Delta variants. If at all a booster vaccine is planned, it makes sense that a new vaccine dose is made which is effective against the presently circulating viruses. The new vaccine will not need elaborate trials as the antibody levels can be taken as surrogates to clinical end points.

To conclude, the inexorable progression of COVID pandemic will make booster doses of the vaccines essential. But present evidence for the efficacy of booster doses is sketchy, our country needs to give out first and second doses to the majority before considering booster dose, and a booster designed for the new variants is better than using the existing vaccines as the third dose. A booster for COVID may be needed in India too, but now is not the time.

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