Need for booster shot may arise even as India struggles to finish second Covid vaccine dose

Booster shots may be critical to inducing a “hyper-antibody response," necessary to enhance protection against the evolving virus

Even as India struggles to vaccinate a majority of the population with the required two doses, evidence suggests that a third dose (or "booster shot") may be required to contain the still-active pandemic.

An ongoing study at Jayadeva Institute of Cardiovascular Sciences and Research suggests that as many as between 15 to 20% of people vaccinated with two doses would definitely require a booster shot. “This is due to the fact that only 80% of 300 fully vaccinated healthcare workers subject to an antibody test showed ‘effective’ levels of antibodies a month later,” explained Dr C N Manjunath, Director of Jayadeva.

At the same time, because it will not be possible to subject all two-dose beneficiaries to antibody tests to determine the level of neutralizing titres in their body, experts believe that the booster shot may become mandatory for everyone.

“A mandatory booster shot follows the established immunological schedule that we have seen with other viral diseases whereby a booster is required six months after the last dose,” explained the noted virologist Dr T Jacob John, former Principal of CMC Vellore.

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He added that the booster shot is critical to inducing a “hyper-antibody response,” necessary to enhance protection against the evolving virus.

The issue of a booster has arisen because the current generation of Covid-19 vaccinations were conceived to defeat the D614G mutation of the wildtype B.1 virus which was largely responsible for the first wave last year. Since then, variants of the novel coronavirus have developed some “immune evasion” characteristics although they do not yet have “immune escape” abilities.

These variants include the baseline “Delta” variant and the P2 “Zeta” variant of interest which has been found in West Bengal and Gujarat. “This ‘Zeta’ variant is found to have a six-fold ability to reduce vaccine effectiveness,” pointed out Dr Vishal Rao, a member of the state’s Genomic Surveillance Committee.

Even the baseline “Delta” is not above inducing worry. A recent ICMR study in pre-publication form found that 16.1% of people who had been given two doses of the vaccine had not developed observable neutralizing antibodies against the Delta variant.

India’s low second dose vaccination coverage could mean that the country will face a more prolonged outbreak than other nations where high proportions of people vaccinated with both doses could be eligible for the booster shot earlier. This means an earlier breaking of the chain of infection. In Karnataka, for instance, only 8.9% of the statewide adult population had received the second dose so far as of Friday.

However, Dr John said that he expects India’s second dose coverage to gather momentum in the coming weeks when vaccine production increases.

Read | How does the Delta variant dodge the immune system?

What form of booster?

Even as discussion about booster vaccines increases, the form of the “boosters” themselves have not yet been finalized. Experts said it could constitute a third dose of the same vaccine used to twice-inoculate a person or could boil down to “dosage mixing” - that is, inoculating a person previously dosed with a viral-vector vaccine such as Covishield with an mRNA vaccine such as the Pfizer vaccine or an inactivated virus vaccine such as Covaxin.

Meanwhile, some vaccine manufacturers such as Moderna are already modifying their existing Spikevax vaccine to serve as a booster shot. IISc’s protein-based thermotolerant vaccine could also be turned into a standalone booster shot.

“The next 3-6 months are going to give us the answers,” said Dr Vishal Rao.