Corbevax as booster dose after Covaxin, Covishield? Top panel on immunisation to discuss possibility

“A discussion on allowing this heterologous vaccine combination for all adults is about to happen, and depending on its recommendations, changes may be permitted in the national COVID-19 vaccination programme,” said a senior government official.

In India, all adults are now permitted to receive a booster dose but as of now the booster has to be the same vaccine that was used in the primary vaccination.

Last week, the health ministry had reduced the gap between the second and booster doses from 9 months to 6 months as only about 5 crore booster or precautionary doses have been administered in the country so far.

Currently, Corbevax is permitted for the 12-14 age group in India and so far 6.2 crore doses of the vaccine have been administered.

Promising results

Last month, after DCGI approval on mixing COVID-19 vaccines, Biological E said in a statement that it had conducted a multi-centre, phase 3, placebo-controlled heterologous booster clinical trial. It was conducted on 416 volunteers aged 18 to 80 who had been previously vaccinated with two doses of either Covishield or Covaxin, at least 6 months earlier.

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The booster dose of Corbevax increased the neutralising antibody titers in the Covishield and Covaxin groups significantly compared to the placebo, it said.

In a subset of subjects evaluated for neutralising antibodies against the Omicron variant, which for several months has been the dominant SARS CoV 2 strain circulating globally, the Corbebax booster shot resulted in a significant increase in the antibody titers.

After the booster dose of Corbevax, neutralising antibodies against Omicron were observed in 91 percent and 75 percent of subjects who had received primary vaccination by Covishield and Covaxin respectively, the firm said.

Importantly, the Corbevax booster dose also resulted in a significant cellular immune response, as indicated by cytokine expression analysis post stimulation of the T-cells, which are part of the immune system.

Better protection?

Many experts feel that offering Corbevax as a precautionary dose may be a better choice compared to a homologous (the same as earlier) vaccine.

Dr Laxman Jessani, consultant, infectious diseases, at Apollo hospitals in Navi Mumbai, pointed out that Corbevax is a “recombinant protein sub-unit” vaccine, which means it is made up of a specific part of SARS-CoV-2 — the spike protein on the virus’s surface.

Other Covid-19 vaccines approved so far globally are either mRNA vaccines (Pfizer and Moderna), viral vector vaccines (AstraZeneca-Oxford/Covishield, Johnson & Johnson and Sputnik V) or inactivated vaccines (Covaxin, Sinovac-CoronaVac and Sinopharm’s SARS-CoV-2 Vaccine–Vero Cell).

“Inactivated vaccines, which include killed particles of the whole SARS-CoV-2 virus, attempt to target the entire structure of the virus,” he said. “On the other hand, Corbevax, like the mRNA and viral vector Covid-19 vaccines, targets only the spike protein, but in a different way”.

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According to Jessani, viral vector and mRNA vaccines use a code to induce cells to make the spike proteins against which the body has to build immunity. “Hence, Corbevax can provide better immuno-protection when given as a booster to those who have received Covaxin or Covishield.”

However, others, including Dr Satish Koul, director, internal medicine, Fortis Memorial Research Institute, pointed out that while mixing Corbevax on top of Covaxin or Covishield is theoretically an exciting option, there needs to be publicly available and scientifically scrutinised data and randomised controlled trials before such a move is approved.