A US-designed and India-made Covid-19 vaccine named Corbevax has become the country’s first “heterologous” booster for those who have received two doses of Covishield or Covaxin, the vaccine maker announced on Saturday.

The Central Drugs Standard Control Organisation (CDSCO), the country’s drug regulatory authority, has approved Corbevax as a heterologous booster dose for people aged 18 years or older after six months of primary series of Covaxin or Covishield, the Hyderabad-based Biological E said.

The company said its clinical trial on the heterologous booster dose with 416 participants has shown that Corbevax booster doses provided “significant enhancement in the immune response and excellent safety profile required for an effective booster”.

The Corbevax booster shots resulted in a significant increase in neutralising antibody levels against the omicron variant, the company said, outlining the results in a subset of clinical trial participants. It said the booster dose was safe with no adverse events of interest in three months of follow-up.

Corbevax is an engineered protein subunit vaccine designed by scientists at the Baylor College of Medicine and Texas Children’s Hospital in the US and developed and produced by Biological E. More than 17 million children between 12 and 15 years in India have received two doses of Corbevax as the primary vaccine.

Nearly 804 million people 18 years or older in India have taken two doses of Covishield or Covaxin, the earliest two vaccines through which the country launched its vaccination campaign last year. But less than 24 million among them have so far opted for booster doses.

The campaign had opened boosters for all adults earlier this year, starting with homologous booster doses, meaning Covishield boosters for those who have received Covishield and Covaxin boosters for those who have received Covaxin.

A clinical trial to assess boosters by mixing vaccines at the Christian Medical College, Vellore, had found that Covishield when used as a booster after two doses of Covaxin significantly enhances the neutralising antibody titres but, health experts say, the gains from such boosts remain to be evaluated.

“It isn’t easy to evaluate the gains from a booster only through the numerical values of neutralizing antibody levels,” said Jacob John, professor of community medicine at the CMC who was involved in the trial.

“If the levels are 30,000 with one vaccine, 5,000 from another, but the level required for protection is between 12 and 30, then the gains from 30,000 remain unclear.”

Biological E has supplied 100 million doses of Corbevax to the Centre.

John said the available data suggest Corbevax is a “great vaccine” and given that its doses are available, it would make sense to introduce it as a booster, although a key question about any heterologous booster is whether it can widen the repertoire of antibodies against the coronavirus.