Boosters: Experts say mixing vaccines may give better immune cover

But some virologists feel two-dose immunity, along with memory-cell action, may be sufficient

A head of a much-anticipated meeting on Covid booster shots, experts feel the way forward is to mix vaccines to give better immune coverage even as others question the need for a booster dose.

In the wake of the recent scare from Omicron, some virologists feel two-dose immunity along with memory-cell action would be sufficient, while others feel that booster shots may be necessary.

An expert group is likely to meet next week to decide on the course of action related to allowing booster shots.

Bharat Biotech’s inactivated whole-virus based Covaxin seems to have an edge here for two reasons — one, a third dose of Covaxin can be given as a booster shot, while for Covishield (the AstraZeneca-Oxford vaccine) giving the same vaccine as a third shot may not prove to be effective. Secondly, experts say Covaxin is likely to have an edge over other vaccines in offering some immunity to a mutating virus as it contains the whole virus.

On whether giving a third dose of Covishield as a booster shot to someone who has already received two shots of the vaccine makes sense, Shahid Jameel, Senior Rese­arch Fellow at Green Temp­leton College at Oxford Unive­rsity, said, “Vector immunity would be dominant. A booster of Covishield in people with two doses of the same is unlikely to give much benefit.”

Covishield is based on a weakened version of a common cold or adenovirus found in chimpanzees. This viral vector contains the genetic material of the Sars-CoV-2 spike protein (that helps the virus to bind with human cells).

Jacob John, senior virologist and former head of the departments of clinical virology and microbiology at Christian Medical College, Vellore, said vector-based vaccines have two immune responses by the body — one is against the vector itself, which is irrelevant, the other is against the spike protein of the coronavirus, which is the relevant immunity.

Jameel feels that a separate vaccine given as a booster to Covishield may work better. “Serum Institute already has a good booster to Covishield.  It’s the Novavax protein vaccine Covovax. But India has not approved it yet,” he adds.

Serum CEO Adar Poonawalla, too, has hinted at developing a new booster to the AstraZeneca shot. “Scientists at Oxford are also continuing their research, and based on their findings, we may come out with a new vaccine that would act as a booster in six months’ time,” Poonawalla said this week.

Meanwhile, his company has approached the drug regulator, seeking approval for Covishield as a booster citing adequate stock.

John seemed to agree with Jameel when he said that “for people who have received two doses of Covishield, Covaxin as the booster shot makes sense, but Covishield as the third shot makes no sense. For people who have received two doses of Covaxin, it makes sense to give another Covaxin shot as a booster as this vaccine is based on the whole virus. So it contains all the parts of the virus, not just spike protein antigens.”

John feels that booster shots are a necessity in the wake of the Omicron variant, which is likely to escape immunity and cause breakthrough infections.

Others like Jayaprakash Muliyil, Chairman of the Scientific Advisory Committee of the ICMR’s National Institute of Epidemiology, don’t think a booster dose is needed.

He says that the Sars-CoV-2 virus is an excellent immunogen and exposure to either the virus or vaccine would work to induce immune response in the case of future exposures. “We have no evidence that a booster dose makes a difference in the case of either Covishield or Covaxin,” he says.

Muliyil, however, says if a booster is given, in the case of Covaxin, giving the same vaccine dose will work and admits that Covishield will have a problem with vector-induced immunity. “Covaxin induces excellent memory in our immune cells.”

Globally, the clamour for boosters is growing. The US Centers for Disease Control and Prevention has recommended that all adults are eligible for a booster shot six months after completing their primary vaccination series if they started with Pfizer-BioNTech or Moderna, or two months after getting the Johnson & Johnson single-shot vaccine. Any vaccine available in the US can be taken as a booster regardless of which vaccine was taken primarily.