The Indian government says people receiving “precautionary” Covid-19 booster jabs, being offered for the over-60s with comorbidities and frontline workers, will get the same vaccinations as they received in the first two rounds.

The decision to administer a booster shot of the same vaccine comes despite studies indicating that mixing and matching vaccines is effective and moreover creates a broader and more potent response.

About 90 per cent of Indians have been vaccinated with Covishield, the locally made version of Oxford-AstraZeneca’s vaccine Vaxzevria. This means their third dose will also be Covishield.

In a new study, AstraZeneca has said that a third dose of its Covid -19 vaccine “significantly” lifted antibody levels against omicron, though the findings have yet to be peer-reviewed but the company said that the researchers who carried out the study were independent from those who developed the vaccine.

The study indicated that after a three-dose course of Vaxzevria (Covishield in India), neutralising levels against Omicron were broadly similar to those against the virus’s Delta variant after two doses.

The UK, where many received the AstraZeneca vaccine last year, decided to offer all adults an mRNA booster shot such as Pfizer to counter the swiftly spreading Omicron variant. The decision was a blow to the Anglo-Swedish drugmaker AstraZeneca which had hoped the UK would opt for its vaccine as a third jab.

The protection afforded by two doses of the AstraZeneca coronavirus shots, including against severe disease, begins to wane three months following the second shot, a British study released late last month said. The Indian government has said that those over 60 and frontline workers can only receive their booster shots nine months after getting their second jab. In the UK, the government is urging all adults over 18 to get a booster jab two months following their second shot.

It’s not known yet how well boosters protect against developing a severe case of omicron but scientists believe getting an extra jab offers significant protection against being hospitalised or dying. But booster shot protection against symptomatic COVID-19 caused by the Omicron variant also appears to fade in about 10 weeks and in Israel, authorities are offering citizens in that country a fourth jab.

A recent study in the UK that evaluated seven vaccines as a third dose following two doses of the AstraZeneca vaccine, found that a booster dose of Novavax vaccine, known as Covovax in India, triggered a higher antibody response than if the booster was the AstraZeneca vaccine.

Novavax, produced by a Maryland-based company, is known as Covovax here and is produced by the Serum Institute that got emergency use approval in India earlier this month. Novavax can be stored in refrigerators, making its transportation and distribution much easier than the mNRA injections that require deep-freeze storage facilities.

Some vaccine makers are working on an omicron-targeted injection. Even so, while antibodies wane, it’s believed that the T-cell response will continue to provide “durable protection against severe disease and hospitalisations,” said Mene Pangalos, the head of AstraZeneca's biopharmaceuticals R&D said. T-cells are part of the immune system that play a key role in fighting infection.

Antibody levels against Omicron after the AstraZeneca booster shot were higher than antibodies in people who had been infected with and recovered naturally from Covid -19, the company said.

AstraZeneca’s rivals Pfizer-BioNTech and Moderna have also found a third dose of their mRNA shots works against Omicron. Neither Pfizer nor Moderna’s vaccines is available in this country as Indian government rejected the companies’ request for immunity from civil lawsuits over any side-effects from the jab.

Dr N. K. Arora, chairman of the country's Covid Task Force, had said that the choice of vaccine for the boosters would be “based on the science of what would be the best one and available experience within the country, and it will be transparently available and informed to the community,"

Serum Institute chief executive Adar Poonawalla said last month the company was sitting on half a billion doses of Covishield and that it was halving its output of vaccines because it had no fresh orders from the health ministry.