It is too early to say if booster shots of the Covid-19 vaccine will be required to target more infectious variants of the coronavirus, World Health Organization Chief Scientist Soumya Swaminathan told Bloomberg in an interview on Sunday.

Swaminathan’s remark came amid concerns about the spread of more transmissible variants of Covid-19 and questions about how long the effects of the current double-dose vaccine regimen would stay.

“We do not have the information that’s necessary to make the recommendation on whether or not a booster will be needed,” Swaminathan told the news website. “The science is still evolving.”

The WHO chief scientist added that calls for using booster shots were premature, since in most countries, those at a high risk of Covid-19 infection hadn’t even been fully vaccinated.

Swaminathan told Bloomberg that WHO would look at data from countries that introduce the booster shot later in 2021 before issuing its guidelines.

Health experts in the United Kingdom have been urging the government to start planning for a rollout of booster shots, according to the BBC. The country is grappling with a surge in cases of the Delta variant, which was first detected in India.

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Francis Collins, director of the National Institutes of Health, said that Covid-19 vaccines approved by the United States worked well against “variants of concern”.

“Nobody is saying you need a booster today,” he was quoted as saying by Bloomberg. “But boosters might very well be in our future at some point, and they might be here sooner if other variants pop up that aren’t covered as well by existing vaccines.”

Last week, the Russian Direct Investment Fund had said that it would offer a booster shot of Sputnik V to other vaccine manufacturers. The shot has been adjusted to work against the Delta variant of the coronavirus disease.

The Delta variant of Covid-19, first detected in India, has caused concern globally. Swaminathan had on Friday said that the variant was becoming the globally dominant variant because of its significantly increased transmissibility.

There are more than 17.84 crore cases of the coronavirus disease across the world, according to Johns Hopkins University. More than 38.64 lakh people have died due to the infection.

WHO chief scientist on mixing vaccines

Swaminathan told Bloomberg that using two different vaccines seemed to generate a stronger immune response.

She cited data from the UK and Germany that showed the “mix-and-match” inoculation regimen caused more minor side effects compared to two doses of the same vaccine.

“It seems to be working well, this concept of heterologous prime-boost,” Swaminathan said. “This opens up the opportunity for countries that have vaccinated people with one vaccine and now are waiting for the second dose they have run out of, to potentially be able to use a different platform vaccine.”