A Covid-19 variant spreading in India is more contagious and may be dodging vaccine protections, contributing to the country's explosive outbreak, the World Health Organization's chief scientist has said.

In an interview with AFP, Soumya Swaminathan warned that "the epidemiological features that we see in India today do indicate that it's an extremely rapidly spreading variant".

India registered more than 4,000 Covid-19 deaths in just 24 hours for the first time yesterday, and more than 400,000 new infections.

New Delhi has struggled to contain the outbreak, which has overwhelmed its healthcare system, and many experts suspect the official death and case numbers are a gross underestimate.

Ms Swaminathan, an Indian paediatrician and clinical scientist, said the B.1.617 variant of Covid-19, which was first detected in India last October, was clearly a contributing factor to the catastrophe unfolding in her homeland.

"There have been many accelerators that are fed into this," she said, stressing that "a more rapidly spreading virus is one of them".

The WHO recently listed B.1.617 - which counts several sub-lineages with slightly different mutations and characteristics - as a "variant of interest".

But so far it has stopped short of adding it to its short list of "variant of concern" - a label indicating it is more dangerous than the original version of the virus by being more transmissible, deadly or able to get past vaccine protections.

Several national health authorities, including in the United States and the UK, have meanwhile said they consider B.1.617 a variant of concern, and Ms Swaminathan said she expected the WHO to soon follow suit.

Soumya Swaminathan

"B 1.617 is likely to be a variant of concern because it has some mutations which increase transmission, and which also potentially could make (it) resistant to antibodies that are generated by vaccination or by natural infection," she said.

But she insisted that the variant alone could not be blamed for the dramatic surge in cases and deaths seen in India, lamenting that the country appeared to have let down its guard down, with "huge social mixing and large gatherings".

Mass election rallies held by Prime Minister Narendra Modi and other politicians have for instance partly been blamed for the staggering rise in infections.

The surge in India is frightening not only due to the horrifying number of people who are sick and dying there, but also because the exploding infection numbers dramatically increase the chances of new and more dangerous variants emerging.

"The more the virus is replicating and spreading and transmitting, the more chances are that... mutations will develop and adapt," Ms Swaminathan said.

"Variants which accumulate a lot of mutations may ultimately become resistant to the current vaccines that we have," she warned.

"That's going to be a problem for the whole world."

Pfizer chief Albert Bourla recently acknowledged that it is likely a booster will be needed to help extend the protection conferred by its vaccine and ward off new variants.

A recent study presented a mixed picture.

It found that the antibody response of current vaccines could fail against variants. However, a second immune response in the form of killer T cells - which attack already infected cells and not the virus itself - remained largely intact.

Several startups are working on developing shots centred on T cells in hopes of producing a jab that would not only provide protection against new virus strains already on the loose, but also variants that don't yet exist.

Alexis Peyroles heads up French biotech firm OSE Immunotherapeutics, which is developing a vaccine that targets T cells that has just begun clinical trials.

"It could offer several years of protection," he told AFP.

Another French firm, Lyon-based Osivax, is also working on a T cell shot, promising a "universal" vaccine that would be effective against any potential variant.

The government of France, which has yet to develop its own vaccine, is supporting the effort with millions in funding.

Such projects are far from widespread. Among the 400 vaccines under development counted by the World Health Organization only a few are aimed at universal use.

The most advanced shot of its kind is the ImmunityBio vaccine under development in the United States. Very preliminary results released last month were mostly encouraging.

No lab foresees a final product before next year and many scientists are sceptical about the usefulness of trying to develop a shot to protect against a virus strain that doesn't yet exist.

"Mass vaccination itself is a form of evolutionary 'selection' pressure," British virologist Julian Tang told AFP, "and this pressure may push the virus to evolve to escape any vaccine protection -- so it can be a double-edged sword."

Other questions involve the extent to which the body will be able to fight the virus with a T cell-based response.

T cells and antibodies work together to form an immune response in the body.

French virologist Yves Gaudin pointed out that if an antibody response fails, "T cells don't serve much purpose".

He said he is "doubtful about the effectiveness of such a vaccine," emphasising that an ideal vaccine would be effective in both areas.

In Europe and the United States the plan for T cell jabs, should they see the light of day, would be to give them to people who had already received the current antibody vaccines.

Peyroles confirmed that OSE's vaccine, should it prove effective in trials, is indeed meant as a way to strengthen current inoculations.

"You would complement and broaden the response created by the first vaccines in terms of scope and time."

He added that T cell vaccines could offer protection to people who have difficulties developing antibodies due to other ailments such as diabetes or cancer.