The delta plus variant, a mutated version of the delta variant of the coronavirus, was first reported in a Public Health England bulletin on June 11.

It is a sub-lineage of the delta variant first detected in India and has acquired the spike protein mutation called K417N, which is also found in the beta variant first identified in South Africa.

Some scientists worry that the mutation, coupled with other existing features of the delta variant, could make it more transmissible.

"The mutation K417N has been of interest as it is present in the beta variant (B.1.351 lineage), which was reported to have immune evasion property," India's Health Ministry said in a statement.

Shahid Jameel, a top Indian virologist, said the K417N was known to reduce the effectiveness of a cocktail of therapeutic monoclonal antibodies.

Where has it been found?

It has been detected in several countries, including Britain, Canada, India, Japan, Nepal, Poland, Portugal, Russia, Switzerland, Turkey and the United States.

The earliest case in India is from a sample taken on April 5.

Britain said its first five cases were sequenced on April 26, and they were contacts of individuals who had traveled from, or transited through, Nepal and Turkey.

What are the concerns?

Studies are ongoing in India and globally to test the effectiveness of vaccines against this mutation.

"WHO is tracking this variant as part of the Delta variant, as we are doing for other Variants of Concern with additional mutations," the World Health Organization (WHO) said in a statement sent to Reuters.

"For the moment, this variant does not seem to be common, currently accounting for only a small fraction of the Delta sequences ... Delta and other circulating Variants of Concern remain a higher public health risk as they have demonstrated increases in transmission," it said.

But India's Health Ministry warned that regions where it has been found "may need to enhance their public health response by focusing on surveillance, enhanced testing, quick contact-tracing and priority vaccination."

There are worries delta plus would inflict another wave of infections on India after it emerged from the world's worst surge in cases only recently.

"The mutation itself may not lead to the third wave in India – that also depends on COVID-appropriate behavior, but it could be one of the reasons," said Tarun Bhatnagar, a scientist with the state-run Indian Council for Medical Research.