Indian health authorities have named a mutation of the Delta strain as a new coronavirus "variant of concern".
The so-called "Delta Plus" variant has so far been found in the Indian states of Maharashtra, where 16 cases were detected on Tuesday, as well as Kerala and Madhya Pradesh.
Genomic sequencing by Indian laboratories has confirmed the Delta Plus variant displays increased transmissibility, stronger binding to receptors of lung cells, and potential reduction in monoclonal antibody response.
The Indian Health Ministry said in a statement on Tuesday night it had advised the three states that the public health response measures "have to become more focused and effective".
State leaders "have been advised to take up immediate containment measures in the districts and clusters including preventing crowds of intermingling of people, widespread testing, prompt tracing as well as vaccine coverage on a priority basis".
The move comes as the already highly infectious Delta strain continues to spread, with a case now confirmed to have visited New Zealand on a flight from Australia.
First detected in India last October, the Delta variant has now spread to at least 62 countries and is behind a growing number of outbreaks across Asia and Africa, the World Health Organisation said earlier this month.
In the UK, the Delta variant – given the new name by the WHO to simplify its scientific name, B. 1.617.2, and to avoid stigmatising countries that detect new strains – now accounts for 99 per cent of cases.
Experts say the new strain was already twice as infectious, and far more likely to land patients in hospital. In India, a huge spike in Delta variant infections was behind the country's horror second wave in April and May.
Crematoriums ran out of space, burning bodies day and night, as gasping patients died outside hospitals, unable to get beds, oxygen and drugs.
India's death toll more than doubled to more than 330,000, according to official figures. Many experts suspect the true toll is over a million.
The rapid spread in Delta cases has put pressure on health authorities around the world to ensure populations receive two vaccine doses as soon as possible, as the vaccines have been found to be much less effective against the new strain.
Figures published by Public Health England last week suggested both the AstraZeneca and Pfizer vaccine only offered about 30 per cent and 36 per cent protection against the Delta variant after one dose.
But protection increased substantially in people who have had two doses, with the Pfizer jab 88 per cent effective and the AstraZeneca 67 per cent effective.
Those figures were behind UK Prime Minister Boris Johnson's decision to delay the country's planned easing of restrictions on June 21 by four weeks, in order to give people more time to get their second shot.
At the time, around 75 per cent of the UK had been vaccinated, but only 40 per cent have had two doses.
In India, only 5.5 per cent of the adult population has had two shots.
Since May, India has vaccinated fewer than three million people per day, falling well below the target of 10 million health authorities say is needed to prevent future surges, Sky News reported.
Sandeep Budhiraja, medical director at Max Healthcare in Delhi, told news agency AFP earlier this week he was concerned to see shopping centres and markets buzzing again.
On some days now there are no funerals for Covid-19 victims in the nation's capital, down from 700 a day during the recent peak.
Budhiraja was surprised at people's short memories. "People are just behaving as if nothing happened just about two, three weeks back," he said. "And this is … amazing,"
But while this will likely lead to a sharp rise in cases, for a new "explosion" a new virus variant would have to take hold, he said.
He noted that the new "Delta Plus" variant had been identified, which appears more transmissible and more resistant to treatment.
One reason for hope, however, is that unlike in January and February, the authorities are preparing the healthcare system for another wave, Budhiraja said.
But vaccinations remain slow.
"Until the country is vaccinated, with over a billion people getting vaccinated, there is no way we can ever think of the pandemic coming to an end," Budhiraja said.