India’s second Covid wave undermines herd immunity theory

India is experiencing a sharp rise in coronavirus infections, increasing pressure on New Delhi to accelerate its vaccine rollout and puzzling scientists after a retreat of the pandemic had buoyed hopes that parts of the country had reached herd immunity.

The nation has reported more than 40,000 daily Covid-19 infections since Friday, up from lows of about 11,000 last month. On Wednesday, India reported 47,000 new infections, the highest since November.

Cases are rising in 23 of the 30 states or union territories for which figures are available, according to a Financial Times analysis, with the percentage of positive tests rising in 21 of those.

Test positivity rates are now doubling every five days in several states, the FT analysis shows. That is faster than the growth rate seen in the UK when the B.1.1.7 variant took off at the end of 2020 and is suggestive of rapid community spread. At the height of the winter surge in London, doubling time was only nine days by comparison.

The resurgence is led by Maharashtra, home to India’s financial capital and economic powerhouse Mumbai, which is registering record daily infections of more than 25,000. Twenty per cent of its tests are coming back positive.

Scientists are unclear what is driving the increase, particularly after widespread infection last year prompted speculation about whether parts of India were enjoying a degree of herd immunity.

One factor being considered was the lifting of most lockdown restrictions, which sent people flocking back to restaurants, weddings and even cricket stadiums. But health experts said new Covid-19 variants could be fuelling the spread, and possibly reinfection.

They said India had to employ more robust monitoring to search for new variants. “We need to know exactly what is happening. Are these variants, and if they are, are the vaccines going to be effective? Are there reinfections?” asked Lancelot Pinto, a respirologist and epidemiologist at Mumbai’s Hinduja Hospital.

Nimalan Arinaminpathy, reader in mathematical epidemiology at Imperial College London, said it was possible that immunity had been waning and that the rise in cases involved a substantial amount of people being reinfected, or that a new, more transmissible variant was driving the surge.

“Perhaps we are seeing an epidemic that is merely shifting to parts of the states that were less affected in earlier waves,” he said. “There might be some combination of all three of these going on at the moment, but it’s hard to say which of them is playing the strongest role.”

India’s health ministry said on Wednesday that it had found 771 instances of “variants of concern” including those first found overseas as well as a “new double mutant variant”.

“These have not been detected in numbers sufficient to either establish [a] direct relationship or explain the rapid increase in cases in some states,” it said.

At its September peak, India was recording nearly 100,000 new infections a day. A nationwide seroprevalence survey conducted in December and January showed that a fifth of Indians had antibodies, with other studies showing exposure to be far higher in metropolises such as Pune or Bangalore.

Soumya Swaminathan, chief scientist at the WHO, told the FT that claims of widespread immunity were probably exaggerated. “India never reached herd immunity at the population level,” she said. “The first wave was focused in the big cities which probably got some level of herd immunity but now it’s more widespread.

“This virus, once in the community, constantly looks for new hosts. Emerging viral variants with specific mutations make it more transmissible, hence the need for stricter public health measures.”

More than half of Maharashtra’s districts now have case rates higher than they did at the September peak. Nagpur, which has gone back into lockdown, is recording 67 new cases per 100,000 people every day — more than three times the rate statewide. Cases there are doubling every 10 days.

India announced on Tuesday that anyone older than 45 would be eligible for vaccination from April, after concern that overly restrictive criteria was slowing inoculation.

Doctors said one encouraging sign was that the wave of infections in hard-hit cities such as Mumbai appeared to be less severe than last year.

Aparna Hegde, at Mumbai’s Cama Hospital, said her ward had about 12 Covid-19 patients, up from three or four in January but well below the 60-plus patients last year.

“From my own experience, it’s not as if it’s the same as before,” she said. “But you have to be careful because it doesn’t take much time to tip over.”

Yet while India’s fatality rate remains low, reported deaths have risen from about 100 a day at their low last month to more than 200.

“I’d be worried about a lot of places in India,” said Reuben Abraham, chief executive of the IDFC Institute think-tank, pointing to the rising Covid-19 reproduction number across the country. “Why this is happening, honestly, we don’t really know.”

Additional reporting by Stephanie Findlay in New Delhi