Doctors, the public and the media point to anecdotal evidence of infection among those vaccinated as well. Scientists say the data is too thin and cite other reasons for the country’s second wave.

NEW DELHI – At the Sir Ganga Ram Hospital, a huge facility in the middle of the Indian capital, 37 fully vaccinated doctors contracted Covid-19 earlier this month.

The infections left the most mild symptoms, but it heightened their growing fears that the virus behind India’s disastrous second wave is different. They wonder if a more contagious variant that evades the immune system could fuel the epidemic in the world’s hardest-hit nation.

So far, the evidence is inconclusive, and researchers are warning that other factors could explain the viciousness of the outbreak that has overwhelmed India’s capital so quickly that hospitals are completely overcrowded and crematoria burn non-stop. However, the presence of the variant could make it difficult to tame the Covid-19 disaster in India.

“The current wave of Covid has a different clinical behavior,” said Dr. Sujay Shad, a senior cardiac surgeon at Sir Ganga Ram Hospital, where two of the doctors needed supplemental oxygen to recover. “It affects young adults. It affects families. It’s a whole new thing. Babies two months old become infected. “

The Indian outbreak worsened Wednesday as authorities reported nearly 3,300 deaths a day. That brings the official total to nearly 201,200 people lost, although experts believe the real number is much higher. The daily new infections also rose to almost 357,700, another record.

With supplies becoming dangerously scarce and hospitals being forced to turn away sick, scientists are trying to determine what role variants of the virus could play. You are working with valuable little data. India, like many other countries, has not built a robust virus tracking system.

India’s concerns have centered on a native variant called B.1.617. The public, popular press, and many doctors have concluded that it is responsible for the severity of the second wave.

Researchers outside of India say the limited data so far suggests instead that a more well-known variant called B.1.1.7 may be a more significant contributing factor. This variant walled the UK late last year, struck much of Europe, and is now the most common source of new infections in the United States.

“While it is almost certainly true that B.1.617 plays a role, it is unclear how much it is directly contributing to the surge and how this compares to other circulating variants, particularly B.1.1.7,” said Kristian Andersen, a virologist at the Scripps Research Institute in San Diego.

“There are many reasons to conclude that B.1.167 is the explanation for what happened,” said Jeffrey Barrett, director of the Covid-19 genomics initiative at the Wellcome Sanger Institute in the UK. “Those other things are probably more of an explanation.”

Initial evidence suggests that the variant still responds to vaccines, albeit a little less. India relies heavily on the Oxford-AstraZeneca vaccine, which clinical studies have shown to be less effective than Pfizer-BioNTech and Moderna vaccines and potentially more susceptible to mutation.

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April 28, 2021, 11:29 a.m. ET

“For the moment the vaccines remain effective, but there is a trend towards less effectiveness,” said Dr. Celine Gounder, an infectious disease doctor and epidemiologist at Bellevue Hospital in New York.

In India, a number of doctors point to isolated evidence that people who have been fully vaccinated get sick. These doctors also say they see children with severe symptoms like severe diarrhea, acidosis, and falling blood pressure, even in otherwise healthy patients.

“This is very different from last year,” said Dr. Soonu Udani, director of intensive care at SRCC Children’s Hospital in Mumbai.

Researchers say other factors could lead to more infections among young people, such as India’s schools, which reopened in the last few months after the country’s first wave.

The variant in India is sometimes referred to as a “double mutant,” although the name is a misnomer as it has many more mutations than two. It got the name because one of its three versions contains two genetic mutations found in other hard-to-control variants of the coronavirus. One of these can be seen in the highly contagious variant that roamed California earlier this year. The other is similar to the variant first identified in South Africa and the vaccines are believed to be slightly less effective.

“There are variants that are more transferable than what we all did a year ago,” said Dr. Barrett on the many varieties that are in circulation in India. “Things can change very quickly. If a country doesn’t react quickly enough, things can go from bad to very bad very quickly.”

Scientists say that different variants appear to dominate certain parts of India. For example, variant B.1.617 was detected in a large number of samples from the central state of Maharashtra.

In contrast, the B.1.1.7 variant is rising rapidly in New Delhi, said Dr. Sujeet Singh, Director of India’s National Center for Disease Control. It was predominant in half of the samples evaluated in late March, up from 28 percent just two weeks earlier. Variant B.1.617 is also in circulation in New Delhi, he added.

Ultimately, however, the data from India is too thin to analyze the distribution of the variants across the country. Despite the large number of new infections, India does very little genome sequencing.

In December, the government tapped into a group of 10 laboratories and set an ambitious goal of sequencing 5 percent of samples across the country each month. However, less than 1 percent of the samples collected have so far been sequenced. A report in The Wire, an Indian online publication, pointed to logistical challenges, red tape and lack of funding as some of the reasons.

“They just aren’t well equipped, as sophisticated as their scientists and doctors,” said Dr. Gounder.

Aside from the UK, few other countries have been watching variants closely. The United States also sequenced less than 1 percent of the samples, and has only stepped up its efforts in the past few weeks.

Officials in India are trying to track how many fully vaccinated people have become sick, a measure known as the breakthrough infection rate. That could suggest how virulent a variant could be in India. They focused on frontline healthcare professionals who were more likely to have received both doses of the AstraZeneca vaccine.

So far, data from the Indian Council for Medical Research through April 21 shows an extremely low breakthrough infection rate, although possibly not as low as that of the United States. The data shows that 0.02 to 0.04 percent of people who are vaccinated get sick. The rate in the United States based on various vaccines is 0.008 percent.

At Sir Ganga Ram Hospital, the 37 post-immunization doctors received their first dose of the AstraZeneca vaccine between late January and early February, and their second dose four to six weeks later. The hospital employs around 500 doctors.

Dr. Shad, the cardiac surgeon, was reluctant to draw any conclusions about variants that break the immunizations. “I don’t think anyone has the serological data,” to respond to, he said.

A widespread lack of data plagues the scientific hunt for variants and whether they contribute to the severity of the Indian crisis. Fast moving mutations make the picture difficult because it is not immediately clear how quickly they spread or how they respond to vaccines.

In India, the health system was not alert to the effects of home-based variants, although they were beginning to spread around the world, said Dr. Thekkekara Jacob John, a senior virologist in the southern state of Tamil Nadu.

“We weren’t looking for variations at all,” he said. “In other words, we missed the boat.”

Hari Kumar contributed to the coverage.