still responsive to vaccines, although slightly less so. India relies heavily on the Oxford-AstraZeneca vaccine, which clinical trials show is less powerful than the vaccines made by Pfizer-BioNTech and Moderna and could perhaps be more easily thwarted by mutations.

“For now the vaccines remain effective, but there is a trend toward less effectiveness,” said Dr. Céline Gounder, an infectious disease physician and epidemiologist at Bellevue Hospital in New York.

In India, a number of doctors point to anecdotal evidence that people who have been fully vaccinated are getting sick. Those doctors also say they are seeing children with serious symptoms, such as severe diarrhea, acidosis and falling blood pressure, even among otherwise healthy patients.

“This is very different from what we saw last year,” said Dr. Soonu Udani, head of critical care services at the SRCC Children’s Hospital in Mumbai.

report in The Wire, an Indian online publication, pointed to logistical challenges, bureaucratic red tape and the lack of funding as some of the reasons.

data from the Indian Council of Medical Research up to April 21 shows an extremely low breakthrough infection rate, though perhaps not as low as that of the United States. The data shows 0.02 percent to 0.04 percent of vaccinated people falling ill. The rate in the United States, which relies on different vaccines, is 0.008 percent.

At Sir Ganga Ram hospital, the 37 doctors who became infected after immunization had received their first dose between late January to early February and then their second dose four to six weeks after that. The hospital employs about 500 doctors.

Dr. Shad, the cardiac surgeon, was reluctant to jump to conclusions about variants breaking through the immunizations. “I don’t think anyone has the serological data” to answer that, he said.

A broad lack of data plagues the scientific chase for variants and whether they are contributing to the severity of India’s crisis. Fast-moving mutations complicate the picture because it isn’t immediately clear how quickly they spread or how they respond to vaccines.

In India, the health care system wasn’t on alert for the impact of variants at home, even as they began to spread globally, said Dr. Thekkekara Jacob John, a senior virologist in the southern state of Tamil Nadu.

“We were not looking for variants at all,” he said. “In other words, we missed the boat.”

Hari Kumar contributed reporting.

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