Herd immunity? India still has a long way to go, scientists say

Herd immunity? India still has a long way to go, scientists say

Science’s COVID-19 reporting is supported by the Pulitzer Center and the Heising-Simons Foundation

Last week, a panel of leading scientists appointed by the Indian government delivered a startlingly optimistic message: The world’s second largest COVID-19 epidemic has rounded a corner. India’s daily number of cases has declined by roughly half in the past month, and a new mathematical model suggests “we may have reached herd immunity,” the panel wrote in a paper published online by The Indian Journal of Medical Research. Assuming measures such as social distancing, wearing masks, and hand washing remain in place, the group said the pandemic could be “controlled by early next year.”

But other scientists say the model overestimates the number of people already infected and warn that with colder temperatures and several religious holidays approaching, India may well see a second wave. The positive national trends hide a more complex picture, suggests Giridhar Babu, an epidemiologist with the Public Health Foundation of India. He believes the virus may have burned through large, densely packed populations but will continue to spread in rural areas, at a lower rate, for many months: “We still have large numbers of people for the virus to go through.”

The encouraging projections come from the National Supermodel Committee, which modeled the past and future of India’s epidemic at the government’s request. Its work suggests 380 million Indians had already been infected by mid-September and that there might be “minimal active symptomatic infections” by late February 2021 if control measures continue. (The study also concluded that by flattening the curve, India’s lockdown in the spring saved up to 2.6 million lives.)

The model did not take regional differences in viral spread into account. But Babu notes that serological surveys—which test for antibodies in a population to gauge the fraction already infected—have found much higher infection rates in Indian cities, and in particular in slum areas. Studies in August, for example, found antibodies in 41% of residents of Mumbai’s slums, compared with 18% elsewhere in the city. “Many dense pockets that can be easily infected have [likely] already been infected,” Babu says. That could help explain the slowdown in new infections but suggests many more people remain vulnerable.

Gautam Menon of Ashoka University, a co-author on several COVID-19 modeling studies, adds that the model suffers from “a lack of epidemiological realism” because it assumes an unusually large fraction of infected people remains asymptomatic. He says 200 million to 300 million is a better estimate for the number of infected people.

Experts agree that spread in rural areas, home to more than half of the population, is a challenge to both monitor and fight. Testing isn’t easily available in many small towns and villages; serosurveys suggest official testing, now at 1 million per day, vastly undercounts actual cases. Some states rely heavily on so-called rapid antigen tests, which range widely in sensitivity. Meanwhile, the health infrastructure in many of India’s rural states is weak, making it harder to treat patients.

More granular data might help scientists better understand the pandemic’s trajectory. The national epidemic is a “figment of statistical imagination,” says T. Jacob John, former head of the department of virology at Christian Medical College; instead, “There are 100 or more small epidemics in different states and cities, rising and falling at different times,” he says.

Babu warns against complacency. “The decline [in cases] is real and valid, but no one should rejoice yet,” he says. “It only means that the first set of formidable challenges is over, and the next set is beginning.”

Vaishnavi Chandrashekhar is a journalist in Mumbai.


Leave a Reply

Your email address will not be published. Required fields are marked *