India’s Covid-19 Cases Drop. A Second Wave May Loom.

NEW DELHI — Two months ago, India looked like a coronavirus disaster zone.

Reported infections neared 100,000 a day, deaths were shooting up, and India seemed ready to surpass the United States in total recorded cases.

Today, India’s situation looks much different. Reported infections, deaths and the share of people testing positive have all fallen significantly. By contrast, infections in Europe and the United States are surging.

But doubts persist about the reasons for India’s drop, and some researchers say the results stem at least in part from a possible change in testing, though researchers say they do not have access to complete data to really know the big picture. The experts generally agree that the number of infections has far outstripped efforts to track them in India, like elsewhere, and that infections in the country could still get considerably worse.

There has also been a shift in collective thinking, and experts worry that India has begun to lower its guard. After an intense lockdown in the spring and restrictions on social gatherings through the summer, the government has been steadily unlocking. There’s no talk of locking down again.

The overall mood seems to be, “Let’s move on.”

Mobility data show that Indians have returned to shopping areas and public spaces. Many are not wearing masks. A large chunk of the population seems resigned to the threat of infection.

“People are saying: ‘What the hell, we have to learn to live with it. God knows how long it will last,’” said Dr. Naresh Trehan, a cardiologist and the head of the Medanta hospital chain, based near New Delhi.

In many places, he added: “People are partying like there is no tomorrow. So if you do things like that, you are bound to suffer.”

Many doctors here believe it’s just a matter of time before the cases start shooting up again. Other countries, including the United States, France and Germany, thought the worst virus days were behind them, only to hit new highs.

Parsing the course of the outbreak is difficult anywhere, and the next few weeks could complicate the picture. Cooler weather could spur a rise in infections. The Hindu holiday season, when millions of people travel to see relatives and flock to stores to load up on gifts for Diwali, Hinduism’s festival of lights, is beginning.

Air pollution in India’s cities is also increasing, as it does every fall and winter, and doctors fear that toxic air could lead to more hospitalizations and deaths from Covid-19. Some areas are seeing spikes, even as the national numbers fall. New Delhi, for example, is currently hitting record highs.

“The unexposed, the elderly, the young who missed the first two waves” could still get sick, said P. Umanath, a doctor and civil servant helping to supply testing kits in Tamil Nadu State.

The Institute for Health Metrics and Evaluation at the University of Washington projects that India’s cases will soon increase again and exceed one million daily infections — many of them never detected — by year’s end, in part on the assumption that India does not widely embrace wearing masks.

Still, for the moment, official numbers suggest the coronavirus is in retreat. From a high point of nearly 98,000 daily infections on Sept. 16, the average dropped to about 46,000 cases per day this past week. The number of daily virus deaths has fallen to around 500 from 1,200 in mid-September, and India’s overall death toll is still much lower, per capita, than that in many other countries. By official figures, India has had about 8.5 million infections, trailing the United States by about 1.5 million.

The government has claimed credit, citing its lockdown in the spring and a public awareness campaign, even as it has urged the Indian people to remain vigilant.

“Things are getting better,” said Harsh Vardhan, India’s health minister. “However, there is no room for complacency.”

Several prominent scientists and doctors have been reluctant to accept that India’s overall infections are dropping, saying the lower numbers could be explained by the increased use of less reliable tests and fewer tests.

The number of tests performed each day varies, but on average it has remained around 1.1 million for the past two and half months, according to the Indian Council of Medical Research, the top government body aggregating Covid-19 data.

And the council points to a drop in the positivity rate across the country, or the share of administered tests that find the coronavirus, to 3.7 percent this week from 8.6 percent in mid-September.

The council also said the country had decreased its reliance on rapid antigen tests, which detect viral proteins called antigens and are considered less sensitive than other tests, to 41 percent now from 47 percent in mid-September.

Data from the more reliable tests — called polymerase chain reaction, or PCR, tests — showed a similar decrease in infections, said the council’s director general, Balram Bhargava.

“We are seeing that downward trend clearly,” Mr. Bhargava said.

Even more broadly, scientists believe that testing cannot capture the full spread of the disease in India or in any other countries.

Surveys of blood samples have suggested that at least 15 percent of residents of certain urban areas may have already contracted the virus. A new blood sample survey estimated that more than one-quarter of people in Karnataka, a large southern state with a population of more than 60 million, had already been infected.

Just how many people in India have contracted the disease, and may have developed protective antibodies, is the big question. A government-backed study based on mathematical projections — employing a model typically used to track the progression of a disease — estimated that nearly one-third of the country had already contracted the virus. The scientists estimated the number of undetected cases for each recorded infection by using data showing the rates of transmission in India, the numbers of infected and the numbers of recoveries.

While other scientists cast doubt on that study, they believe infections have reached 200 million, or at least 15 percent of the population, undetected.

“The government and the public have focused on the recoveries and low fatalities and decided to let the virus take its own natural course and cruise toward herd immunity, if that can be attained,” said Bhramar Mukherjee, a professor of epidemiology at the University of Michigan’s School of Public Health who tracks India closely.

Controlling the outbreak in India would be difficult under any circumstances. Much of its population lives in close quarters. The health care system is vastly underfunded. The government is a freewheeling democracy that doesn’t exert the same level of control of a nation like China.

Like many other countries, India imposed a broad lockdown that brought the economy to a halt. But much of it was lifted after two months, when officials concluded that the restrictions were killing the economy.

India has room for spread. Though some people work from home, doing so is a luxury that most can only dream of. Countless millions have to circulate every day on the streets to feed their families.

“India could light up like a Christmas tree in the next three or four months,” said Michael Osterholm, an epidemiologist at the University of Minnesota. “We welcome, obviously, the decrease in cases, but realizing just as every other country that as soon as you let off the brake, then it comes.”

Hari Kumar contributing reporting.