NEW DELHI — It is a race against time, with consequences for millions of people in India and beyond.

After falling steeply for months, coronavirus cases are surging in the world’s second-most-populous country, pitting a new wave of infections against India’s vaccine rollout.

On Wednesday, India recorded 53,476 new cases, the highest figure since October. The speed of India’s second wave has alarmed experts, who say it probably reflects changes in behavior, waning immunity among those previously infected, and the influence of new variants. Deaths from the virus are also on the rise in India, with 250 reported on Wednesday.

Compared to many countries, India is in an enviable position when it comes to vaccine supply, thanks to its robust domestic manufacturing capacity. About 50 million people in India, or 4 percent of the population, have received at least one shot. But India has also sent 60 million doses to other countries, a strategy that has raised questions about the pace of India’s rollout at home.

As cases increase, India is racing to ramp up vaccinations. All Indians 45 and older will be eligible for vaccines starting April 1, the government said this week.

Here’s what you need to know about the challenge ahead for India.

How big is the new wave of cases, and what is driving it?

In January, new daily cases in India dropped below 10,000. Antibody surveys showed that a large proportion of people in major cities — including Delhi, the capital — had already been exposed to the virus, dampening its spread. But infections started to rise last month, particularly in the western state of Maharashtra, home to Mumbai.

Now new infections are accelerating at their fastest rate since May, said Bhramar Mukherjee, a biostatistician at the University of Michigan who models India’s outbreak. Elections will soon take place in several large Indian states while a major religious festival is expected to draw as many 10 million pilgrims to the banks of the Ganges River, all events with the potential to drive infections, she noted.

“You can see that the tidal wave is coming,” Mukherjee said. “It would be foolish to be in denial.”

Almost exactly a year after India instituted the world’s largest lockdown, localized versions of such restrictions are being applied in a handful of areas. Delhi and Mumbai, meanwhile, have begun randomly testing people in public places without their consent.

Scientists say that as cases declined and pandemic-related restrictions were lifted, Indians seized the chance to return to something resembling normal life. While the surge may be driven by several factors, “the main cause is laxity and people letting their guard down,” said Rakesh Mishra, director of the Center for Cellular and Molecular Biology in Hyderabad, one of the laboratories screening for new variants.

There are signs that the virus is spreading among new populations. In Mumbai, about 90 percent of the cases in the second wave are from neighborhoods other than the city’s densely populated slums, which were hit hard early in the pandemic, said I.S. Chahal, the city’s municipal commissioner.

Contact tracing is very difficult, he added, particularly given the massive movement each day of millions of residents on buses and commuter trains. On Wednesday, Mumbai reported more than 5,000 cases, the highest daily total to date in the pandemic. The silver lining is that a large majority of the new cases in the city appear to be asymptomatic, Chahal said in an interview last week. “This is a great blessing for us or there would have been chaos,” he said.

What is the role of variants in the surge?

It is not clear yet whether variants are to blame for India’s second wave of cases. In certain regions, they appear to be playing a notable role. In the state of Punjab, where infections are rising rapidly, 80 percent of the samples sent for genetic sequencing from February to early March showed the presence of the variant first detected in the United Kingdom, considered more transmissible and possibly more deadly than earlier versions of the virus.

Variants from South Africa and Brazil are also present in India, but for now they do not seem to be a driving force behind infections. On Wednesday, the government said that a new variant had been detected in India that combines two mutations that “confer immune escape and increased infectivity.” The double-mutant variant was found in about 20 percent of new cases in Maharashtra. Health authorities emphasized that it was too soon to say whether any variant could explain the recent increase in infections in the country.

How fast is India vaccinating its population?

India began vaccinating people on Jan. 16, but the effort got off to a slow start, compounded by vaccine hesitancy and a lack of urgency among some Indians to get immunized as cases declined. Health-care workers were the first in line, followed by people older than 60 and those older than 45 who suffer from co-morbidities. The push is now gaining pace: Over the past week, India has vaccinated more than 2 million people a day on average. But it will need to accelerate if India is to meet its target of vaccinating 300 million people by the summer.

The vast majority of those vaccinated so far have received the AstraZeneca vaccine, which is being manufactured locally by Serum Institute of India. A smaller proportion have received Bharat Biotech’s Covaxin, India’s indigenous entry in the global vaccine race.

Some experts and local politicians say the central government’s one-size-fits-all approach — it alone determines who is eligible to be vaccinated nationwide — has impeded the rollout. This week, the top politician in Delhi declared that he could vaccinate all adults in the capital in three months if the government relaxed eligibility requirements.

“This hierarchy of power and control is not conducive to public health,” said Mukherjee of the University of Michigan. “You needed to start aggressive vaccinations when case counts were low.”

Increasing the pace of vaccinations will help, but getting people fully immunized still takes weeks or months, depending on the interval between the two shots, said Shahid Jameel, an Indian virologist and professor at Ashoka University. Meanwhile, curbs on movement and gatherings will have to return, he said, including limited lockdowns in specific areas.

Local officials are bracing themselves for what comes next. In the city of Haridwar in the foothills of the Himalayas, millions of pilgrims are expected to arrive next month to bathe in the Ganges River as part of the Kumbh Mela, a major Hindu festival.

“Our worries are growing,” said Harbeer Singh, a senior member of the organizing committee. “It is possible that the Kumbh will potentially become a superspreader event.”

Masih reported from Mumbai. Taniya Dutta contributed to this report.