As India confronts a devastating coronavirus outbreak where thousands are dying each day, the country desperately needs to vaccinate its population as soon as possible. Yet the vaccine drive is stumbling just when it is most crucial. Over the past six weeks, the number of vaccinations per day has fallen by about half, from a high of 4.2 million per day on April 2 to 2 million on Thursday.

Vaunted as the largest in the world, India’s vaccine program is being hobbled by supply shortages and an abrupt shift in procurement policy that appears to be without parallel. The woes of the inoculation drive are especially striking given India’s unique advantages, including a large vaccine industry and a record of mass immunization campaigns.

Just months ago, public health experts were counting on India to play a crucial role in supplying coronavirus vaccines to the developing world. The government was so confident of its ability to meet domestic demand for vaccines that it allowed more than 60 million doses to be exported or donated to other countries between January and March.

Now the vaccine exports are a source of increasing anger. In Delhi, posters appeared with a question for Prime Minister Narendra Modi: “Why did you send our children’s vaccines abroad?” Police arrested and questioned more than 20 people in connection with the posters over the weekend.

Faced with surging infections, India suspended exports at the end of March and has begun importing Russia’s Sputnik vaccine to try to alleviate the shortfall in local production. Last week, the government acknowledged that vaccine shortages in India will persist at least until July.

“To cover the entire nation, please remember it will take a little while,” V.K. Paul, a senior health official, told reporters on Thursday. “We should accept that as a reality with humility.”

The inability to accelerate the vaccination drive in the short term means that immunizations will do little to blunt the ferocity of the current wave of infections: To date, only 10 percent of Indians have received at least one shot.