NEW DELHI — India granted emergency approval Sunday to its first vaccines — Oxford-AstraZeneca and homegrown Covaxin — as it prepares to undertake an unprecedented immunization program for the country of more than 1.3 billion.

The announcement of India's approval of the Oxford-AstraZeneca vaccine came days after regulators in Britain greenlighted it and marks a big step for the country that is the world's second-worst affected by the coronavirus pandemic. India aims to administer the vaccine to 300 million people in the first phase, and distribution could begin in the coming days.

The Oxford-AstraZeneca vaccine, known as Covishield in India, is being produced locally by the Serum Institute of India, the world’s largest vaccine manufacturer. It has stockpiled 40 million to 50 million doses and plans to produce 300 million doses by July. Its billionaire owner, Adar Poonawalla, has pledged 50 percent of its production for India. On Sunday, Poonawalla said on Twitter that his company’s risks had “paid off” and that the vaccine is “ready to roll-out.”

Interim results published by Oxford-AstraZeneca researchers in the United Kingdom showed that the vaccine was 62 percent effective among those given two doses. A different dosage proved 90 percent effective; scientists are actively studying that dosage and its efficacy. Pfizer-BioNTech and Moderna, which have rolled out vaccines in the United States, reported efficacy rates of about 95 percent.

According to a statement by the Indian drug regulator, the Serum Institute conducted Phase 2 and 3 trials on 1,600 participants in the country, and the data was found to be “comparable” with the data from overseas studies. The approval is subject to regulatory conditions, though details of those were not shared immediately.

Prime Minister Narendra Modi said it was a matter of pride for every Indian that both vaccines are “made in India.”

But the announcement was marred by questions about the fast-track approval of Covaxin, which has not completed its third phase of human clinical trials. That vaccine is being developed by Hyderabad-based Bharat Biotech in collaboration with two government-backed institutes. The regulator said the trials for the first two phases demonstrated the vaccine was safe, but it did not share efficacy data.

“Detailed analysis documents need to be put in the public domain,” said Giridhar Babu, a public health expert, adding that terms like “restricted use” in the announcement need to be explained.

“Regulatory conditions need to be spelled out much more clearly,” he said.

Shashi Tharoor, a member of the opposition party in the Indian Parliament, tweeted that the approval of Covaxin was “premature and could be dangerous.”

Health Minister Harsh Vardhan late Sunday night clarified that all Covaxin recipients will be tracked and monitored “as if they’re in trial,” calling it a strategic decision.

Bharat Biotech said in a statement that the vaccine addresses “an unmet medical need” and that its goal is to provide “global access to populations that need it the most.”

India aspires to a big role in the supply of coronavirus vaccines to the developing world through its homegrown vaccine candidates. Even as wealthy countries such as the United States and Britain have snapped up vaccines for their entire populations, poorer regions are struggling to secure supplies.

Both the vaccines require two doses — as does the ­Pfizer-BioNTech vaccine — but are cheaper and do not require extremely low storage temperatures.

Vardhan had earlier said the vaccine will be free for the health-care and other essential workers who will be the first in line to get vaccinated.

With more than 10 million infections, India’s coronavirus caseload is second only to that of the United States. More than 149,000 people in India have died after contracting the virus. However, the number of daily infections in India has fallen drastically since the early stages of the pandemic. The country has confirmed the presence of the new U.K. strain of the virus among recent arrivals from Britain, which has led to worries about a possible resurgence of infections.

For the program’s rollout, India will first vaccinate 30 million health-care workers and other first responders, including police and members of the armed forces. Next will be those over 50 and with co-morbidities — an estimated 270 million more people.

India’s health-care system is patchy, especially in rural areas and places under strain from the pandemic. In September, at the peak of the pandemic, hospitals in some states faced oxygen shortages because of lack of storage and delivery facilities. The country also lacks the infrastructure needed to store vaccines such as the one by Pfizer at extremely low temperatures.

But the challenges are outweighed by other factors, experts say.

“India’s biggest advantage is its vaccine-manufacturing capacity,” said Shahid Jameel, director of the Trivedi School of Biosciences at Ashoka University near Delhi. “India will have sufficient doses of vaccine of various types without spending huge amounts of money to preorder vaccines.”

India is a global leader in vaccine manufacturing and claims to produce 60 percent of the world’s vaccines. Its experience in running large-scale immunization programs will also be valuable. Its child immunization program targets 27 million newborn babies annually.

For effective distribution of the coronavirus vaccine, India is looking to its experience in conducting the world’s largest elections every five years.

“The procedure of the vaccination drive is the same as the election,” Health Minister Vardhan said Friday, involving thousands of workers who receive training in a standardized operating procedure carried out nationwide. Step-by-step processes have been outlined and roles demarcated for vaccination teams.

For seamless tracking and delivery of vaccine doses, India has developed a digital platform called Co-WIN, which will maintain a database of recipients, cold-chain points and post-vaccination symptoms. Identification of beneficiaries, physical sites for administering doses and training of vaccination workers is being carried out by state governments.

According to government guidelines, about 100 people will receive vaccination shots at every site in a day. Last week, four state sites held dry runs that went off smoothly.

Rajesh Bhaskar, the officer in charge of pandemic management in Punjab, where one of the four sites is located, said the state had identified 150,000 health-care workers for the first round who can be vaccinated within two days.

In the southern state of Andhra Pradesh, Mohammed Imtiaz, a senior official, said the test run was conducted in rural and urban areas at government and private facilities. The focus was on ensuring that vaccination officers knew the protocols for validating identities, following biomedical-waste norms and monitoring for adverse reactions.

The rest of the country conducted similar test runs this week.

Jameel, the virologist from Ashoka University, said there may not be a need to vaccinate everyone. Areas where 70 percent of the population have antibodies for the coronavirus won’t require vaccinations, while those with a lower percentage should be given priority, he said. The government plan for vaccination will take nine to 12 months to complete, and scientists should conduct periodic surveys to assess the antibody levels. “That will guide us,” he said.