On Thursday, India celebrated passing the milestone of administering 1 billion COVID vaccine doses.

To mark the occasion, Prime Minister Narendra Modi visited health care workers at New Delhi's Ram Manohar Lohia Hospital. The Health Ministry also held events with music and entertainment across the country.

"We are witnessing the triumph of Indian science, enterprise and collective spirit of 1.3 billion Indians," Modi tweeted.

However, nine months after launching the world's biggest vaccination drive, experts say it will be early next year before its adult population is fully vaccinated.

Currently, 30% of India's adult population is fully vaccinated and 74% have been given one dose of a two-dose regimen.

Vaccination milestone follows deadly second wave

India launched the vaccination drive in stages starting on January 16, 2021. Health care workers were given priority.

Several weeks later, eligibility was expanded to anyone above 60 years old, and those above 45 years old with comorbidities.

In May, everyone 18 years and over was made eligible to receive a vaccine.

However, during the peak of a devastating outbreak in spring 2021, India experienced widespread vaccine shortages as new daily COVID cases at one point topped 300,000.

The daily death toll during this so-called second wave was in the thousands. Crematoriums in cities across the country were operating beyond capacity, with makeshift funeral pyres being set up.

"Most of us hadn't dealt with a pandemic in our lifetime so preparation and anticipation were not easy," infectious disease expert Priscila Rupali from the Christian Medical College told DW.

At the time, the government was criticized for having sent largeshipments of vaccines abroad, a policy it was forced to turn around as Indian producers struggled to pump out doses.

"From challenges of availability of vaccine doses initially, we have come a long way and now we have to vaccinate those left behind," said N K Arora, head of the COVID working group at India's National Technical Advisory Group on Immunization.

"It is not an easy task. Community engagement with dispelling myths, suspicions and anxiety has helped us reach the billion mark," he told DW.

According to a survey conducted earlier in October, only 7% of India's adult population reported being hesitant towards taking a COVID vaccine. The survey was carried out by the online community platform LocalCircles, and received 12,800 responses from over 300 districts.

This survey shows one of the lowest rates of vaccine hesitancy in India since the vaccine rollout began in January.

It cited quick clinical trials, hasty vaccine approvals, safety concerns, and side-effects as having been among the reasons behind apprehension towards taking a vaccine. 

What is the current state of India's vaccination drive?

The Covishield vaccine, manufactured by the Pune-based Serum Institute of India, accounts for nearly 90% of vaccine doses in India. Covishield is the brand name in India for the Oxford-AstraZeneca vaccine.

Covaxin, a vaccine developed in India by Bharat Biotech, has emergency use authorization in India, but still has not been able to meet production expectations.

Despite the initial vaccine shortages experienced during the second wave, supply lines of vaccines seem to have vastly improved in the past three months.

In early March, there were an estimated 3,000 vaccination centers in India. There are now nearly 100,000 across the country.

Moving forward, epidemiologists and public health experts argue that authorities need to prioritize administering second doses, as the Covishield vaccine requires two doses to provide maximum protection.

With the domestic situation vastly improved, India has also resumed contributing to international supplies of vaccines.

Exports are expected to increase significantly in the next few months as domestic supply is secured and most of the population has received at least one dose of a vaccine.

Drive to get Indians fully vaccinated

However, India's disparity between partially and fully vaccinated is among the highest in the world.

Health Secretary Rajesh Bhushan has told state governments to target districts with low vaccination rates and push people to take a second dose.

"Covering the last mile is difficult in any country. We are already seeing difficulty getting complete coverage done even in the US and other developed nations," epidemiologist Giridhar Babu told DW.

"There is no point in having an arbitrary deadline," he added.

Babu believes India should continue strengthening local planning and implementation in areas and population groups that are difficult to reach, even if it takes a little longer.

India set a single-day vaccination record, using the occasion of Modi's birthday on September 17 to launch a vaccination drive, administering more than 25 million shots.

"That we have enough infrastructure to give that number of shots in a day is commendable," Vineeta Bal, scientist at the National Institute of Immunology, told DW.

India is aiming to fully vaccinate its entire adult population by the end of this year, which means an additional 900 million doses would need to be administered between now and December 31.

"As immunity from the first dose or natural infection fades, we need to ensure a second dose is administered to all eligible persons by the first quarter of next year, especially with exports resuming and those below 18 likely to be included," Srinath Reddy, President of the Public Health Foundation of India, told DW.

According to census figures, 41% of India's 1.3 billion-plus population is younger than 18 years old. Authorities are expected to announce protocols for the rollout of vaccines for children over 12 soon.