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Less than two weeks ago, India celebrated administering its 1 billionth COVID-19 vaccine in a campaign that has partially inoculated nearly 70% of the country’s 900 million adults. Now, a new reality is worrying its health authorities: More than 100 million people have not turned up for their scheduled second vaccine dose, leaving the country vulnerable to a third wave.

India’s daily COVID cases have dropped to around 12,500 from a peak of 400,000 in early May. But the problem of missed second doses is raising concerns about a fresh surge of infections in the world’s second most populous nation, especially as crowds flood markets and mingle freely during the ongoing festival season and as Indian schools and colleges prepare to return to in-person instruction after nearly two years of remote learning.

States such as West Bengal and Assam have already witnessed a spike in infections in the past month after relaxing pandemic restrictions during religious festivals.

Nearly 70% of India’s population have received at least a single dose of vaccine, but just 30% are fully vaccinated with both doses. India is expected to miss its target of fully vaccinating the country’s entire adult population by year’s end.

“We have seen people suffering and dying during the second COVID wave. We know that fatality drops to nearly zero after the second vaccine dose. Why are we not working hard to give the second dose to people?” says Amir Ullah Khan, an economist and former policy adviser for the Bill & Melinda Gates Foundation. “We can’t pat ourselves on the back for a job half-done, because that sends a wrong message.”

Most COVID-19 vaccines require at least two doses and the interval between the two jabs depends on which one is being used, according to the World Health Organization. The Covishield vaccine—the Oxford-AstraZeneca jab manufactured locally by the Serum Institute of India—makes up the majority of vaccines being administered in India. While there is no firm data showing exactly how well one-dose and two-dose recipients fare against the Delta variant, Khan says anecdotal evidence shows that fatality rates do fall among the population with two doses.

The Indian government has launched public health advertisements on television and radio that caution people about the need to maintain safety protocols like wearing masks as well as the need for vaccination, but health experts say the public doesn’t sufficiently understand the importance of the second vaccine dose. Misinformation about the COVID-19 vaccines—especially in remote rural areas—have hampered efforts to achieve a higher full-vaccination rate.

At the height of India’s second wave in May—when the virus was rampant and jabs were in short supply—the country increased the waiting period between the first and second doses of its Covishield vaccine from four to six weeks to 12 to 16 weeks to prioritize first doses, which offer partial protection.

There is no shortage of vaccines now, but many Indians are unsure about when to receive their second dose because the time frame shifted, says Khan. Everyone over age 18 is eligible for a jab in India, but Indians are required to register themselves for vaccination on a CoWin government app.

The information about the schedule of the second dose is available on the app, but low smartphone penetration and low literacy levels mean not everyone can access the information, says Gajendra Singh, an independent health expert. Just over 50% of India’s 1.4 billion population own a smartphone.

Indian health professionals are expected to soon start a door-to-door drive to administer second doses, similar to a decades-long national campaign against polio. The task is harder with COVID-19 because there’s a risk workers will get infected, says Singh.

The Indian government also has not used results from a national survey of blood tests, which estimates around 70% of Indians are positive for COVID-19 antibodies, to target its vaccination drive at people more vulnerable to the virus, says Barun Mitra, an independent public policy consultant, to Fortune.

Indian health authorities could have rationed vaccines to prioritize distribution to those with low antibodies who needed it most, says Mitra. Had health authorities done that, the country could have reached universal immunity—through infection or vaccination—by now, he says.