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SINGAPORE-When the highly infectious Omicron variant reached Asia a few months ago, India and Indonesia had a major gap in their defenses: Two-thirds of their populations were yet to be fully vaccinated.

But the countries are emerging from their most recent Covid-19 waves with a fraction of the deaths they recorded during the onslaught of the earlier Delta variant. Their deaths per capita are even lower than more vaccinated places that have better healthcare systems such as South Korea, Japan and Hong Kong.

A big part of the reason, epidemiologists say, is that developing nations hit hard by the Delta wave last year acquired high levels of immunity through infection. And that protection appears to have endured. Studies in India, Indonesia and South Africa show widespread prevalence of Covid-19 antibodies, far outstripping their vaccination rates.

This is a breakthrough for the developing world. Relatively slow-moving vaccination drives had sparked fears that another tsunami of cases would overwhelm hospitals and drive up deaths, Omicron's relative mildness notwithstanding. But past Covid-19 infections-far more widespread than official case counts depict-have helped them achieve immune cover broad and protective enough to hold off the most feared outcome. Although immunity will wane, it buys governments more time to expand coverage of vaccines and boosters before the virus mutates again.

Vaccination campaigns have lagged behind in much of the developing world, as many countries struggle to distribute shots outside major urban areas. In Asia, countries like India, Indonesia and the Philippines have fully vaccinated half their populations. That is higher than the 13% of people living in Africa who are vaccinated, with 4% immunized in Nigeria, the continent's most populous country. Latin America has done better. Many countries, including Brazil and Peru, have vaccinated more than 70% of their people.

Natural immunity, which refers to antibodies acquired through infection, was widespread in Indonesia when Omicron arrived. One study from October to December of roughly 20,000 Indonesians found that 74% of unvaccinated Indonesians had protective antibodies, according to Pandu Riono, a University of Indonesia epidemiologist who worked with government researchers on the study.

Based in part on the results, Mr. Riono and his colleagues estimate that 70% of Indonesians had been infected with Covid-19 as of December, far higher than the 34% of Americans who had been infected by then, according to a survey by the U.S. Centers for Disease Control and Prevention.

Government serological surveys in India conducted last year have similarly shown that 97% of New Delhi residents and 87% of Mumbai residents have antibodies against Covid-19.

"Hybrid immunity, which is some combination of having exposure to infection and immunization, has been high in this part of the world," said Ramanan Laxminarayan, director of the Center for Disease Dynamics, Economics and Policy, a public-health research organization based in Washington, D.C., and New Delhi. "South and Southeast Asia did have a worse Delta wave."

The situation is similar to South Africa, where Omicron was discovered and caused a surge of cases but relatively few deaths compared with previous waves. In a study published last month in the New England Journal of Medicine, South African researchers showed that even though 36% of people who were 12 years and older had received at least one vaccine dose in Gauteng province around the time Omicron began circulating, Covid-19 antibodies were widespread. Of unvaccinated survey participants in the province, 68% had Covid-19 antibodies, along with 93% of those who were vaccinated, the study showed.

Indonesia recently recorded caseloads higher than during its Delta peak, but it averaged 270 deaths a day compared with 1,800 in August, according to Our World in Data. In India, deaths during the Omicron wave peaked at an average of 1,100 a day in early February, compared with 4,200 during the height of the Delta wave in May.

Like much of the world, both countries are undercounting Covid-19 deaths. But the picture the data presents is corroborated by how their healthcare systems have fared. Unlike last year when hospitals ran out of oxygen and medicines, medical facilities this time around are less crowded, with many Covid-19 patients recovering at home. In the Indonesian capital, Jakarta, roughly 50% of Covid-19 hospital beds were unoccupied in mid-February, as cases were peaking.

"Deaths aren't as heavy as they were before," said Tri Maharani, an emergency-room doctor.

Some research suggests that immunity from infection lasts longer than immunity from vaccination. Dorry Segev, a professor of surgery and population health at NYU Langone Health, an academic medical center, led a team that published a research letter last month in the Journal of the American Medical Association that found that unvaccinated people maintained natural immunity up to 20 months after infection. He said that further research under review shows that this immunity was substantially protective against Omicron. Research from Israel and the U.K. show waning immunity from vaccination after a few months.

The U.S. CDC says fully vaccinated people and those previously infected with Covid-19 are at low risk of subsequent infection for at least six months.

Mr. Segev said the durability of natural immunity, alongside active vaccination campaigns, bodes well for countries like India and Indonesia. "We know the strongest immunity is hybrid immunity. Previous infection plus vaccination gives you epic levels of protection," he said.