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.
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.
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.
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.