India's death toll from Covid-19 has crossed 500,000, a level many health experts say was breached last year but obscured by inaccurate surveys and unaccounted dead in the hinterlands, where millions remain vulnerable to the disease.

The country, which has the fourth-highest tally of deaths globally, recorded 400,000 deaths by July last year after the devastating outbreak from the Delta variant of the coronavirus, according to official data. Some experts believe the figures were much higher.

“Our study published in the journal Science estimates three million Covid deaths in India until mid-2021 using three different databases,” Chinmay Tumbe, an assistant professor at the Indian Institute of Management, Ahmedabad who co-authored the study, told Reuters.