Millions of coronavirus cases may have gone undetected in India earlier this year, a research paper estimates, saying that for every confirmed case in May, there were 82 to 130 infections that were not recorded.

The paper, published in the Indian Journal of Medical Research, estimated that the country had nearly 6.5 million cases as early as May. Total cases stood at more than 180,000 in late May, according to the health ministry.

The research paper, written by government scientists and other experts and published late on Thursday, said large numbers of cases could have gone under the radar earlier this year because testing was limited to symptomatic patients or states had varying testing rates.