Up to two-thirds of India's population may have been infected with Covid-19, a government study suggested on Tuesday.

The blood serum survey of about 29,000 people in June and July showed 67.6 percent of those tested had antibodies.

The results of the study by the Indian Council of Medical Research (ICMR) show the impact of a devastating surge of cases in April and May, when India reported about 400,000 cases and 4,000 deaths per day.

In December and January, less than 25 percent of those in a similar study had positive antibody tests.

ICMR chief Balram Bhargava said that the study shows "there is a ray of hope".

"But there is no room for complacency. We must maintain Covid-appropriate behaviour," he said in New Delhi on Tuesday.

The hundreds of millions in India without antibodies are more susceptible to severe infection.

Only 13 percent of the adults in the study were fully vaccinated. 

India, home to 1.3 billion people, has only vaccinated about eight percent of eligible adults, triggering fears of an imminent wave of cases as it eases Covid curbs.

India's official Covid death toll of 418,480 is the world's third-highest after the United States' 609,000 fatalities and Brazil's 544,000.

But a US research group report, also released on Tuesday, estimated that the actual death toll could be up to 10 times higher, citing the number of extra people who died compared with pre-crisis figures.