A study conducted by the Regional Medical Research Centre, based in the eastern city of Bhubaneswar in India, found a "significant" drop in COVID-fighting antibodies within four months of the first shot.

The Indian study of 614 fully vaccinated health workers in India, published in the Research Square pre-print platform but yet to be peer reviewed, is one of the first such done in the country involving its main two vaccines, Covishield, a licensed version of the AstraZeneca shot, and domestically developed Covaxin.

These findings could help the Indian government decide whether to provide booster doses as some Western countries have done.

The director of a state-run institute that did the study, Sanghamitra Pati, told Reuters: "After six months, we should be able to tell you more clearly whether and when a booster would be needed. And we would urge similar studies in different areas for pan-India data."

Waning antibodies do not necessarily mean that immunised people lose their ability to counter the disease, as the body's memory cells may still kick in to offer substantial protection, Pati added.

Researchers in the UK had said last month that protection offered by two doses of the Pfizer (NYSE:PFE)/BioNTech and the AstraZeneca COVID-19 vaccines begins to fade within six months.