The protection, including against severe disease, after two shots of the Oxford-AstraZeneca coronavirus vaccine starts to wane three months after the second dose, a study published on Monday in medical journal The Lancet found.

Pune-based Serum Institute of India has partnered with AstraZeneca and Oxford University to produce the vaccine in the country under the brand name Covishield.

“We found waning vaccine protection of ChAdOx1 nCoV-19 [scientific name of the vaccine] against Covid-19 hospital admissions and deaths in both Scotland and Brazil, this becoming evident within three months of the second vaccine dose,” the researchers wrote. “Consideration needs to be given to providing booster vaccine doses for people who have received ChAdOx1 nCoV-19.”

The study cited national analyses of the data received from Scotland and Brazil. It found that “risks of severe Covid-19 outcomes [defined as hospital admission or death due to Covid-19] increased with time since receiving a second ChAdOx1 nCoV-19 vaccine dose”.

The study was conducted when the Delta variant was prevalent in Scotland from May 19, 2021, and in Brazil when the Gamma strain was dominant from January 18, 2021 until October 25, 2021.

“Quantifying the exact magnitude of waning is challenging, and vaccine effectiveness estimates should be considered with caution given the difficulty of estimating risk among unvaccinated people,” the study said. “However, our findings consistently show substantial waning in both countries.”

Medical experts had been hopeful that two doses of a Covid-19 vaccine would protect against severe disease as the number of antibodies does not dictate immunity in an individual.

However, the recent study has raised concerns about vaccine efficacy without booster shots. Retired consultant in communicable disease control Peter English told the Financial Times that the paper casts doubts on the assumption that protection against sever disease would continue for many years.

“It documents a gradual decrease in prevention against severe disease over time, with severe disease becoming about five times more likely, compared to shortly after vaccination, after just a few months,” English told the newspaper.

Penny Ward, a visiting professor at King’s College in London, told the Financial Times that booster doses were needed for higher levels of protection. But she added that there was still sustained effectiveness of the Covid-19 vaccines against deaths and hospitalisations.

University of Edinburgh Director Professor Aziz Sheikh said that the waning effectiveness of the vaccines had been a concern for a while. “By identifying when waning first starts to occur in the Oxford-AstraZeneca vaccine, it should be possible for governments to design booster programmes that can ensure maximum protection is maintained,” he told IANS.

Booster shots are jabs given to ramp up decaying antibodies against the coronavirus. Almost 40 countries, mostly the rich and developed ones, are allowing citizens to get inoculated with a third vaccine dose.

India, which is yet to vaccinate its entire population and those under 18 years old with two shots of a Covid-19 vaccine, has not made any preparations for booster doses yet. But calls for rolling out additional vaccine doses, at least for vulnerable groups, have grown in the light of the emergence of the Omicron variant of Covid-19.

The Indian Council of Medical Research had on December 12 said that the rollout of booster shots can wait as it is important to first inoculate eligible adults with the first two doses of the Covid-19 vaccine.

India’s genome sequencing body, the Indian SARS-CoV-2 Genomics Consortium, had also said on December 4 that more scientific experiments were needed to assess the impact of booster doses of Covid-19 vaccines. However, a week before that it had suggested that booster shots be considered for people above 40 because of the Omicron variant.