The protection afforded by two doses of the Oxford/AstraZeneca coronavirus shots, including against severe disease, begins to wane three months after the second shot, according to a new study that highlights the importance of booster programmes.

Researchers analysed data for 2m people in Scotland and 42m people in Brazil who had received two shots of the vaccine. Their findings were published in a new peer-reviewed paper in The Lancet late on Monday. The Omicron variant was not circulating during the study.

“We found waning vaccine protection 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,” they wrote. “Consideration needs to be given to providing booster vaccine doses for people who have received [the vaccine].”

Researchers were able to compare the Scottish and Brazilian data because a similar interval of three months was used between doses, and because those at highest risk of severe disease and healthcare workers had been prioritised for vaccination in both places.

During the study, viral variants — Delta in Scotland, Gamma in Brazil — were circulating which, with vaccine protection waning, “likely” contributed to the decline in effectiveness.

The UK, where many received the AstraZeneca vaccine earlier this year, has moved to offer all adults an mRNA booster shot to counter the rapidly spreading Omicron variant.

Other wealthy countries have ramped up their booster programmes to limit the spread of the strain. Early, preliminary evidence suggests that a two-shot course of widely used vaccines generates a smaller amount of antibodies against Omicron than for previous variants, and that a third mRNA shot can increase their level. However, it is not yet clear how this translates to effectiveness.

The head of the European drugs regulator has said that it would take some time to reach a consensus on whether Omicron-targeted shots would become necessary, the Financial Times reported on Monday.

Scientists have been hopeful that two shots will still continue to protect against severe disease, because immunity is not just a function of the number of antibodies. However, some experts who were not involved in the study expressed concern that this could change that notion.

Peter English, a retired consultant in communicable disease control, said the prevailing assumption so far had been that protection against serious illness and more severe outcomes, for example by T-cells, would persist, “perhaps for many years”.

“This paper casts doubt on these assumptions,” said English. “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.”

“This is very concerning,” he said. “In the short term, it emphasises the need for an extra dose, and in the medium to longer term it raises questions about how effectively the current generation of vaccines can prevent future illness.”

One other external expert, Penny Ward, a visiting professor in pharmaceutical medicine at King’s College London, said there was still “sustained vaccine effectiveness” in preventing hospitalisation and death from Covid of at least 50 per cent across the follow-up period. But she agreed that a booster shot was required to confer high levels of protection.

Aziz Sheikh, an author on the study and head of Edinburgh university’s Usher Institute, said that by identifying when waning first started to occur in those who had received the AstraZeneca vaccine, “it should be possible for governments to design booster programmes that can ensure maximum protection is maintained”.

“If eligible for a booster and you have not had yet had one, I would highly recommend that you book one soon,” he said.

Pascal Soriot, AstraZeneca’s chief executive, had last month suggested that use of the vaccine could help stave off severe illness in older people for longer. He said, however, that there was no proof and more data was needed.

Oxford and AstraZeneca did not respond to requests for comment.