The protection offered by the Oxford/AstraZeneca Covid vaccine declines after three months of receiving two doses, a new study has said.
The findings, which were drawn from datasets in two countries and published in The Lancet, suggest that booster programmes are needed in order to help maintain protection from severe disease in those vaccinated with the AstraZeneca jab, experts have said.
Researchers from Scotland and Brazil analysed data for two million people in Scotland and 42 million people in Brazil who had been vaccinated with the AstraZeneca vaccine.
In Scotland, when compared with two weeks after receiving a second dose, there was approximately a fivefold increase in the chance of being hospitalised or dying from Covid-19 nearly five months after being double vaccinated.
Experts said the decline in effectiveness begins to first appear at around three months, when the risk of hospitalisation and death is double that of two weeks after the second dose.
The risk then increases threefold just short of four months after the second vaccine dose, with similar numbers seen for Brazil.
Researchers compared data between Scotland and Brazil as they had a similar interval between doses - 12 weeks - and initial prioritisation of who was vaccinated - this being people at highest risk of severe disease and healthcare workers.
However, the dominant variant was different in each country during the study period – Delta in Scotland and Gamma in Brazil – which means the decline in effectiveness is likely because of vaccine waning and the impact of variants.
The study also estimated vaccine effectiveness at similar fortnightly intervals by comparing outcomes of people who have been vaccinated with those who are unvaccinated.
However, experts warned these figures should be treated with caution because it is becoming harder to compare unvaccinated people to vaccinated people with similar characteristics, particularly among older age groups where so many people are now vaccinated.
‘Our work highlights the importance of getting boosters’
Professor Aziz Sheikh, Director of the University of Edinburgh’s Usher Institute and EAVE II study lead, said: “Vaccines have been a key tool in fighting the pandemic, but waning in their effectiveness has 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.
“If eligible for a booster and you have not yet had one, I would highly recommend that you book one soon.”
Professor Vittal Katikireddi at the University of Glasgow said: “Our analyses of national datasets from both Scotland and Brazil suggest that there is considerable waning of effectiveness for the Oxford/AstraZeneca vaccine, with protection against severe Covid-19 falling over time.
“We studied two million people in Scotland and over 42 million people in Brazil who had received two doses of the Oxford-AstraZeneca vaccine. More than four months after receiving a second dose, the risk of experiencing either a Covid-19 hospitalisation or death was approximately five times greater than the period of maximum vaccine protection after accounting for changes in infection rates and a range of other factors.
“Our work highlights the importance of getting boosters, even if you’ve had two doses of the Oxford AstraZeneca vaccine, as soon as you are able to.”