If you were looking for yet another reason on why to get a COVID-19 booster, here’s one. The protection given from the Oxford-AstraZeneca vaccine starts declining after three months of getting the second dose, according to a new study. The researchers analyzed data from 42 million vaccinated people in Brazil and two million in Scotland, finding that a quick booster is important. 

Three months after the second AstraZeneca dose, the risk of hospitalization and death is double that of two weeks after the second dose, the researchers found. The risk increases threefold four months after the second dose and fivefold after five months. The study showed similar numbers in Brazil and Scotland. 

Both countries had a similar 12-week interval between the two doses, and both prioritized health workers and people at risk of severe disease. However, the two countries had a different dominant variant, which suggests that the decline in effectiveness was likely due to vaccine waning and not due to variations in one variant or the other. 

“By drawing on findings from data sets in two countries with differing dominant COVID-19 variants, the researchers have been able to disentangle vaccine waning from the effects of changes in variants – strengthening the evidence for the ongoing booster programme,” Andrew Morris, Director of Health Data Research UK, said in a statement. Still, the figures should be treated with caution, the researchers argued, as it’s difficult to compare unvaccinated to vaccinated people with similar characteristics, especially among older age groups with more people vaccinated.

The growing need for boosters

Professor Aziz Sheikh, Director of the University of Edinburgh’s Usher Institute and study lead, said that thanks to the findings, governments will now be able to design booster programs to ensure maximum protection remains. “If eligible for a booster and you have not had yet had one, I would highly recommend that you book one soon,” he added. 

The call for booster COVID-19 shots has been intensified amid the expansion of the new Omicron variant, first identified in South Africa in late November. Researchers and doctors are now trying to grasp the risk that the new variant represents, with initial data suggesting that it is spreading faster than the previous Delta variant.

In fact, Omicron seems to be displacing Delta, as it’s much more contagious. The variant is already present in 89 countries, according to the World Health Organization (WHO), with cases doubling every 1.5 to 3 days where there’s community spread.

Both Pfizer/BioNTech and Moderna already said that two doses of their COVID-19 vaccines don’t provide enough protection against Omicron, but three doses do. So if you need another reason to get a booster as quickly as possible, there it is.

The study behind the AstraZeneca vaccine was published in The Lancet.