he protection offered by the Oxford-AstraZeneca COVID-19 vaccine declines after three months of receiving two doses, a study says.

The findings—drawn from datasets in two countries—suggest that booster programs are needed to help maintain protection from severe disease in those vaccinated with Oxford-Astra Zeneca, experts say.

Researchers from Scotland and Brazil analyzed data for two million people in Scotland and 42 million people in Brazil who had been vaccinated with the Oxford-Astra Zeneca vaccine.

In Scotland, when compared with two weeks after receiving a second dose, there was approximately a fivefold increase in the chance of being hospitalized or dying from COVID-19 nearly five months after being double vaccinated.

The decline in effectiveness begins to first appear at around three months, when the risk of hospitalization and death is double that of two weeks after the second dose, experts say.

The risk increases threefold just short of four months after the second vaccine dose. Similar numbers were seen for Brazil.

Researchers were able to compare data between Scotland and Brazil as they had a similar interval between doses—12 weeks—and initial prioritization of who was vaccinated—people at highest risk of severe disease and healthcare workers.