People who had received a first dose of the Oxford-AstraZeneca COVID-19 vaccine and received an mRNA shot for their second dose had a lower risk of infection compared to people who had received both doses of only the Oxford-AstraZeneca vaccine, according to a study.

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The study, published in the journal The Lancet Regional Health - Europe, points out the high effectiveness of mix-and-match COVID-19 vaccines.

A team from Umea University, Sweden conducted a study of about 700,000 individuals and showed a 67 per cent lower risk of infection for people who received the combination of Oxford-AstraZeneca plus Pfizer-BioNTech, and a 79 per cent lower risk for Oxford/AstraZeneca and Moderna, both compared to unvaccinated individuals.

For people having received two doses of the Oxford-AstraZeneca vaccine, the risk reduction was 50 per cent. These risk estimates were observed after accounting for differences regarding date of vaccination, age of the participants, socioeconomic status, and other risk factors for COVID-19.

"Having received any of the approved vaccines is better compared to no vaccine, and two doses are better than one," said Peter Nordstrom, Professor of geriatric medicine at Umea University. "However, our study shows a greater risk reduction for people who received an mRNA vaccine after having received the first dose of a vector-based vaccine, as compared to people having received the vector-based vaccine for both doses," he added.

Importantly, the estimates of effectiveness apply to infection with the Delta variant, which was dominating the confirmed cases during the follow-up period. There was a very low incidence of adverse thromboembolic events for all vaccine schedules.

Previous research has demonstrated that mix-and-match vaccine schedules generate a robust immune response. However, it has been unclear to which extent these schedules may reduce the risk of clinical infection.

"The findings of this study suggest that the use of heterologous Oxford-AstraZeneca and mRNA prime-boost vaccination is an effective alternative to increase population immunity against COVID-19, including against the Delta variant which dominated the confirmed cases during the study period," the researchers said.

"These findings could have important implications for vaccination strategies and logistics, and consequently in the battle against the COVID-19 pandemic," they added.