People who have been double-vaccinated against Covid-19 with different vaccines enjoy a lower risk of infection, a study in Sweden has shown.
Protection was higher for people receiving an Oxford-AstraZeneca injection with one Moderna dose, or one Pfizer-BioNTech dose, than for people given two AstraZeneca vaccinations.
Researchers at Umea University in Sweden carried out the research after the use of AstraZeneca’s vector-based vaccine was stopped for people under 65.
“Having received any of the approved vaccines is better compared to no vaccine, and two doses are better than one,” says Peter Nordström, professor of geriatric medicine at Umea.
“However, our study shows a greater risk reduction for people who received an mRNA vaccine after having received a first dose of a vector-based, as compared to people having received the vector-based vaccine for both doses.”
The study showed a 67 per cent lower risk of infection for the combination of Oxford-AstraZeneca and Pfizer-BioNTech, and a 79 per cent lower risk for Oxford/AstraZeneca and Moderna, both compared with unvaccinated people.
For people having received two doses of the Oxford-AstraZeneca vaccine, the risk reduction was 50 per cent.
The effectiveness estimates also applied to infection with the Delta variant, which was dominating Sweden when the study was carried out.
“The results of the study may have implications for vaccination strategies in different countries,” says Marcel Ballin, doctoral student in geriatric medicine and co-author of the study.
There was also very low incidence of adverse thromboembolic events for all vaccine schedules, or for cases severe enough to need inpatient hospital care.
Previous research has demonstrated that mix-and-match vaccine schedules generate a robust immune response against Covid-19 but the extent of the reduction in risk of clinical infection has been unclear.
The study is based on nationwide registry data from the Public Health Agency of Sweden, the National Board of Health and Welfare, and Statistics Sweden.
In the main analysis, about 700,000 people were included. The risk estimates were calculated after accounting for differences in vaccination dates, age of the participants, socioeconomic status, and other risk factors for Covid-19.
In the US, Merck has applied for emergency use authorisation of its oral anti-Covid drug, a major step towards finding a simple pill to treat the disease.
In the UK, infections in unvaccinated children aged between 12 and 15 have risen considerably, with one in 12 now suffering from coronavirus, the Office for National Statistics reported last week.
Across Europe, life expectancy fell by more than a year in some areas during the first year of the pandemic, with especially sharp drops in England, Spain, Italy and Poland.
