As we learn more about vaccines against the virus that causes COVID-19, researchers have studied if mixing and matching vaccines can reduce the risk of side effects. For example, there are concerns about rare blood clotting problems in younger people who receive two doses of the Astra-Zeneca vaccine.

AstraZeneca first, then Pfizer or Moderna

In a new study in Sweden, researchers studied the effectiveness of different combinations of vaccines. They looked at people who had received the Astra-Zeneca vaccine as a first dose followed by either the Pfizer or the Moderna vaccine as the second dose. They compared this group to people who had received two doses of the Astra-Zeneca vaccine. This approach of mixing and matching vaccines was linked to 67% (Pfizer as second dose) to 79% (Moderna as second dose) effectiveness against COVID-19 infection. So mixing and matching better protects against symptomatic COVID-19 infection than two doses of the Astra-Zeneca vaccine (at 50%).

Risk of abnormal blood clots very rare

There was a very low incidence of side effects due to abnormal blood clotting using this mix and match approach.

Combining vaccines in this way appears to increase immunity against COVID-19. This includes the Delta variant (the main variant among the confirmed cases during the study period). These results are important for vaccination strategies in the battle against the pandemic.


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