By Roxanne Khamsi
Ms. Khamsi is a science journalist covering Covid-19.
At the start of the Covid-19 pandemic, it was unclear whether researchers would be able to create a single working vaccine, which makes it all the more surprising that the latest immunization dilemma arises from having multiple vaccine options.
Because of unpredictable supply and some concerns about an exceedingly rare but serious clotting risk from the AstraZeneca vaccine, public health officials in some parts of the world that have relied heavily on that shot have recently issued new guidance on mixing and matching different Covid-19 vaccines.
Recently, for example, Canada’s National Advisory Committee on Immunization updated its guidance to say that people who received the AstraZeneca vaccine as their first dose can receive that same vaccine as their second dose or get a follow-up shot of Pfizer-BioNTech or Moderna instead. The committee also said that it was possible to receive the Pfizer-BioNTech and Moderna vaccines interchangeably as first and second doses. Countries ranging from France to Finland to China to Bahrain have also outlined possible scenarios for combining different vaccines. Even the Centers for Disease Control and Prevention has interim guidance saying this is acceptable in “exceptional situations,” such as if the same vaccine is not available.
While this guidance may seem confusing, especially when the initial vaccine guidance told people to get the same shot for both doses, it does provide an opportunity to understand the safety of using mismatched vaccines, and to measure whether mismatched vaccines offer any advantage.
One recent small and not yet peer-reviewed study of 26 people who received the AstraZeneca shot followed by one from Pfizer-BioNTech suggested, based on blood tests, that those with mismatched vaccines had at least as strong an immune response as people who got both Pfizer shots. The National Institutes of Health recently began a clinical trial that will examine the effects of different combinations of Covid-19 vaccines.
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