What Mixing Covid Vaccines Could Mean for the Pandemic

Most of the Covid-19 shots that have proved effective have been two-dose vaccines: People get two portions of the same formulation injected into their arm, weeks or months apart. Months into the rollout of the vaccines, a number of countries wound up administering a different second shot in some cases. Soon, some people were actively seeking out this method, citing research that says mixing vaccine types can provide an advantage. With vaccine shortages holding up a return to normalcy for much of the Covid-plagued world, mixing shots could become a strategy to help end the pandemic.

It mostly happened unintentionally. In March, concerns about rare blood clots associated with AstraZeneca Plc’s vaccine triggered several nations, mainly in Europe, to pause use of the shot. Some European countries soon said health-care professionals could administer a different second dose to those in certain age groups who’d had a first AstraZeneca shot, even though this combination hadn’t been tested in clinical trials. German Chancellor Angela Merkel was among those affected by this policy, receiving a Moderna Inc. vaccine after an Astra shot. Other countries, such as Canada, have cited restricted supplies of the Astra vaccine as a reason for mixing shots. This could become more common in poorer countries struggling to secure a steady pipeline of doses. Finally, some people -- including pharmaceutical executives -- have purposefully switched up their own shots based on evidence that mixing vaccine types can produce stronger immunity.