Germany and France both say that mixing two types of coronavirus vaccine can be effective.
They are recommending that some who took a first dose of AstraZeneca get a different second shot.
Several countries are dialing back use of the AstraZeneca shot, fearing very rare but dangerous blood clots.
Countries are beginning to explore an usual way of using coronavirus vaccines, in order to avoid giving some groups a second dose of the AstraZeneca shot: mixing and matching with another type of vaccine.
Germany and France have both recommended that some groups take a different second dose, citing a link between the AstraZeneca shot and very rare blood clots. Canada and Norway are considering doing the same.
Germany recommended on April 1 that those under 55 receive an mRNA vaccine alternatives, such as the Pfizer and Moderna vaccines, as a follow up shot. France recommended the same on Friday for people under the age of 60.
The European Medicines Agency (EMA) announced on Wednesday that unusual blood clots with low platelets should be listed as a "very rare side effect" of the AstraZeneca vaccine.
The EMA says that the benefits of the vaccine outweigh its risks, and experts say that a definite link between the shot and the blood clots has not been established.
Canadian health authorities are examining the evidence for dose mixing, Dr. Howard Njoo, Canada's deputy chief public health officer, said on Thursday.
Separately from recommending dose-mixing, countries using the AstraZeneca vaccine are changing their recommendations to avoid giving it to younger populations.
Although it is not widely recommended yet for COVID-19 vaccines, mixing doses from different manufacturers is not unusual, one expert said.
"Mixing different manufacturers vaccines is called 'heterologous prime boost' and is something that has been looked into long before COVID," Prof. Paul Hunter, professor of medicine at the University of East Anglia, told Insider in an email.
It "often appears to give better protection," although it is not known whether this holds true in the case of COVID-19, Hunter said.
"Personally, I would not worry too much", he said, but "before recommending this on a wider scale I would prefer to await the results" of trials like the one Norway is watching.
The UK is not recommending dose-mixing, and says that people who have taken one shot of AstraZeneca's vaccine should go ahead and get the second.
Health experts told Insider that in spite of the unusual blood clots being listed as a very rare side effect of the vaccine, getting the shot is still safer than flying in a plane.
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