A growing number of countries are looking at switching to different Covid vaccines for second doses amid supply delays and safety concerns that have slowed their vaccination campaigns.
Several medical studies to test the efficacy of switching Covid vaccines are under way.
The following are countries that are weighing, or have decided to adopt, such a solution:
Canada will recommend to mix and match a first dose of the AstraZeneca vaccine with a second shot of either Pfizer or Moderna vaccines, CBC News reported on June 1st.
The country's National Advisory Committee on Immunization will also advise that recipients of a first dose of Moderna or Pfizer can get either of the two as a second shot.
Chinese researchers in April were testing the mixing of Covid-19 vaccine doses developed by CanSino Biologics and a unit of Chongqing Zhifei Biological Products, according to clinical trial registration data.
China's top disease control official said on April 12th the country was “formally considering” mixing vaccine doses developed with different technologies to boost their efficacy.
Finland's Institute of Health and Welfare said on April 14th recipients of a first dose of AstraZeneca's vaccine who were younger than 65 might get a different shot for their second dose.
France's top health advisory body has recommended in April that people under 55 injected with AstraZeneca first, should receive a second dose with a so-called messenger RNA vaccine, although dose-mixing has not yet been evaluated in trials.
Norway said on April 23rd it would offer those who have received a dose of the AstraZeneca vaccine an injection with an mRNA vaccine as their second dose.
Russia put on hold the approval in the country of clinical trials combining AstraZeneca and Sputnik V vaccines, after the health ministry's ethical committee requested more data, AstraZeneca official told Reuters on May 28th.
South Korea said on May 20th it would run a mix-and-match trial, mixing AstraZeneca doses with those developed by Pfizer and other drugmakers.
Spanish health minister Carolina Darias said on May 19th the country would allow those under 60, who got an AstraZeneca shot first, to receive a second dose of either AstraZeneca or Pfizer's vaccine.
The decision followed preliminary results of a study by state-backed Carlos III Health Institute, which found that following up an AstraZeneca vaccine with a Pfizer shot was safe and highly effective.
Sweden's health agency said on April 20th that people under 65, who have had one shot of AstraZeneca's vaccine, would be given a different vaccine for their second dose.
Britain said in January it would allow people to be given a different vaccine for a second dose on extremely rare occasions, for example if the first vaccine was out of stock.
The first findings of an Oxford University-led study released on May 12th found that people who received Pfizer's vaccine followed by a dose of AstraZeneca, or vice versa, were more likely to report mild or moderate common post-vaccination symptoms than if they received two doses of the same type.
Novavax said on May 21st it would take part in a mix-and-match Covid-19 vaccine trial to test the use of an additional vaccine dose from a different producer as a booster. The trial will start in June in the UK.
In January, CNBC reported the US Centers for Disease Control and Prevention (CDC) had updated its guidance, allowing a mix of Pfizer/BioNTech's and Moderna's shots with a gap of at least 28 days between the two shots, and in “exceptional situations”.