A growing number of countries are looking at switching to different Covid-19 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-19 vaccines are under way.
The following are countries that are weighing, or have decided to adopt, such a solution:
* The country's National Advisory Committee on Immunization said on June 1 that recipients of a first dose of AstraZeneca's vaccine can choose to receive a different vaccine for their second dose. It added that vaccines from Pfizer and Moderna could be used interchangeably.
* Chinese researchers in April were testing the mixing of vaccine doses developed by CanSino Biologics and a unit of Chongqing Zhifei Biological Products , according to clinical trial registration data.
* Finland's Institute of Health and Welfare said on April 14 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 23 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 28.
* South Korea said on May 20 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 19 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, after preliminary results of a study by state-backed Carlos III Health Institute.
* Sweden's health agency said on April 20 that people under 65, who have had one shot of AstraZeneca's vaccine, would be given a different vaccine for their second dose.
* Novavax said on May 21 it would take part in a mix-and-match 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 United Kingdom
* The first findings of an Oxford University-led study released on May 12 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. https://bit.ly/3fraZz9
The US National Institutes of Health (NIH) said on June 1 that it started a clinical trial on fully vaccinated adults to evaluate the safety and immunogenicity of a booster shot of a different vaccine.