Countries Weigh ‘Mix and Match’ COVID-19 Vaccines

(Reuters) – 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 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:


Officials said in May that people who were inoculated with AstraZeneca’s COVID-19 shot may be offered a second dose of another vaccine, a move based on supply concerns as well as the rise in incidence of rare blood clots linked to first doses produced by the Anglo-Swedish drugmaker.


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 12 the country was “formally considering” mixing COVID-19 vaccine doses developed with different technologies to boost their efficacy.


Finland’s Institute of Health and Welfare said on April 14 that recipients of a first dose of AstraZeneca’s vaccine who are aged under 65 may get a different shot for their second dose, as authorities warned about delays to the country’s rollout


France’s top health advisory body Haute Autorite de la Sante (HAS) recommended in April that people under 55 injected with a first shot of the AstraZeneca vaccine 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.


South Korea said on May 20 it would run a mix-and-match trial of COVID-19 vaccines, mixing AstraZeneca doses with those developed by Pfizer and other drugmakers.


Spanish Health Minister Carolina Darias said on May 19 the country would allow people under 60 years old, who have received a first dose of AstraZeneca’s vaccine, to receive a second dose of either the AstraZeneca shot or Pfizer’s vaccine.

A study (CombivacS) on mixing COVID-19 vaccines, run by Spain’s state-backed Carlos III Health Institute, found that giving a dose of Pfizer’s shot to people who have already received a first shot of AstraZeneca’s vaccine is safe and highly effective, preliminary results showed on May 18.


Sweden’s health agency said on April 20 that people under 65 years of age, who have had one shot of AstraZeneca’s vaccine, would be given a different vaccine for their second dose.

UNITED KINGDOM * 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 12 ( found that people who received Pfizer’s vaccine followed by a dose of AstraZeneca’s, or vice versa, were more likely to report mild or moderate common post-vaccination symptoms than if they received two of the same type.

Novavax said on May 21 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 United Kingdom.


In January, CNBC reported the U.S. 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 inoculations, and only for “exceptional situations”.

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