Factbox - Countries weigh 'mix and match' COVID-19 vaccines

A growing number of countries are looking at switching to different COVID-19 vaccines for second doses or booster shots 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. A mixed schedule, where a shot of Pfizer's vaccine is given after an AstraZeneca shot, produced more antibodies than two AstraZeneca shots, an Oxford study said on June 28.

The following are countries that are weighing, or have decided to adopt, such a solution:

BAHRAIN

* Bahrain said on June 4 that eligible candidates could receive a booster shot of the Pfizer/BioNTech or the Sinopharm vaccine, regardless of which shot they had initially taken.

CANADA

* The country's National Advisory Committee on Immunization said on June 17 that the provinces should offer recipients of a first dose of AstraZeneca's vaccine a different shot for their second dose, CBC News reported.

The committee had said earlier this month that people first injected with an AstraZeneca shot could choose to receive a different vaccine for their second dose.

ITALY

* Italy's medicine agency AIFA said on June 14 that people under the age of 60 who were inoculated with a first dose of the AstraZeneca shot can receive a different second shot.

RUSSIA

* Russia may start trials on a COVID-19 vaccine combining its Sputnik V vaccine and various Chinese shots in Arab countries, the Interfax news agency quoted Russia's RDIF sovereign wealth fund as saying on June 4.

The RDIF also said that no negative side-effects were found during clinical trials combining COVID-19 vaccine using the AstraZeneca and Sputnik V shots, Interfax reported.

SOUTH KOREA

* South Korea said on June 18 that some 760,000 people who were inoculated with a first dose of AstraZeneca's will receive Pfizer's jab as a second shot due to shipment delays by global vaccine sharing scheme COVAX.

SPAIN

* Health Minister Carolina Darias said on May 19 Spain would allow people under 60 who received an AstraZeneca shot first to get a second dose of either the AstraZeneca or Pfizer vaccine, after preliminary results of a study by the state-backed Carlos III Health Institute.

UNITED ARAB EMIRATES

* The United Arab Emirates have made the Pfizer/BioNTech coronavirus vaccine available as a booster shot to those initially immunised with a vaccine developed by the China National Pharmaceutical Group (Sinopharm).

A representative of Mubadala Health, part of the state fund, said a different vaccine could be provided as a booster shot but this was at the recipient's discretion and health professionals did not make recommendations.

UNITED KINGDOM

* Novavax said on May 21 it would take part in a mix-and-match vaccine trial starting in June in the United Kingdom to test the use of an additional vaccine dose from a different producer as a booster.

UNITED STATES

* The U.S. National Institutes of Health (NIH) said on June 1 it had started a clinical trial on fully vaccinated adults to evaluate the safety and immunogenicity of a booster shot of a different vaccine.