(Reuters) - A growing number of countries are looking at switching to different COVID-19 vaccines for second doses or booster shots after supply delays and safety concerns have slowed their vaccination campaigns.
The World Health Organization said on July 12 the practice was a "dangerous trend" since there was little data available about the health impact, while Europe's drug regulator on July 14 made no definitive recommendations on switching doses.
The following are countries that are considering, or have decided to adopt, such a solution:
* Bhutan's Prime Minister Lotay Tshering said on June 24 he was comfortable about mixing-and-matching COVID-19 vaccine doses to immunize a population of about 700,000 people.
* The 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.
CHINA * Chinese researchers are running an early-stage, small-scale trial on the mixed inoculation of one dose of the vaccine from CanSino Biologics (CanSinoBIO), followed by one dose of the shot provided by a unit of Chongqing Zhifei Biological Products, clinical trial registration data showed in April.
* Researchers are also running a trial using one dose of CanSinoBIO's shot as a booster for those who have received one or two doses of an inactivated vaccine, clinical trial registration records in June showed.
* Indonesia is considering offering a booster shot to its healthcare workers immunized with Sinovac's vaccine, as thousands of them are testing positive for COVID-19.
* 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 approved on July 26 clinical trials combining an AstraZeneca shot with Sputnik V vaccine, according to the country's state drug register, after the approval process was suspended in May by the health ministry's ethical committee due to a lack of data.
* A study from South Korea found in July that a mixed vaccination of an AstraZeneca shot first and then a Pfizer one boosted neutralizing antibody levels six times more than two AstraZeneca doses.
In June, a British study showed that an AstraZeneca shot followed by a Pfizer one produced the best T-cell responses, and a higher antibody response than a Pfizer shot followed by an AstraZeneca one.
* Thailand said on July 12 it would use AstraZeneca's shot as a second dose for people first inoculated with Sinovac's vaccine, in an attempt to increase protection.
The move is the first publicly announced mix-and-match of a Chinese vaccine and a Western-developed shot.
* Vietnam said on July 13 it would offer the mRNA vaccine jointly developed by Pfizer and BioNTech as a second dose option for those who received the AstraZeneca vaccine as their first dose.
(Reporting by Federico Maccioni; Editing by Gareth Jones, Nick Macfie, Barbara Lewis)