The study, which involves about 600 participants 21 years or older, is expected to conclude in April 2023. PHOTO: ST FILE

SINGAPORE - Researchers in Singapore are studying the impact mixing different Covid-19 vaccines, including some not currently under the national vaccination programme, can have on the body's immune system.

The Pribivac study will compare the effect on individuals given three doses of the same vaccine, including the booster, against those given two doses of the same vaccine and a different one for their booster shot.

Details of the study were included on the United States National Library of Medicine's database of clinical studies - the largest biomedical library in the world.

The research is being sponsored by Tan Tock Seng Hospital in collaboration with the Agency for Science, Technology and Research, Duke-NUS Medical School, and KK Women's and Children's Hospital.

It is being led by Dr Barnaby Young of the National Centre for Infectious Diseases (NCID), who previously researched immune response in vaccinated individuals with Covid-19 and the impact of long Covid on patients.

Dr Young is also head of the Singapore Infectious Diseases Clinical Research Network.

The latest study involves about 600 participants 21 years or older, who had their second dose of Moderna's or Pfizer-BioNTech's vaccines at least six months before enrolling. Among other criteria, they must also not have previously been infected with Sars-CoV-2, which causes Covid-19, or Sars-CoV-1, which causes severe acute respiratory syndrome.

Participants will be divided into five groups.

Individuals in the first will receive a third dose of an mRNA vaccine they had previously taken, such as Moderna.

Those in the second group will be given an mRNA vaccine that differs from the one they previously took, which could mean a combination of vaccines by Moderna and Pfizer-BioNTech.

The remaining three groups will be given a booster shot of a non-mRNA vaccine.

While the US biomedical library did not specify what vaccines will be administered, the study's description stated: "This may include vaccines that have received full or interim authorisation from (the Health Sciences Authority), are available under the Special Access Route for vaccines that are part of the WHO (World Health Organisation) Emergency Use List, or are currently under clinical development."

The Straits Times understands that the study may include Covaxin, a vaccine developed in India that received WHO emergency-use listing late last year.

Covaxin is an inactivated virus vaccine, which uses a version of the virus that has been disabled to teach the body to fight Sars-CoV-2. It is not currently part of the national vaccination programme here.

China's Sinovac-CoronaVac, which also uses an inactivated virus, received interim authorisation for use here as a Covid-19 vaccine under the Pandemic Special Access Route in October last year.

The study is expected to conclude in April 2023.