SINGAPORE, Feb 12 — Getting boosted with a different brand of messenger ribonucleic acid (mRNA) vaccine has been linked to lower Covid-19 infection rates than if one received the same jab as their first two doses, a Singapore study of more than 700,000 people has found.

The study, published in the peer-reviewed Journal of the American Medical Association yesterday (February 11), also found that for those who got two doses of the Pfizer-BioNTech shot in particular, getting a Moderna booster jab meant a lower likelihood of infection.

Among persons who received the Pfizer-BioNTech shot for their primary series of jabs, the incidence of confirmed infections is 227.9 per million person-days if they had taken a Pfizer-BioNTech booster, and 147.9 per million person-days for those who got Moderna as their third dose.

In epidemiology, the number of incidents per million person-days is a way for researchers to measure risk while taking into account time and sample size. A smaller number indicates a lower risk of infection.

Still, one finding was clear: Getting any brand of an mRNA vaccine as a booster shot would work better at reducing infections and severe disease than not receiving a booster jab at all.

Yet, despite the large number of samples studied, researchers could not conclude whether getting two doses of the Moderna shot would lead to more or fewer instances of severe infections if the booster shot was from Pfizer-BioNTech.            

Three doses of Pfizer-BioNTech’s vaccine give 73 per cent more protection than two doses of Pfizer-BioNTech

•            Pfizer-Pfizer-Moderna gives 82 per cent more protection than two doses of Pfizer

•            Three doses of Moderna gives 80 per cent more protection than two doses of Moderna

•            Moderna-Moderna-Pfizer gives 86 per cent more protection than two doses of Moderna

The study looked at data from September to October 2021, during the emergence of the Omicron variant in other parts of the world. Singapore recorded its first cases of the highly infectious coronavirus strain in early December last year. 

Seven Singapore researchers, including Professor Wang Lin-Fa from Duke-NUS Medical School and the director of the Ministry of Health’s (MOH) policy, research and evaluation division Kelvin Tan, authored the study.

Official healthcare data was used in the study, which was exempt from ethical review and informed consent by MOH, as it was carried out under the Infectious Diseases Act, based on a research letter published in the journal.

Asked how significant the study was for policymaking, one of the authors, Associate Professor Alex Cook, told TODAY that the study was conducted with the "express purpose” of informing policy decisions. He noted that Associate Professor Benjamin Ong, who chairs MOH’s expert committee on Covid-19 vaccination, was one of the authors.

About 62 per cent of Singapore’s population have received their vaccine booster shots. Those aged 18 and older can choose either brand of the mRNA vaccines as their booster, while those under 18 may pick only the Pfizer-BioNTech shot.Assoc Prof Cook, vice-dean of research at the Saw Swee Hock School of Public Health, said that the study was done at a time when Singapore was still early in its national booster roll-out, which began in September 2021. 

"At (that) point, it was unclear whether mixing brands or keeping to the same one would give better protection,” said Dr Cook. 

In November last year, Health Minister Ong Ye Kung referred to an MOH study, which found that two doses of the Pfizer-BioNTech vaccine followed by a Moderna booster shot had a "slight edge” in reducing the risk of infection, when compared with taking three doses of Pfizer-BioNTech.

With the study now peer-reviewed, Dr Cook said that the findings published yesterday showed a few things. 

"First, that receipt of a booster is protective against both diagnosed infections and severe disease, and this holds regardless of which of the mRNA vaccines you had for the primary series and for the booster,” he said.

Second, the improvement in protection is "not very different” between the brands of booster jabs used. That said, mixing a Moderna booster after two doses of Pfizer-BioNTech had led to a lower risk of confirmed infection, added Dr Cook.

Dr David Lye from the National Centre for Infectious Diseases (NCID), a co-author of the study, summed it up on Facebook: "Moderna booster protected from infection better than Pfizer booster for Pfizer-vaccinated. 

"With Omicron surge, get boosted. Healthcare workers in primary care, emergency and hospitals will thank you,” wrote Dr Lye, who is also a member of the expert committee on Covid-19 vaccination.

Nevertheless, Dr Cook said that there were limitations to the study, given the relatively low numbers of people who got Moderna’s vaccine for their first two jabs when the study was carried out.

This also meant that there was a low number of severe Covid-19 outcomes for this group. 

Eight events of serious infections were detected among the group that had not received their booster after two shots of the Moderna jab. There were no severe infections found for those who got a Pfizer-BioNTech or Moderna booster after two shots of Moderna.

Hence, the researchers could not conclude which booster would be better for those who had two jabs of the Moderna vaccine. 

Other limitations of the study include "potential confounding” from unobservable individual characteristics that may influence each person’s choice of booster jab, a lack of data from younger age groups, and a short follow-up period.

The mixing of vaccine boosters is not unique to Covid-19. It has been done in the past for hepatitis B and the human immunodeficiency virus, which can lead to Aids (acquired immunodeficiency syndrome). 

For Sars-CoV-2, the virus that causes Covid-19, scientists around the world have also been carrying out research on vaccine mixing for jabs available in their countries. 

Some studies entail taking blood samples from research participants at regular intervals to test for their antibody counts after they receive their booster shot. 

Earlier this year, Singapore researchers embarked on a study to look at the impact of mixing Covid-19 boosters on the immune system. These include vaccine brands that are not in Singapore’s national vaccination programme.

Called the Pribivac study, it is led by NCID’s Dr Barnaby Young, and researchers are looking to recruit 600 participants aged 21 and above who had their second dose of an mRNA vaccine at least six months before but have not received their booster shot.

. — TODAY