SINGAPORE - Those here who are eligible for a fourth dose of the Covid-19 vaccine should get it, as it has been shown to protect against severe illness, say experts.

This comes after a recent study from Israel found that a second booster shot of Pfizer-BioNTech's Covid-19 vaccine helped provide additional protection against infection with the Omicron variant, as well as severe illness among older adults.

But the study also found that the booster's effectiveness against infection wanes after four weeks and almost disappears after eight weeks.

However, protection against severe illness did not ebb in the six weeks following the extra dose.

On March 24, Singapore's multi-ministry task force tackling the coronavirus had accepted the recommendation by the Expert Committee on Covid-19 Vaccination (EC19V) that everyone aged 80 and above, those living in aged care facilities, and medically vulnerable people should receive a second booster dose from about five months after receiving their first booster dose.

The EC19V had also previously recommended that immunocompromised people receive three doses as part of their primary series and get a booster dose around five months after the third dose of their primary vaccination series.

Asked about the local health authorities' recommendations in light of the Israeli study, Professor Dale Fisher, chair of the World Health Organisation's Global Outbreak Alert and Response Network, highlighted that the study also showed a reduction in severe Covid-19 disease in people who had received a fourth dose, compared with those who had received only three doses, with the last dose at least four months ago.

He noted that in the study, which focused on adults aged 60 and above, those aged 80 and over received the most benefits, with the fourth dose significantly reducing the incidence of severe disease.

Prof Fisher, who is also a senior consultant at the National University Hospital's Division of Infectious Diseases, said those aged 80 and over, as well as the immunocompromised, "should definitely have the fourth dose".

Assistant Professor Hannah Clapham, from the National University of Singapore's (NUS) Saw Swee Hock School of Public Health, agreed.

"Vaccinations can offer protection against infection, symptoms, and severe disease and death. What we are consistently seeing is that even though protection against infection may be waning in the time since vaccination, protection against severe outcomes is waning less and holding up well over time," she said.

Prof Fisher, who is also a professor in the Department of Medicine at the Yong Loo Lin School of Medicine in NUS, said that vaccine efficacy waning is a "gradual continuous occurrence".

He explained: "It occurs at different rates for different people, and there is a huge difference between mild or asymptomatic infection and severe infection. Protection against very mild disease goes away first with a small number of people becoming reinfected even within some weeks after being fully vaccinated.

"As time goes by, more people will become less immune. Protection against severe disease lasts longer, meaning the immune system can't prevent infection but it kicks in after infection to stop progression."

Adding that the EC19V looks at evidence both from Singapore and around the world when deciding on booster shots, Prof Clapham said: "I think protection (offered) against severe disease is particularly important here. Therefore, yes, I would certainly recommend those who are eligible to get the second booster shot."

Noting that this was not the first study Israel had conducted on the efficacy of boosters, Prof Fisher added: "It is possible that further studies by Israel will show a fourth dose helps reduce severe disease incidence in those aged below 80, and that when that happens, there will be a recommendation for other age groups in Singapore to get a fourth dose. It is also likely that the timing of the fourth dose could vary for different age groups."

This article was first published in The Straits Times. Permission required for reproduction.