Experts question move to study giving Covid-19 vaccine boosters to younger adults

A student getting her jab at the Bishan Community Club Vaccination Centre in June, 2021.
A student getting her jab at the Bishan Community Club Vaccination Centre in June, 2021.PHOTO: LIANHE ZAOBAO

SINGAPORE - Local infectious diseases experts have questioned the Government's plan to study giving Covid-19 vaccine boosters to younger adults to reduce infection rates, saying there is currently no evidence that this group of people need boosters.

They also reiterated that the purpose of the Covid-19 vaccine is to prevent severe illness rather than infection.

Their comments come after Finance Minister Lawrence Wong on Monday (Sept 6) said the authorities are studying the possibility of giving booster jabs to younger adults to help slow down local transmission rates.

This was one of several measures that the co-chairman of the multi-ministry task force handling Covid-19 had outlined in response to the latest surge in Covid-19 cases. Singapore recorded 328 local cases on Tuesday (Sept 7).

Professor Dale Fisher, a senior infectious diseases consultant at the National University Hospital, said Singapore's current stance should not be to return to a full prevention of Covid-19 cases.

"It is understandable that with a strategy to exit the pandemic slowly, it's nice to put the brakes on for a bit while we gauge the impact of high levels of community transmission on hospitalisations," he said.

However, while more vaccinations may help to slow transmission, the process takes months and it is "not nimble enough" for an immediate slowdown.

"The ultimate aim... is to prevent severe disease and death. Vaccination is excellent at doing this, and very few people for this purpose would benefit from a third jab," he added.

Professor Ooi Eng Eong from the Duke-NUS Medical School's Programme in Emerging Infectious Diseases said preventing infection would be "a bonus and not a necessity".

Data has shown that the Moderna vaccine has a 76 per cent efficacy rate in preventing infection from the Delta variant, while the efficacy rate is only 42 per cent for the Pfizer vaccine.

Professor David Allen, associate vice-president (health innovation and translation) 
at the National University of Singapore, cited a recent study from Israel, which has not yet been peer-reviewed, that found the Pfizer booster dose is able to decrease the risk of infection by 70 per cent to 84 per cent after 14 to 20 days.

However, he noted that more than 80 per cent of the patients in the study were above 60, and it is not known how long this boost in preventing infection will last.

Prof Ooi said that while vaccinated individuals who are asymptomatic can still transmit the virus, the likelihood of viral transmission is still a lot lower than those who are unvaccinated.

He pointed to a recent study that showed it is difficult to find infectious Sars-CoV-2 virus in the airway of vaccinated Covid-19 cases - meaning the immune response produced by vaccination is effective in killing the virus.

"This reduces the risk of severe Covid-19 greatly... With the highest vaccination rate in the world... Singapore can live with endemic but mostly mild Covid-19," said Prof Ooi.

While silent viral transmission could still occur with high vaccination rates, especially to those who are more vulnerable and at risk of severe Covid-19, Prof Ooi pointed out that there is now a whole arsenal of Covid-19 therapeutics, ranging from antibody treatment to antivirals.

"Early diagnosis of Covid-19 and early treatment have been shown to be effective in reducing the risk of severe disease. Unlike in 2020, Covid-19 is now a vaccine-preventable and treatable disease," he said.

Prof Allen reckons the Government's aim in giving boosters to younger adults could be to "maximise whatever benefit a third dose can provide in preventing transmission to others, (as well as) to prevent mild disease and the risk of longer-term effects of Covid-19 in those already vaccinated".

While he is in favour of boosting the protection of the immunocompromised and possibly those 60 and above, Prof Allen said he is not aware of any data that shows boosters for healthy, younger people have an overall benefit to the individual or society, compared with the regular two doses.

Mr Wong on Monday said those aged 60 and above who had received their second jab six to nine months ago will begin receiving invitations for booster shots in two weeks' time.

Prof Ooi also called for a closer look at whether there are any benefits to booster shots for younger adults.

He noted that the second dose of the mRNA vaccines produces more, albeit temporary, side effects such as pain and fatigue. There is insufficient evidence to determine if these side effects would be more severe following the booster, he added.

"There is also no evidence that booster shots in younger adults, given so soon after the completion of a two-dose series, provide any additional benefit. Despite the vaccine's usefulness, it should only be used when its benefit outweighs the risk," he said.

Instead, he said, it would likely be more effective for countries with high vaccination rates and spare vaccines to help inoculate the unvaccinated in other parts of the world, said Prof Ooi.

Doing so would reduce the global Covid-19 burden and facilitate countries, including Singapore, to open up their borders safely, he added.

The World Health Organisation has called on countries to halt giving booster doses until every country has vaccinated at least 10 per cent of its population.

Singapore has participated in global programmes and bilateral arrangements to make vaccines available to both its neighbours and the global community, noted Prof Allen. For example, the Republic has given 500,000 doses of the Pfizer vaccine to Australia, and these will later be returned.