• After a second mRNA vaccine jab, it is still unclear how many months one should wait before getting a booster shot
• Infectious disease experts believe that vaccine boosters may not be for everyone
• It should be used to reduce the risk of deaths or serious illnesses, they said
• It is justified only for the younger, healthy population if herd immunity is compromised, one said

SINGAPORE — The national vaccination programme to administer booster shots of Covid-19 vaccines got going on Wednesday (Sept 15) for seniors and those with weakened immune systems, but questions hang in the minds of some people. For instance, why are booster shots needed and are they really necessary at this point? 

There are no straightforward answers as scientists around the world voice differing views.

One camp is sceptical about whether it is the most efficient use of vaccine supplies, while the other side points to the waning immunity of messenger ribonucleic acid (mRNA) vaccines over time. In Singapore, mRNA vaccines by manufacturers Pfizer-BioNTech and Moderna have been the mainstay of the national immunisation exercise.

As the Government works towards living with Covid-19 as an endemic disease circulating among the population like influenza, TODAY explains what roles vaccine boosters would play in that strategy and why it might not be for everyone.

What are vaccine boosters?

A vaccine booster is any extra dose of a vaccine that is given to maintain or augment the protection provided by the original dose or doses. This is because vaccines may confer less protection over time. 

The extra dose brings immunity levels back up and may be needed months or several years after the initial series of vaccine doses are given. 

For instance, booster shots for the vaccine that prevents tetanus (a serious disease of the nervous system caused by a bacteria), diphtheria (a serious bacterial infection that affects the mucous membranes of the throat and nose) and whooping cough (a highly contagious respiratory disease caused by a bacteria), are administered once every 10 years, while influenza shots should be taken yearly.

How often are booster shots needed for Covid-19 vaccines?

It is unclear yet how often one should get booster shots against Covid-19. 

Studies have found differing answers between the Moderna and Pfizer-BioNTech vaccines as well.

Data from several recent studies suggested that Moderna’s vaccine protection lasts longer than a similar shot of Pfizer’s, which was said to wane within six months after injection.

One such research published late last month in the Journal of the American Medical Association found that Belgian healthcare workers who took the Moderna vaccine had more than double the number of antibodies two months after their second dose than those who got Pfizer’s.

A theory for the longer protection is because Moderna manufactured the vaccine with a higher dose of mRNA, which is the genetic code that teaches the immune system how to recognise the coronavirus’ spike protein and fend it off.

This mRNA component is three times higher than Pfizer’s.

Another could be the time between doses: Four weeks is recommended for Moderna’s, whereas it is three weeks for Pfizer’s.

However, Moderna on Wednesday released an analysis that showed higher rates of infection among people vaccinated roughly 13 months ago, compared with those vaccinated about eight months ago.

Between July and August, 88 cases of infection were detected among those who got the two shots more recently in the US, versus 162 cases among those who were vaccinated last year. The analysis is yet to be peer-reviewed.

The company also pointed to a trend towards a lower rate of severe cases among the more recently vaccinated, although the finding was not statistically significant.

On the same day, Pfizer released data showing how vaccine protection declined six to eight months after the second dose, with vaccine efficacy falling about 6 per cent every two months after the second dose.

The firm also referred to data from Israel showing that a third booster jab restored protection up to 95 per cent.

At around the same time, Public Health England, a government agency in the United Kingdom, published data suggesting that protection from vaccines against severe illness and death wanes beyond three months after the second dose. 

What are the suggested standards elsewhere?

At least 10 countries, including Canada, France and Germany, have jumped on the bandwagon to offer mRNA vaccine boosters, but there is still no consensus as to when is a good time to introduce these jabs.

Israel, the first country to administer them, gives the booster jab as long as five months have passed since the second dose.

The United States’ Centers for Disease Control and Prevention recommends at least eight months, the authorities in England advise at least six months, and the authorities in Hungary let people take a booster four months after their second dose.

Who is being offered booster shots in Singapore?

• People who are moderately to severely immunocompromised: This group, which includes transplant patients on medications that suppress the immune system, and cancer patients on active chemotherapy treatment, can get a third dose of the same mRNA vaccine two months after their second dose

• People aged 60 and above, as well as residents of aged-care facilities: They can get a booster dose of an approved mRNA vaccine six to nine months after their second dose

When making the recommendation earlier this month, the expert committee on Covid-19 vaccination under the Ministry of Health said that this would help ensure “high levels of protection” against the coronavirus and severe disease.

Seniors were included because they are also at risk of more severe disease in case of an infection, it said.

“Seniors may develop a lower immune response to the initial two doses of vaccines, and immunity has also been observed to decline over time,” the committee added.

From Tuesday, the day registration for these booster shots opened, about 140,000 seniors were being invited to fix an appointment, Health Minister Ong Ye Kung said.

In an update on Wednesday, he said that more than 12,000 seniors had fixed their appointments to get the booster vaccine shots within the first 24 hours of registration opening.

About 3,200 seniors, including those in nursing homes, started getting their booster shots on Wednesday.

Must one stick to the same vaccine brand for booster jabs?

It is likely that Singapore will allow people to take their booster shots from a brand different from their initial doses, if they wish.

This means that someone who took doses of the Pfizer-BioNTech vaccine may be able to take Moderna for their booster jab, and vice versa.

When asked about this during a press briefing by the national Covid-19 task force on Friday, Associate Professor Kenneth Mak, director of medical services at the Ministry of Health, said that the authorities were expecting the expert committee to make its recommendation on this “very soon”.

“Once that comes out, we will take that on board, and we may adjust our strategies accordingly,” he added.

He said that the committee had advised the Government that mRNA vaccines can be treated as equivalent to each other for the purpose of the country’s booster vaccination strategy. It has been studying if mixing vaccines provides a more optimal augmentation of the immune response, compared with using the same vaccine. 

Are Covid-19 vaccine booster shots really necessary?

Infectious disease and public health experts approached by TODAY said that there was no credible data backing the need for young and fairly healthy people to get booster shots if they took mRNA vaccines.

Dr Alex Cook from the National University of Singapore’s Saw Swee Hock School of Public Health said that it was justified for people with weak immune systems because they may not have benefited as much from the first shots and another dose now might confer further protection.

People in the older age groups do tend to have a faster waning of protection from vaccination as well, which is why they are also being given booster shots now, he added.

Beyond these two groups, Dr Cook wonders if it would be necessary for younger people to take the shots.

He believes that they need to take them only if it serves to raise or maintain herd immunity among the population. 

It is important to note that vaccines have differing protection against infection, symptoms, severe disease and transmission, he added.

“It doesn’t make sense to simply talk about falling protection without specifying protection against what.”

Pointing out that the mRNA vaccines are “reasonable at preventing infection, good at preventing illness, and very good at preventing severe illness”, he said: “This tiering is why it is misleading to say that the efficacy falls 6 per cent every two months.

“There’s a big difference between effectiveness against infection falling by 6 per cent and effectiveness against death falling by 6 per cent.”

For now, in the absence of more concrete answers to these questions, Dr Cook said that Singapore could watch places such as Israel and see how the roll-out of booster shots divert the epidemic trajectory.

Associate Professor Lim Poh Lian, director of the high-level isolation unit at the National Centre for Infectious Diseases, said that based on data, older adults and immunocompromised persons were at greater risk of severe Covid-19 and were more likely to have waning protection from the vaccine.  

“So vaccine boosters are necessary and recommended for these groups. We are closely monitoring data for boosters in other groups and will make a careful decision based on emerging data,” Assoc Prof Lim told TODAY in an emailed response on Sept 21. She is also a member of Singapore’s expert committee on Covid-19 vaccination.

On whether it might be too early for Singapore to give out booster shots, Assoc Prof Lim said that it was not, given that Covid-19 cases under intensive care had doubled and those needing oxygen help to breathe had tripled compared with a week ago. 

“Data has shown that immune protection against Covid-19 infection starts to wane by six months, whether the immunity is from virus infection or from vaccination,” she added.

“Older adults started getting vaccinated around February 2021, so many will be at least six months out from their Covid vaccinations, so the time is right to give out booster doses.”

Dr Leong Hoe Nam from Rophi Clinic at Mount Elizabeth Novena Specialist Centre is of the view that vaccine boosters should be used whenever it can reduce deaths or serious illnesses.

He finds the decision to give boosters to seniors aged 60 and above “a rather rough tool”.

Although he recognises that a booster will help a diabetic suffering from kidney failure, it is “unlikely to really benefit” a 69-year-old man with no history of chronic illnesses.

So, for those aged 60 and under, Dr Leong does not think that the benefit of preventing infection or severe illness with a booster shot is “worth it”.

“Should the vaccines be donated to other countries (that need them more)?

“The data for those healthy and under 60 years old is unclear. No deaths have occurred in this group yet. I cannot advocate for this group since there is no data.”

In the meantime, he warns against taking booster jabs too often because the body may treat the vaccine as "self", learn to tolerate it and then not respond to fight the virus.

“That would be the greatest disaster. The body’s immune system would then ‘welcome’ the real infection with no resistance,” he said. 

Similarly, Dr Paul Tambyah, president of the Asia Pacific Society of Clinical Microbiology and Infection, does not see booster shots as necessary for the entire population for now, because there are still many unknowns such as how long protection from a booster shot would last.

He pointed out that an Israeli study published in the New England Journal of Medicine looked only at the first 12 days after the third shot and it showed that the third dose was around 90 per cent effective against both severe disease and any confirmed infection. 

“I think that there are other options such as studying a longer interval between doses or a mix-and-match approach, and these were studied in the UK and South Korea,” he said. 

“We need to see the data before making broad decisions. For now, I think focusing on the immunocompromised is reasonable.”