Seniors, immunocompromised people, antibody levels: What you should know about Singapore's Covid-19 booster shots

With some countries such as the US, Britain and France announcing plans to roll out Covid-19 booster shots as soon as this month, the kiasu Singaporean in you might have been wondering when it'd be your turn.

Well, if you're 60 and above, or immunocompromised, you could be receiving your third jab soon. 

Singapore will be rolling out its Covid-19 booster shots for seniors, residents of aged-care facilities, as well as those who are moderately to severely immunocompromised from this month, the Ministry of Health (MOH) said in a press release today (Sept 3).

Here's all you need to know about booster shots - what they are and when the rest of us might be likely to get them. 

What is a booster shot?

According to WebMD, a booster shot is an additional dose of a vaccine that will help to boost your immunity. This is needed for some vaccines as their effects can diminish over time.

Booster shots aren't anything new. In fact, you've probably already received a few, such as the hepatitis B, polio, and tetanus shots.

In the case of Covid-19, booster shots are important to "pre-empt a very sharp rise in breakthrough infections", Health Minister Ong Ye Kung said today.

Who should get the Covid-19 booster shot?

A third shot is recommended for those who were immunocompromised at the time of their first two Covid-19 vaccine doses.

This includes those on cancer treatment or immunosuppressive therapy, as well as transplant patients and end-stage kidney disease patients on dialysis.

"Because of their conditions, these persons react much less to vaccination, even after two doses, meaning they cannot produce as much antibodies or activate the necessary mechanisms to fight the virus," Ong had previously said on Aug 19. "Hence, a third dose of vaccine is necessary for them."

According to MOH, the immunocompromised should receive a third dose of the same mRNA vaccine two months after their second dose as part of their primary course of vaccination. 

Seniors are also another group recommended to receive booster shots as they are at risk of severe Covid-19 infections. Furthermore, as seniors were among the first groups vaccinated in Singapore, there is an "expected decline" of their immunity to the virus over time, MOH noted. 

They should receive a booster mRNA shot six to nine months after their first two doses. 

As for the rest of the population, we'd likely have to wait and see. According to Ong, the Expert Committee on Covid-19 Vaccination is still observing the results of vaccination and booster programmes in other countries.

When will it be rolled out in Singapore?

With the first batch of seniors completing their primary courses of vaccination around March this year, they should be set to receive booster shots sometime this month, MOH said. 

More details on the implementation of the booster shot programme are to be announced later, the ministry added. 

What if I got an inactivated virus vaccine like Sinovac? How long should I wait?

If you received an inactivated virus vaccine such as Sinovac or Sinopharm instead of the Pfizer-BioNTech/Comirnaty and Moderna mRNA vaccines used in Singapore's national vaccination programme, you might be due for a booster much sooner than the rest of the population.

The neutralising antibody levels of those with the Sinovac jabs appear to drop rapidly in six months, Associate Professor Hsu Li Yang, the vice-dean of Global Health and Programme Leader of Infectious Diseases at NUS Saw Swee Hock School of Public Health, told The Straits Times.

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Neutralising antibody levels can be an indicator — though not the only determinant — of the level of protection you're getting from your Covid-19 vaccine.

To increase your protection against the virus, booster shots — either of the same vaccine or an mRNA vaccine — are an option, said Assoc Prof Hsu.

Infectious diseases specialist Dr Leong Hoe Nam told the South China Morning Post that he has seen an increasing number of patients opting for the Pfizer-BioNTech/Comirnaty jab as their third dose after receiving the Sinovac jabs.

"They took the Sinovac shots, did the blood test and saw low antibody levels, then opted for Pfizer as the third dose," said Dr Leong.

According to Dr Leong, the normal antibody level for someone with two Pfizer-BioNTech/Comirnaty doses is between 1,300 and 2,000 international units per millilitre (IU/mL). However, some who took the Sinovac jab have around zero to 40 IU/mL while others have 200 to 300 IU/mL, Dr Leong added.

Will my booster shot be the same vaccine as my first two doses?

Mixing and matching vaccines is not unheard of — some countries such as South Korea, Vietnam and Cambodia are currently mixing doses of different vaccines as part of their inoculation programmes.

While most of us here in Singapore would have gotten the same vaccine for our first two doses, it's possible that we might be offered a different vaccine for our booster shot when our turn comes around, according to a report by The Straits Times.

Associate Professor Lim Poh Lian, a member of the Expert Committee on Covid-19 Vaccination, told the newspaper that there are pros and cons to getting a different vaccine for one's booster shot, and emphasised that it "may not be a one-size-fits-all recommendation".

kimberlylim@asiaone.com