6.OCT.2021 7 MIN READ | 7 MIN READ

In Singapore, our vaccine booster programme has been rolled out, starting with seniors above 50 years old and individuals who are moderately to severely immunocompormised.

What is a booster shot?

A COVID-19 booster shot refers to another dose of a COVID-19 vaccine that is given to individuals whose protection against COVID-19 has decreased over time after completing the two-dose vaccination regime.

Booster shots are not a new phenomenon. They are commonly used for viral infections such as the flu, as well as other diseases such as Hepatitis A. Most vaccine boosters are identical to the previous doses, but some are modified to enhance their efficacy.

Why do we need COVID-19 booster shots?

Why do I need a COVID-19 booster shot?
Some may be wondering if the introduction of booster shots means that the current COVID-19 vaccines are not effective enough. This is not the case. While the COVID-19 vaccines effectively protect against COVID-19 and its variants, the level of protection can decrease over time. Receiving a booster shot can increase an individual’s level of immunity and protection from COVID-19.

Israeli health officials have said that the effectiveness of the second dose of the Pfizer-BioNTech vaccine decreased 6 months after it was given, necessitating the administration of a booster dose to provide increased protection. Similarly, a Chinese study that has yet to be peer-reviewed reported that antibodies from Sinovac’s CoronaVac COVID-19 vaccine declined below a key threshold from around 6 months after the second dose was administered for most recipients. However, a third dose of CoronaVac administered 6 or more months after the second dose resulted in a significant increase in antibodies, effectively strengthening immune protection against COVID-19.

Besides the general population, individuals who are severely immunocompromised, such as cancer patients, transplant patients and patients on immunosuppressive therapy are particularly in need of a COVID-19 booster shot. This is because such individuals do not have as strong an immune response to the vaccinations as the typical population does. A booster shot would help to increase their immune response to an adequate level.

Nevertheless, the data shows that while protection against COVID-19 infection wanes after 6 months, immune protection against severe disease, hospitalisation and death remains high, even after 6 months, for both COVID-19 and its more contagious variants. A recent study in The Lancet which was conducted by scientists, including some from the World Health Organisation (WHO), concluded that the immune response mounted by current vaccines are sufficient to protect against the current variants. Furthermore, the body’s immune system has other defences besides antibodies that continue to protect the individual from being seriously ill, even after antibody levels have dropped. This has led these same scientists to argue that vaccine booster shots are not needed at this time for the general public. Furthermore, the duration of protection given by these booster shots remains to be seen.

When will booster shots for COVID-19 be available?

Some countries have already started to administer COVID-19 booster shots to their population. Israel is the first country to administer booster shots for its population in end-July, and started giving a third dose of the Pfizer-BioNTech vaccine to adults above 60. It has recently expanded the eligibility to individuals above 12 years old. Around 2.6 million people out of its population of over 9 million have received their third dose of the Pfizer-BioNTech vaccine.

The US will also be offering Pfizer-BioNTech booster doses to its population this month. Other countries such as Indonesia and Thailand have already offered Moderna and Pfizer-BioNtech booster shots to medical workers. Elsewhere in Asia, Cambodia has started giving AstraZeneca booster shots to its population since August.

In Singapore, booster shots will be offered to individuals who are moderately to severely immunocompromised, adults above the age of 50, and residents of elderly care facilities starting from September. These groups are at a higher risk of severe illness and death due to COVID-19.

Are COVID-19 booster shots effective against COVID-19 and its variants?

Are the booster shots effective?
Findings from Israel’s booster shot roll-out has shown that the third dose of the Pfizer-BioNTech has significantly increased protection from infection and serious illness in individuals aged 60 and older compared with those who received only 2 shots. The study showed that with a Pfizer-BioNTech booster dose, there was a 11.4-fold reduction in the risk of getting infected with COVID-19, and more than a 10-fold decrease in the risk of serious illness.

Similarly, initial data from an early-stage trial from the companies behind the Pfizer-BioNTech vaccine show that the third dose provided significantly higher neutralising antibodies against the COVID-19 virus, as well as against the Beta and Delta variants.

Are COVID-19 booster shots safe?

Data recently published by Pfizer and BioNTech suggests that the side-effects of their third vaccine dose were typically mild to moderate, with the frequency of such side-effects being similar to or better than the second dose. The most common side-effects experienced by recipients included injection site pain, fatigue, headache, muscle and joint pain, and chills. Individuals who have experienced an allergic reaction or anaphylaxis to the vaccines, or myocarditis or pericarditis after getting the vaccines, should not be vaccinated with a COVID-19 booster shot.

However, some experts have expressed concern about the risks to distributing boosters prematurely, including significant adverse reactions such as the rare heart inflammation condition, myocarditis, which is more common after the second mRNA vaccine doses.

Further clinical studies about the safety, efficacy and possible side-effects of these booster shots are ongoing. Singapore’s Expert Committee on COVID-19 Vaccination is also studying the actions that can be taken to remove or mitigate the risk of adverse reactions from booster shots.

When should I take my COVID-19 booster shot?

Currently, the recommended timeline for administering COVID-19 booster shots varies across different countries.

In Singapore, those above 50 years old are recommended to receive a booster dose of an mRNA vaccine at least 5 months after completing their two-dose vaccination regime. On the other hand, immunocompromised individuals are recommended to receive their third dose of the same mRNA vaccine 2 months after their second dose. Singapore is also likely to extend the booster shots to healthcare and frontline workers in the near future.

In Israel, booster shots have been given only to those who received their second vaccine shot at least 5 months before.

Can I use a different vaccine for my booster shot?

Which COVID-19 booster shot should I choose?
Research is ongoing to investigate the safety and effectiveness of receiving booster doses from a COVID-19 vaccine different from the original one administered. Different countries appear to be taking different approaches. Britain is exploring mixing vaccines for its booster doses, while Israel has stuck to using the same mRNA vaccine for its booster shots.

In Singapore, both approaches are being considered, and data is being collected on which would be more effective in protecting against current and future virus strains, as well as the effectiveness for various segments of the population. At present, only the Pfizer-BioNTech and Moderna mRNA vaccines are being offered as booster shots.

Following Singapore’s Expert Committee on COVID-19 Vaccination recommendations, the booster shot for the Moderna vaccine is half the dosage (50mcg) used for the first two shots (100mcg each), except for immunocompromised individuals who will continue to receive a third 100mcg dose for their booster shot. For the Pfizer-BioNTech vaccine, the dosage used for the booster shot is the same as the first two doses.

Will booster shots have to be taken each time a new COVID-19 variant emerges?

It is still unclear if there will be multiple booster shots targeted at protecting against different COVID-19 variants. However, Pfizer and BioNTech have announced that they are developing an updated version of their COVID-19 vaccine to specifically target the Delta variant.

Experts argued in the recent Lancet study that it might be better to deliver specially modified vaccine boosters targeted at new variants, rather than a third dose of the existing vaccines. Other experts say that it might be impractical and costly to expect an updated vaccine with every COVID-19 variant that appears. Furthermore, present data shows that although breakthrough infections appear to be increasing due to the rise of COVID-19 variants, current vaccines still remain effective, especially at preventing hospitalisation and death.


Article reviewed by Dr Edwin Chng, medical director at Parkway Shenton, One Raffles Quay


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