covid vaccination booster shots combination

COVID-19 Vaccines and Booster Shots: Which Combination is Most Effective?

Is it safe to mix and match your COVID-19 vaccines and booster shots and which combination will induce a better immune response?

by Grace Koh

Published on: 30 December 2021

In the past two years living with COVID-19, numerous measures have been taken to defend against the virus, and the COVID-19 vaccines are one of them. The Singapore government has been rolling out its vaccination campaign, providing free vaccination shots to residents. Locally, the vaccines available under the national programme are Pfizer-BioNTech/Comirnaty, Moderna and Sinovac-CoronaVac COVID-19 vaccines. Now, the vaccination programme includes the addition of a booster shot. Now that booster shots are available, some of you may be considering whether to take the plunge and take the shot – but questions abound. Who is suitable? Should I take the shot? And more importantly, can I mix and match my vaccines? 

We find out more about booster shots and see which combinations are the most helpful, especially for those who have taken their primary vaccination shots from the national programme.

Why do I need to take the booster shot?

After taking the initial doses of vaccination, their effectiveness starts to wane over time. This is when a booster shot comes into play.

A booster shot is an additional dose of a vaccine given, after the protection of the original shot(s) begins to decrease. It helps people maintain immunity longer against specific diseases or illnesses the vaccine is protecting against. In this case, the booster shot would protect against COVID-19 and the risk of severe infection or complications if contracted.

Booster shots would also help with reducing transmission rates in the community, as more individuals are protected and vaccinated against COVID-19. The Ministry of Health (MOH) shares that vaccine protection from the first two vaccination doses decreases over time –  people vaccinated only with the first two doses may incur a 40% risk for infection with COVID-19, and 90% risk for a severe infection as time passes. This calls for the need to consider a booster shot, which can help to lower the risk of infection. Taking a booster means you are 70% less likely to be infected, and 90% less likely to get symptoms and severe infections from COVID-19, the MOH states. 

Am I eligible to take the booster shot and how do I make an appointment?

Under the National Vaccination Programme, those who are eligible for the booster shot are individuals who are above the age of 18 years, and have taken their primary shots about five months ago. The booster shots under the National Vaccination Programme are the two messenger RNA (mRNA) vaccines, Pfizer-BioNTech / Comirnaty and Moderna. 

If you are eligible for a booster dose, a text message will be sent to you inviting you to receive the vaccination. This text message should be anticipated about five months after your second vaccination dose and will contain a personalised booking link which you can use to register for your booster jab appointment. 

If you are aged between 30 and 59 years old, and have completed your second vaccination shot five months ago, you do not need to wait for the text message invite. You may walk in to any Vaccination Centre without an appointment made to take your booster.

For those aged 60 years and above, and have completed your vaccination second dose five months back, likewise, you do not need an appointment – just walk in to any of the Vaccination Centres, participating Public Health Preparedness Clinics (PHPCs) or Polyclinics to receive your booster dose without any prior appointment.

When should I take the booster shot? 

(Updated 13 Jan 2022)

Depending on the vaccines you took, here is the recommended timeline to take your booster shot:

  • Pfizer-BioNTech / Comirnaty or Moderna: Five months after your second dose
  • Sinovac or Sinopharm: Three months after your third dose

From Feb 14, in order to be considered as fully vaccinated, Singapore will require those aged 18 and above to get your booster shot 270 days (9 months) after your last jab. Once your fully vaccinated status has expired, you will have restricted access to vaccination-differentiated venues, amongst other restrictions. Thus, if you are eligible, it is recommended that you take your booster shot as soon as you are able to.

Do take note that you will be considered as fully vaccinated on the day that you receive your booster jab. Individuals who are medically ineligible for booster shots will also not be affected by the new rule (i.e. their fully vaccinated status will not expire).

Should I take the booster shot if…

I am immunocompromised? 

An immunocompromised person refers to someone whose immune system does not work as well as it should to protect from infections. This makes them more vulnerable to infections and diseases. Immunocompromised people usually are those who have a genetic mutation or a disease like Human Immunodeficiency Virus (HIV), where their condition causes a specific loss of immune function; and those who are taking certain medications to treat specific diseases. For immunocompromised people, Amesh A. Adalja, a senior scholar with the John Hopkins Center for Health Security in the United States, explains that immunocompromised people are less likely to respond to standard vaccination regimens, and hence would benefit from an additional mRNA vaccine dose to be given to them in addition to the primary doses. 

So yes, if you are immunocompromised, you can still take the booster shot. 

In Singapore, persons who are immunocompromised, and have received the three-dose enhanced primary series of vaccination shots, are recommended to receive a booster dose of the mRNA vaccine – a fourth shot of the vaccine. These people are eligible for the booster dose from five months after completing their third dose in the primary series of vaccinations. 

I am a senior? 

Data from MOH has shown that seniors unvaccinated against COVID-19 hold a higher risk of a serious outcome from contracting the illness. The data shows that for seniors in their 80s, one in two unvaccinated seniors, versus one in eight vaccinated seniors, will require intensive care, oxygen, or succumb to COVID-19. Taking the vaccination would markedly lessen the risk of severe consequences of COVID-19 for seniors. Knowing that vaccination efficiency wanes over time, it is hence important that seniors get the booster shot as well. Booster shots are also available to seniors. Anyone, who is above the age of 60 years and has completed their second primary vaccination dose five months ago, can walk-in to a polyclinic, vaccination centre, or PHPC to get their booster dose without needing to register in advance. 

I am under 18 years old?

Currently, individuals under the age of 18 years are not eligible to take a booster dose. 

However, individuals between the age of five to 17 years are now eligible for vaccination. Under the Ministry of Education (MOE) vaccination exercise, parents or guardians of these children will receive a text message with a link to book an appointment for their child or ward. If the child is not under a part of the MOE vaccination exercise, parents and guardians can register on behalf of their child or ward here

I have had COVID-19? 

If you were fully vaccinated before being infected with COVID-19, currently, you would not need a booster dose. However, people who were partially vaccinated or unvaccinated before their COVID-19 infection, should receive a dose of mRNA vaccine at least three months after their infection. This helps to reduce their risk of reinfection, and also complete their primary series of vaccination. No booster dose is indicated for this population presently.

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Will I get any symptoms from the booster shot?

Like other vaccines, you may experience some side effects from the booster shot. Most side effects are mild or moderate, and get better within a few days. Some common side effects include:

  • Pain
  • Redness
  • Swelling at injection site
  • Fever
  • Chills
  • Headache
  • Muscle pain
  • Joint pain
  • Tiredness
  • Lymph node swelling at neck or arms

These are easily treated with rest, and by giving some time for the symptoms to fade. Paracetamol can be taken for pain, fever, chill, and headaches as well. However, do observe and see a doctor if: 

  • The side effects persist or get worse
  • The fever persists for more than 48 hours (2 days)
  • Respiratory symptoms such as cough, runny nose, sore throat, shortness of breath or loss of sense of taste and smell develops
  • Heart or chest symptoms such as chest pain, shortness of breath or abnormal heart beats develops

Very rarely, someone may have a severe allergic reaction to the vaccine, which may appear as difficulties with breathing, swelling of the face, throat, eyes, or lips, a fast heartbeat, dizziness and weakness, and rashes all over the body. Should there be any of such signs observed, do call 995 or go to the nearest A&E immediately. 

Will the booster shot protect me from the Omicron?

A study from Pfizer-BioNTech noted that individuals who have received a third dose of the Pfizer vaccine appeared to have better defense against the Omicron variant, as compared to individuals who only had two shots of the vaccine. A CNBC news article has also stated that a third dose of the Moderna vaccine appears to provide significant protection against the Omicron variant. From the report, the authorised booster is reportedly able to “boost neutralising antibody levels (offer protection against COVID-19) 37-fold higher than pre-boost levels.”

However, at present, there is insufficient data to conclude vaccination effectiveness against the Omicron variant. It is a considerably new variant of the virus, and time is needed for studies to be done to uncover the effectiveness of booster shots on this variant. What is known is that an accelerated response towards vaccination should continue to be the focus of a public pandemic response to encourage widespread immunity. Danny Altmann, immunologist at Imperial College London, notes that increasing antibody levels with booster shots should help protect against Omicron, just as boosters have improved protection against the Delta COVID-19 variant. So, go ahead to get your booster shots for a better safeguard against Omicron. 

What are the different combinations and which is the most effective?

In line with the local National Vaccination Programme, the booster shots can be ‘mixed and matched’ according to your choice. That means if you had Pfizer vaccination shots for your first two jabs, you can opt for an authorised and permitted alternative. What is the logic behind mixing and matching vaccines? Dr Asok Kurup, from the Academy of Medicine’s Chapter of Infectious Disease Physicians in Singapore, explains, ‘Rather than from one particular way (only using one type of vaccination), a (mixed) vaccination means you are stimulating (the immune system) in different ways so that you get a more diverse mix of immune cells… in theory.’ Do note that the only shots available as booster shots in Singapore are the mRNA vaccines, Pfizer and Moderna. 

Here are explanations for some terms used in the below comparisons:

  • International Units Per Millilitre: An international unit is an amount of a substance agreed upon by scientists and doctors. 
  • Neutralising antibodies: An antibody that is responsible for defending cells from organisms which cause disease. They can be triggered by infections or by vaccines. 
  • Binding Antibody Units Per Millilitre: Unit of measurement determined by the World Health Organisation (WHO) to measure the amount of anti-COVID-19 immunoglobulin. 



Studies have found that this combination allowed for 62% reduction of infection risk upon getting the booster. There were also 446.7 international units per ml of neutralising antibodies, and 3409.1 binding antibody units per ml. 


For individuals who are thinking of taking Moderna as a booster shot on top of their Pfizer vaccinations, current studies show that this combination has a 72% reduction of infection risk, and also comes with 785.8 international unit per ml of neutralising antibodies, making it comparatively higher than the Pfizer-only combinations. There are also 6155 binding antibody units per millilitre, which is higher than the Pfizer-only combination as well. 



The Moderna-only route shows to have 901.8 international units per millilitre of neutralising antibodies, and 6799.8 binding antibody units per milliliter.


For taking the Pfizer vaccine as a booster on top of the Moderna vaccines, current studies have shown that this combination shows 677.9 international units per millilitre of neutralising antibodies, and 5195.6 binding antibody units per millilitre. 


For Sinovac, MOH has mandated that to be considered as a fully vaccinated individual, three doses of the Sinovac vaccine must be administered. The second dose should be taken 28 days after the first dose, while the third dose should be taken 90 days after the second dose. After the three doses of Sinovac, booster shots are recommended five months after completing the primary series of vaccination. The recommended booster shots are mRNA vaccines available in the National Vaccination Drive, Pfizer and Moderna.


Currently, there is little research on this combination. However, a research study from Turkey has suggested that people fully immunised with Sinovac may have greater protection from taking a booster shot of Pfizer. The study compared groups of healthcare workers who took different vaccine combinations, and found that health care workers who received a booster shot of the Pfizer vaccine, versus those who took a third dose of Sinovac, were found to have higher antibody levels which is thought to be a marker of vaccine efficacy. Moreover, for the former combination, there were a smaller percentage of healthcare workers infected with COVID-19 than those who received the Sinovac. However, more research is to be done in the Sinovac-Pfizer combination for conclusive results. 


Like the above combination, little research has been done on the Sinovac-Moderna combination. However, what we know from present research is that mRNA boosters, like Moderna, offer significant protection against the new omicron variant of COVID-19, though booster effectiveness may be quick to decrease. 

With the many vaccination combinations available, what we know is one thing for sure: getting a booster shot will help to increase your protection against COVID-19. As the COVID-19 situation unfolds rapidly, with various new variants in a short span of two years, perhaps the safest thing to do is to get a booster shot irregardless.

Key Takeaways

Despite the existence of different vaccines against COVID-19, our safety is still uncertain. Thus, following proper precautionary measures is also one of the best way to protect ourselves from the threat of the virus. Also, if you are not sure on what vaccine to use or mix, you can always consult a doctor to address any concerns you have.

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About the Writer
Grace Koh
Grace is a healthcare writer who has experience in hospital settings and community agencies. Apart from reading, singing, and plodding up muddy trails, Grace enjoys scribbling notes and thinking up a storm.
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