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To achieve herd immunity, at least two-thirds of S'pore’s population must be vaccinated against Covid-19: Experts

SINGAPORE — At least two-thirds of Singapore’s population, or about 3.8 million people, will need to be vaccinated in order for the community to achieve herd immunity against Covid-19, several infectious diseases experts said. This is if available vaccines can stop vaccinated persons from infecting others.

Medical experts talk about the safety concerns over whether Covid-19 vaccines were rushed into use, and whether it would be better to wait and see before getting the jab.

Medical experts talk about the safety concerns over whether Covid-19 vaccines were rushed into use, and whether it would be better to wait and see before getting the jab.

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  • If vaccines can induce “sterilising immunity”, at least two-thirds of Singapore can be vaccinated to reach herd immunity, experts said
  • Sterilising immunity refers to the ability of a vaccine to prevent viral transmission from a vaccinated person to someone else
  • Experts said that the speed of development does not mean safety corners were cut
  • Vaccine sceptics are a cause for concern, for they could prevent Singapore from attaining sufficient immunisation coverage in its population


SINGAPORE — At least two-thirds of Singapore’s population, or about 3.8 million people, will need to be vaccinated in order for the community to achieve herd immunity against Covid-19, several infectious diseases experts said. This is if available vaccines can stop vaccinated persons from infecting others.

However, if vaccines are not able to prevent further spread or if they are less effective against Covid-19 than believed, then nearly the entire population will need the jab before a victory against the coronavirus can be declared, they told TODAY.

Professor Ooi Eng Eong from the Duke-NUS Medical School, who is deputy director of the emerging infectious diseases programme there, said that if vaccines are able to induce “sterilising immunity” — a term that means the vaccines will effectively prevent viral transmission — such immunity in two-thirds of the population will be enough to protect the remaining one-third from Covid-19.

“(This) two-thirds ratio refers to herd immunity. Current trials have not been able to ascertain if either Pfizer-BioNTech’s or Moderna’s vaccine can also effectively prevent infection,” he said of the two firms producing vaccines that are now being used by a few countries.

His colleague, Professor Eric Finkelstein from the school’s health services and systems research programme, said: “If uptake is well below that, then Covid-19 is likely to (continue) proliferating in the community. Therefore, getting a high vaccine uptake is important.”

On Monday, Singapore’s Health Sciences Authority (HSA) approved the Pfizer-BioNTech vaccine for pandemic use, and the authority also identified that it has been looking at vaccines by American biotech firm Moderna and China’s Sinovac Biotech. 

Health Minister Gan Kim Yong said on the same day that there is no specific immunisation coverage target that the authorities are looking at, other than to have a take-up rate that is “as high as possible”.

It is not yet known if the three vaccines, or any of the other 40-odd vaccine candidates now undergoing trials, can prevent viral transmission.

Senior consultant Ling Li Min from Rophi Clinic, which specialises in infectious disease, explained that for Covid-19, sterilising immunity will mean that the vaccine can prevent asymptomatic spread and also protect the infection of upper airways, which would effectively reduce the risk of transmission.

“Although the current vaccine candidates have demonstrated the ability to reduce symptoms and the number of viruses in the lower respiratory tract, there is as yet no evidence of sterilising immunity in the upper respiratory tract,” Dr Ling said.

In other words, the Covid-19 vaccines available now primarily protect the lungs rather than the nasal area. The lungs are where the coronavirus causes the most damage, while the nose is one of the channels for the disease to spread to others.


The medical experts are confident of HSA’s process of approving the use of the Pfizer-BioNTech vaccine against Covid-19, noting that the primary goal of a vaccination programme is to protect the individual from the coronavirus, but not necessarily to achieve sterilising immunity.

Prof Ooi said that it would be a bonus if vaccines are able to achieve sterilising immunity, because then, vaccinated individuals would serve as a dead-end for virus transmission.

“Without sterilising immunity, more people would need to be vaccinated against Covid-19 in order to prevent another Covid-19 outbreak from re-emerging in Singapore,” he said.

One other thing to note: Vaccines allow the immune systems of vaccinated persons to produce antibodies that can defeat the virus, so they are considered immune to the virus. Usually, it takes some time for the body to produce antibodies and build immunity after vaccination, so it is possible a person could be infected with the virus right before or just after vaccination and fall sick.

This is because the vaccine has not had enough time to teach the body to build up immunity and provide protection.

Apart from the ideal vaccination coverage needed for Singapore’s population, several other questions remain at the top of Singaporean’s minds following Monday’s announcement that the first shipment of vaccines will arrive here by the month’s end.

These include safety concerns over whether the vaccines were rushed into use, and whether it would be better to wait and see before getting the jab.

TODAY put these questions to the experts to clarify any misconceptions.

Q: Are the vaccines safe to use? Aren’t they made too quickly?

As with all types of medicine, there are benefits and risks. Common side effects of vaccinations in general include pain and redness on the injection site, as well as mild headache, body ache and fever, Prof Ooi of Duke-Nus Medical School said.

For the Pfizer-BioNTech vaccine specifically, HSA found that its safety profile was also generally consistent with other registered vaccines against other diseases and can produce similar effects as highlighted by Prof Ooi.

These side effects usually resolve within a few days and are an expected part of the body’s natural response.

However, just like other vaccines, the vaccine may — in rare instances — cause severe allergic reactions such as breathing difficulties, wheezing and swelling. 

That is why HSA does not recommend those with a history of anaphylaxis — a condition marked by a rapid onset of severe allergic reactions — to get the Pfizer-BioNTech vaccine.

Pregnant women, people whose immunity is compromised and those under the age of 16 should also not receive the Pfizer-BioNTech vaccine because testing data on these groups is still unknown, HSA said.

While vaccines usually take years to be produced and distributed, this is often due to manufacturing and distribution reasons. Vaccines are also typically tested in consecutive phases, whereas for Covid-19, the process has been so quick because the different phases, such as animal testing and clinical trials, were all done concurrently.

Dr Shawn Vasoo, clinical director from the National Centre for Infectious Diseases, said that novel vaccine technologies such as mRNA-type vaccines that Pfizer and Moderna developed are also faster to make than traditional vaccine types. 

This type of vaccine has had a history of development to combat cancer and is considered “new” only because it is now used to fight an infectious disease.

Dr Vasoo said: “Again, while it may be faster to develop mRNA vaccines, it does not necessarily guarantee clinical efficacy and if a vaccine candidate fails in terms of efficacy or safety — it would be back to the drawing board again.”

Ultimately, this does not mean that pharmaceutical firms or regulators are cutting corners, the experts said.

Q: How can I trust HSA’s approval process?

Prof Ooi said that HSA’s emergency authorisation of the Pfizer–BioNTech vaccine is based on a full set of data from well-conducted clinical trials that demonstrated both safety and efficacy of this vaccine in protecting against Covid-19. 

“Not only were the trials well conducted, the data showing the ability of this vaccine to protect against Covid-19 is also very impressive. I am thus confident that this vaccine would benefit those who choose to receive it. 

“I, myself, would get vaccinated at the earliest possible opportunity,” he added.

HSA has said that it granted the Pfizer-BioNTech vaccine the interim authorisation because the vaccine meets the required safety, efficacy and quality standards, and that the benefits of the vaccine outweigh the known risks.

Dr Mimi Choong, HSA’s chief executive officer, said on Monday that the Singapore regulator was able to complete the evaluation of the Pfizer-BioNTech vaccine in the shortest possible time by working on the available “rolling” data, instead of waiting for the full data set to be submitted before starting its evaluation.

The same rigorous processes are used to register vaccines in normal times, she added.

Q: If herd immunity is possible with two-thirds of the population vaccinated, why can’t I let others get the jab instead?

The logic of becoming a vaccine “free-rider” is flawed because there is no guarantee that herd immunity can be created by vaccination.

Furthermore, the two-thirds ratio to achieve herd immunity applies only to within a certain population, such as within Singapore and not beyond.

New research from the Johns Hopkins Bloomberg School of Public Health in Baltimore of the United States found that a quarter of the world's population will not have access to a Covid-19 vaccine until 2022, with people in poorer countries expected to face a long wait even if manufacturers meet all their production goals.

This means that even with herd immunity, those who have not been vaccinated will still remain vulnerable to Covid-19 for a long time.

“Vaccines do not save lives. Vaccination does,” Prof Ooi said. 

“Misinformation on the safety and efficacy of vaccines can thus do much harm, especially if it prevents sufficient vaccination coverage in the global population, including Singapore, to bring the Covid-19 pandemic under control.”

Q: Is it a cause for concern that there are so many vaccine-sceptics around? Why not make it mandatory?

On Monday, Health Minister Gan said that few vaccinations are made mandatory because the Government wants to respect the people’s choices, adding that the authorities will undertake public education efforts to encourage people to be vaccinated.

In a commentary in TODAY, Dr Hsu Li Yang from the Saw Swee Hock School of Public Health pointed out that even though Singapore does not have an active and coordinated anti-vaccination movement here, exposure to vaccine misinformation via social media is still prevalent.

The experts said that such vaccine hesitancy can be a cause for concern and could prevent Singapore from attaining sufficient immunisation coverage in its population.

And with the country having had low infection rates among its population, this could lead some people to think that a vaccine is not necessary since the chances of them being infected is low.

Prof Finkelstein explained that there will be people who will get vaccinated no matter what because they are risk-averse to Covid-19, and then there will be the group who will never vaccinate under any circumstances.

There is also a large third group of people who fall between these two extremes, depending on how they perceive the risks of Covid-19 and the vaccine.

A September poll on vaccine uptake rates by Prof Finkelstein and his colleagues at Duke-NUS Medical School found that for a vaccine that cost S$100:

  • 22 per cent of the respondents said that they will never be vaccinated

  • 42 per cent said that they will be vaccinated immediately if it were free 

  • 35 per cent chose to wait and see, and pay for the vaccine if need be

He believes that without a vaccine mandate in Singapore, uptake will be “slower than hoped”.

“Providing evidence that vaccines are safe and effective will help, but the reality is we won’t know for sure until there is more experience... Only when there is strong evidence that the vaccine is safe and effective, and if the vaccine remains inexpensive, will uptake go up to herd immunity levels,” Prof Finkelstein added.

Related topics

Covid-19 coronavirus coronavirus vaccine herd immunity sterilising immunity

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