SINGAPORE - Thousands of older people around the world have begun receiving Covid-19 booster shots to further protect them against infection and severe illness - especially as immunity has been shown to wane over time.
But should healthy young adults be next in line? While the jury is still out on that subject, some local scientists believe that the risks may currently outweigh the benefits.
The topic came up during a webinar on Thursday (Sept 16) on living with Covid-19 during which three experts at the forefront of Singapore's Covid-19 fight shared their insights. The session was organised by The Straits Times and moderated by senior health correspondent Salma Khalik.
In Singapore, only two groups of people are being offered booster shots for now: those aged 60 and above, and the immunocompromised.
For those with weakened immune systems, the data is clear. As they tend to produce fewer antibodies than healthy people after vaccination, a third shot is recommended.
The same logic applies to the elderly, who are also believed to have a lower immune response to the vaccine - although the scientific evidence for this proposition is less strong, said Professor Ooi Eng Eong of Duke-NUS Medical School's Programme in Emerging Infectious Diseases.
He believes there is "no harm" in booster shots for this group, but cautioned against administering these jabs to younger adults, who tend to react more strongly.
"Already, the second dose gives quite a lot of side-effects," he said. "I'm not sure what a third dose is going to do."
The experts added that Singapore's eventual decision on whether to extend booster shots to most of the population will depend on what it hopes to achieve through vaccination.
If the goal is to keep people out of hospital, the standard two doses will sufficiently protect most people against severe disease, noted Associate Professor Hsu Li Yang, an infectious diseases expert at the National University of Singapore's Saw Swee Hock School of Public Health.
But if Singapore wants to prevent infection altogether, it will have to make sure most people have high levels of antibodies. Doing so, however, comes with its drawbacks.
Protection against the virus
Why do Covid-19 vaccines protect against severe disease, but seem to do less well at preventing infection?
At present, it is clear that higher antibody levels lead to better protection against infection, said National Centre for Infectious Diseases executive director Leo Yee Sin.
"In other words, it can be Delta, it can be Alpha - if your antibodies are sustained at a very high level, you have a better ability to resist the infection," she said.
People's antibody levels tend to spike after vaccination.
But a person's immune system does not maintain these high levels unless it keeps getting exposed to the same virus, Prof Ooi said. "It's like bodybuilders. If you don't keep exercising, you will lose your muscle mass."
Instead, the body remembers the virus and will mount an immune response - which typically takes four to five days to ramp up - once it recalls this memory. This means it is essentially a race between the immune system and the Delta variant, which has an incubation period of around three days, he added.
A person could therefore start falling ill before the immune system can catch up, preventing him from developing severe symptoms. So before the immune system catches up, he could already be spreading the virus to others.
Studies have shown that both vaccinated and unvaccinated patients have high viral loads in the early days of their illness, although these levels quickly decrease in vaccinated persons.
Is it possible to give booster shots every few months, in order to keep antibody levels high?
No, Prof Ooi said. "Keeping the immune system activated and seeing the same antigen has its price. The price is that the immune system can sometimes think that you are trying to educate it to tolerate this. That becomes a disaster."
This is similar to how food allergies can be treated by gradually exposing a person to increasingly larger amounts of the trigger food, until the body is completely desensitised to it.
In this case, however, the virus can wreak havoc if the immune system does not recognise it as a threat.
In short: Vaccinations and booster shots are important, especially for older people and the immunocompromised, for whom "every little bit matters". But to prevent the virus from spreading in the wider community, they have to be paired with other infection control measures.
Prof Leo, who is in her early 60s, will take her booster jab soon. She encouraged others in her age group - especially those who have not been vaccinated at all - to follow suit.
"My team was the first team to get the mRNA vaccine. We had to overcome that fear, which is a personal fear," she recounted. "For us, we had the advantage of knowing the science."
"It doesn't mean that we are fearless," she added. "We have to make a rational decision based on the knowledge that we have."