SINGAPORE - Covid-19 booster shots are necessary to raise antibody levels and strengthen the body's defences against infection, but questions remain regarding who needs them most.

This is even as the Omicron variant has caused concern globally and upended expectations for transitioning to living with Covid-19.

Only live viral vaccines can confer long-term immunity against viral infection. However, making a live vaccine against Sars-CoV-2 - the virus responsible for Covid-19 - is highly challenging and will take many more years, said Professor Ooi Eng Eong of Duke-NUS Medical School.

Hence the need for booster shots to bolster immunity.

But Prof Ooi noted that the groups who need to take booster shots should be carefully considered.

"Currently, my opinion is that we should actively get our older adults and those who are immunocompromised fully vaccinated and boosted. Young adults living or caring for older adults should also be boosted to reduce the likelihood of infection and transmission to the vulnerable whom they care for," he said.

"However, to me, the benefit of booster shots to young adults who are not in close contact with those vulnerable is less clear. They have lower risk of severe Covid-19 compared with older adults even before vaccination, and vaccination has reduced that risk even further."

It is also still unclear whether booster shots have the same risk as normal jabs of rare but severe side effects such as myocarditis, but it would be prudent to assume that the risk and rarity have not changed, Prof Ooi added. Myocarditis is inflammation of the heart muscle.

However, some experts have argued that booster shots may offer the best defence against the new variant. The extra doses may slow its spread, or at least buy time for vaccine makers to develop an Omicron-specific formulation, if needed. Due to the possibility of immune evasion with the Omicron variant, some doctors have decided to err on the side of giving the booster, The New York Times reported last Wednesday.

In Singapore, those who are eligible to receive their Covid-19 booster shots can now get them five months after completing their primary two-dose regime.

The eligible age group has also been expanded to include adults above the age of 30, except for healthcare workers, front-line workers or those belonging to institutional settings, who are at high risk of infection. It is recommended that members of these groups receive their booster shots if they are 18 years old and older.

Essential Ministry of Defence (Mindef) and Singapore Armed Forces (SAF) personnel who have been assessed to have a higher risk of Covid-19 infection will also be offered booster shots. They include personnel under the age of 30 from operational units, active units undergoing operational training, and SAF training schools such as the Officer Cadet School and Specialist Cadet School, Mindef said last Thursday.

Singapore's Expert Committee on Covid-19 Vaccination has assessed that the benefits of another dose outweigh the risks for the recommended groups. Most side effects are mild or moderate, and usually subside within a few days. There is a small risk of serious adverse events, but countries that have been providing booster doses have not reported any safety concerns.

However, as the pandemic rages on on a global scale, the need for vaccine equity is more pressing.

The world would benefit more from having the vaccine distributed everywhere, regardless of whether a country is rich or poor, instead of worrying about booster vaccination rates in young adults, Prof Ooi noted.