SINGAPORE - A Covid-19 vaccine booster dose is not yet required for all, although it can restore vaccine effectiveness for some people such as the elderly, says the World Health Organisation's (WHO) chief scientist.

With many vaccines in use and the emergence of different strains of the Covid-19 virus, Dr Soumya Swaminathan said that there is a dearth of high-quality studies on the duration of the protection of vaccines and the resulting need for booster shots.

"Today, Delta is the dominant variant across the world, but a few months ago, different countries had different variants circulating... There are also different demographics and underlying illnesses," she said.

Dr Swaminathan was speaking at the virtual Singapore Health and Biomedical Congress on Thursday (Oct 7).

She noted that much of the vaccine effectiveness data currently available is from a small number of countries, including Britain, Chile and Israel.

The scientist was speaking during a panel discussion moderated by Associate Professor David Lye, the director of the Infectious Disease Research and Training Office at the National Centre for Infectious Diseases (NCID).

Dr Swaminathan said most of the vaccines in use now still protect the majority against severe disease.

But a few specific sub-populations - the elderly, people with weakened immune responses and those using certain specific vaccines, where it appears that immunity is waning more rapidly - may be at risk.

"Therefore, we may see Sage (Strategic Advisory Group of Experts on Immunisation) making recommendations now on third doses for those groups of people, but those groups are still small, and it's certainly not time for boosters for the general population," she added.

Sage advises the WHO on overall global policies and strategies in relation to vaccines and immunisation.

The WHO, which has warned that inequitable Covid-19 vaccine distribution is a threat to all nations, has called on wealthy nations to stop distributing booster doses so that there will be more shots available for those in poorer countries.

Some countries, including the United States, France and Singapore, have started giving booster doses to the elderly. In Singapore, those aged 50 and above, including those with weakened immune responses, and residents of aged-care homes are offered the shots.

Dr Swaminathan said the WHO does expect some waning of immunity to occur over time, though more studies are needed.

There are a large number of studies that show that high levels of protection continue against severe illness, she noted.

"So the question now really is... do we use our vaccines today to keep raising the antibody levels of people who've already been vaccinated in the hope that it will also reduce infections, or do we go ahead and vaccinate people who have no immunity?"

Another panellist, Dr Richard Hatchett, chief executive of the Coalition for Epidemic Preparedness Innovations (CEPI), agreed that the broad use of boosters in populations at this point is not justified.

CEPI is an international coalition set up in 2017 to develop vaccines to stop future epidemics.

Dr Hatchett said that although current vaccines, which almost exclusively target the spike protein of the Sars-CoV-2 virus, are very effective, scientists are already developing second-generation vaccines.

"Some, for example, could provide mucosal immunity that might prevent transmission, and help with reducing the emergence of variants," he said. The mucosal immune system is the largest component of the entire immune system.

"We don't want to be chasing the variant vaccine. So even as we contemplate - do we use the current vaccines for boosters - we also need to be exploring, studying and optimising their use to produce the broadest, deepest and most enduring immunity and investing in developing next-generation vaccines so that we can coexist with Covid-19 for the long term," added Dr Hatchett.

Professor Leo Yee Sin, NCID's executive director and the third of four panellists, said it is clear that post-infection, an individual can develop a very high immune response in terms of antibody levels with one dose.

There is also data to suggest that these patients will get an extremely mild infection should they be infected again.

But different countries have different experiences.

Prof Leo said that countries with very high levels of natural infection can bring up the level of protection across the entire population effectively, if they have just one dose of a vaccine.

A second group concerns areas like Singapore with a very low rate of natural infection. These places are vaccinating the population and using boosters to elevate the level of antibodies and sustain cellular immunity for as long as possible, said Prof Leo.

The third group would be those with a zero-Covid-19 policy. These places want to use vaccines to protect the entire population over a long period of time, for instance.

Professor Peter Piot, from the London School of Hygiene and Tropical Medicine, said the current immunity in Europe is very high because there is protection from natural infection plus vaccination.

Prof Piot, who is also the special adviser on Covid-19 to the president of the European Commission, said that in Europe, countries with high vaccination rates of 70 to 80 plus per cent may actually be better off than thought.

He added that in Singapore, given the current surge of new infections, natural immunity will likely gradually build up.

Before the panel discussion, Professor Tan Chorh Chuan, chief health scientist at Singapore's Ministry of Health (MOH) and the executive director at the MOH Office for Healthcare Transformation, spoke about a new paradigm of epidemic control that could bring together approaches used against the severe acute respiratory syndrome (Sars) and influenza.

The panel discussion was held as a hybrid event, with speakers at Max Atria @ Singapore Expo and those online holding discussions, while participants tuned in virtually.