SINGAPORE - Recovered Covid-19 patients have shown strong immune response to the coronavirus for more than 300 days.

But studies done by the National Centre for Infectious Diseases on recovered cases in the community as well as in the dormitories have found that the Covid-19 antibody levels in some of the patients do wane over time.

Hence, it is possible for a recovered person to be reinfected later, said the Health Ministry's director of medical services, Associate Professor Kenneth Mak.

He noted that many of the recovered migrant workers in Singapore have reached the 300-day milestone after being infected with Covid-19.

"It's now good for us to start monitoring very closely for the risk of reinfection taking place, and this would be the basis for the enhanced posture or testing, not just around workers but also for workers that come into Singapore.

"We will continue to be vigilant in looking out for reinfected cases as they arise," said Prof Mak.

As for vaccinated individuals, he said the authorities believe that they may be protected for between 15 and 18 months. But beyond that, it is still an uncertain situation.

He said the authorities will study whether additional vaccine shots might be needed for those who are already vaccinated.

"Just as recovered workers may have gradually waning protection, this may also apply to those who have been vaccinated," he added.

Prof Mak said the authorities will conduct follow-up tests on individuals who have been vaccinated.

"If we find that their immune levels start to drift downwards, it would then be the right time to start planning to vaccinate these people as well," he added.

Prof Mak said the other consideration for additional vaccination shots would be in response to Covid-19 variants.

He said the Pfizer-BioNTech vaccine as well as the Moderna vaccine have demonstrated effectiveness in protecting against Covid-19 virus variants of concern.

But he added that there may be future variants that the current vaccines might not be effective against, which would make booster vaccine shots necessary.

"This is something we are studying very closely and we have discussed this with our counterparts in other countries," he said.

Vaccine manufacturers are also looking into producing vaccines that have enhanced protection against emerging Covid-19 variants of concern, he added.

The authorities would look into plans to make such new vaccines available as booster doses for those who have been vaccinated, said Prof Mak.