SINGAPORE - People who have had Covid-19 can get it again as acquired immunity against the virus wanes over time, Professor Leo Yee Sin, executive director of the National Centre for Infectious Diseases (NCID) told The Straits Times on Tuesday (March 8).

"It is imperative to stress that having had Covid-19 does not make one immune to Sars-CoV-2 forever," she said, adding that the waning of protection after infection has been well documented in previous waves of the virus.

And while most reports to date suggest that, in general, Covid-19 reinfections tend to be milder, Prof Leo added that even a less severe variant like Omicron carries with it a risk of severe illness.

So vaccines will remain one of the key sources of protection against the disease.

Prof Leo cited two studies from Qatar, which she said has high vaccination coverage like Singapore, but also a larger number of infections due to the Alpha, Beta, Delta and Omicron variants of the coronavirus.

One of the studies found that previous Covid-19 infection was 90.2 per cent effective against preventing reinfection by the Alpha variant, 85.7 per cent against Beta, 92 per cent against Delta, and 56 per cent against Omicron.

When it came to protection against hospitalisation, the effectiveness was 69.4 per cent, 88 per cent, 100 per cent and 87.8 per cent against Alpha, Beta, Delta and Omicron respectively.

Noting that severe illness among the reinfected was rare, Prof Leo said: "The lower protection against the Omicron variant is likely due to its immune-evasive ability, even in people with prior infection who have been vaccinated against Covid-19."

She also noted that Qatar's population is young, with only nine per cent being above the age of 50.

The second paper Prof Leo cited studied two sub-lineages of Omicron - BA1 and BA2 - between December 2021 and February 2022, when Qatar experienced a surge of infections fuelled by Omicron.

During this period, 0.9 per cent of those infected with one Omicron sub-lineage were reinfected with another.

The study estimated that being infected with BA1 was about 94 per cent effective in protecting against reinfection with BA2, while being infected by BA2 was 85.6 per cent effective in protecting against reinfection with BA1.

The authors concluded that either sub-lineage infection appeared to induce strong but incomplete protection for at least several weeks, but the longer-term effects remained unknown.

"These two studies demonstrated the existence of reinfection in a highly vaccinated population against the background of high incidence of natural infection. This also demonstrates the nature of the virus, which is to continue to mutate and select the fittest," said Prof Leo, noting that the latter study was conducted over a short period in a young population.

She added that having caught the disease in the past does not make one immune from the coronavirus forever.

When it comes to future variants, the risk of reinfection depends on two main factors.

First, how well the new variant can transmit itself compared with existing ones. The new variant could have immune evasion like Omicron, or it could alter its host's immune response. It could also have a shorter incubation period.

Second, the virus could mutate and result in more severe illness, said Prof Leo. But to date, in general, most Covid-19 reinfections tend to have milder outcomes.

However, stressing that protection levels wane after infection, she cautioned that even less severe variants such as Omicron carry a risk of severe illness.

"Therefore, effective vaccines, with the possibility of future boosters, would remain the mainstay of protection in addition to non-pharmaceutical measures to keep one safe from Covid-19," she added.

Non-pharmaceutical measures refer to actions, aside from getting vaccinated and taking medicine, that slow the spread of illnesses.

"These also act to reduce virus transmission to slow down virus mutation, hence reducing the likelihood of new and more dangerous Covid-19 variants," said Prof Leo.

This article was first published in The Straits Times. Permission required for reproduction.