Singapore has seen 26 cases of Covid-19 reinfection as at Monday, Health Minister Gan Kim Yong said in a ministerial statement in Parliament yesterday.

Such reinfections are known to be possible both overseas and here, he said, but it is still unclear why they occur.

The minister said that investigations are ongoing to determine if reinfections are due to a general waning of immunity levels over time, or a lack of cross-protection - despite a previous infection - against specific variants of the coronavirus that a person is exposed to for the first time.

Mr Gan, who is co-chair of the multi-ministry task force tackling Covid-19 here, noted that Singapore has detected 10 different variants of the virus among local and imported cases.

The emergence of new strains is not surprising as it is in the nature of viruses to mutate, he said.

Their appearance in Singapore is also not surprising as the nation cannot completely shut its borders, he added.

"Singapore is not self-sufficient in many things, and we need to maintain our supply lines and global connections to survive. The virus will then find ways to infiltrate us."

However, Mr Gan said, some variants like the B16172 strain are of greater concern because of their increased transmissibility, the severity of the illness they cause and their potency in resisting vaccines.

The strain is believed to have sparked the clusters of cases at Tan Tock Seng Hospital and the Tuas South community care facility.

Three cases linked to Changi Airport were also found to have been infected with the B1617 strain.

The B16172 strain is a sublineage of the B1617 strain, which was first detected in India last October.

The B1617 strain has been dubbed the "double mutant" because it has two major mutations on the spike protein of the coronavirus.

"While the definitive characteristics of this variant remain unknown, the large clusters and rapid transmission we see locally are concerning as they suggest that infections of this strain can spread quickly and widely," Mr Gan said.

"This is similar to the overseas situation where resurgence of cases is likely driven also by new variants."

He said that Covid-19 vaccines, while not 100 per cent efficacious, can still prevent severe reactions to the infection and are likely to reduce the chances of an infected person passing the virus on to another person.

As at Monday, there have been 56 cases involving people who were infected despite being fully vaccinated. Of these, 30 were local cases and 26 were imported.

About 57 per cent of local cases involving those who were vaccinated were asymptomatic, and none had severe disease requiring more intensive care, Mr Gan said, adding that this is consistent with international experience and emerging evidence.

"Data which came out from Israel last week showed that infection rate amongst unvaccinated persons was 91.5 per 100,000 person-days, while that for fully vaccinated persons was 3.1 per 100,000 person-days.

"Hospitalisation rates were 2.7 for unvaccinated and 0.2 for the fully vaccinated (per 100,000 person-days)," said Mr Gan.

Person-days is a measure of the number of people who participated in a study, multiplied by the number of days they were observed.

Mr Gan said: "Overall, the benefits of the Pfizer-BioNTech and Moderna Covid-19 vaccines continue to outweigh the known risks, and we must continue to encourage Singaporeans to be vaccinated."

He also noted that data on the need for Covid-19 vaccine booster shots is still emerging from both international and local studies.

"We will share more when we know more. Meanwhile, get the first two shots first."