​When it comes to the Delta variant of COVID-19, one thing has become clear: Vaccination is not enough.

This means Singapore cannot let up in its fight against the disease, said Professor Leo Yee Sin, executive director of the National Centre for Infectious Diseases (NCID).

"If I did 100 per cent in 2020, this year I have to do 200 per cent - because that is what the Delta variant demands of us," she told The Straits Times in an interview to commemorate the centre's second year of operations. "We cannot solely rely on vaccines."

In other words: Keep your mask on, hands clean and guard up, even if you have been vaccinated against COVID-19.

Three characteristics make the Delta variant - which accounts for 99.6 per cent of the infections successfully sequenced by NCID between end April and end August this year - more contagious than its wild-type predecessor, Prof Leo said.

First, infected people emit larger quantities of the virus.

International studies have shown that the Delta strain can better attach to a person's upper respiratory tract, making it more easily transmitted to others.

And it has a reduced incubation period of three to five days, meaning it can be passed to others more quickly.

If the outbreak is not properly controlled, Singapore could see an exponential surge in cases, Prof Leo added. "That is something that we have to remember - that this virus can cause a very rapid surge of cases in the community."

Already, Singapore has seen a spike in new infections over the past month, with several large clusters emerging at bus interchanges, migrant worker dormitories and a shopping mall.

Between February and December last year, the country had about 200 community cases a month, observed Adjunct Associate Professor Matthias Toh, director of the National Public Health and Epidemiology Unit at NCID. This figure went up to 590 between January and August this year, driven in part by the Delta strain.

The number of new cases in the community has since increased to 1,325 in the past week, compared with 723 the week before.

On Monday (Sept 6), Finance Minister Lawrence Wong announced new measures to slow transmission and buy time for more people to get vaccinated. This is because existing vaccines protect against severe illness, but do less well at preventing people from becoming infected, especially by the highly contagious Delta strain.

Health Minister Ong Ye Kung has also said that vaccines are about 40 per cent effective at preventing infection, with the strength of protection likely to wane several months after people have received their jabs.

Even so, Prof Leo said every percentage point increase in the country's vaccine coverage can translate into "significant benefits" in protection, especially for Singapore's seniors.

"So even though it's a last-mile effort to get to older individuals who have not got their vaccines yet, we should still try."

During the hour-long interview, the experts addressed a range of topics, including the transmissibility of the Delta variant and the impact vaccines have had on infection here.

Overseas research has found that the Delta strain has had an R0 of 5 to 8, meaning that one person typically transmits the virus to between five and eight other people. This is around the same as chickenpox.

In Singapore, the R0 is estimated at 1.45, taking into account existing safe management measures, Prof Leo said, adding that this figure is significant.

The Delta variant is estimated to be more than two times as contagious as the original wild type. This means the R0 would increase significantly if safe distancing measures were removed.

NCID treated about 1,100 community cases admitted between end April and end August this year, sequencing as much genetic material as it could. Two-thirds of the cases were successfully sequenced, with 99.6 per of cases in this group caused by the Delta variant. The remainder were not sequenced because the genetic material collected was inadequate.

As a general rule, unvaccinated people who contracted this variant developed more serious symptoms than those who caught the wild type last year, Prof Leo said. There were also more severe cases involving younger people in their 40s and 50s.

The good news: Even though more unvaccinated people required supplementary oxygen, fewer people died of the disease. This suggests early detection and intervention are key, she added.

Another silver lining: While both vaccinated and unvaccinated people have high viral loads, vaccination helps people to recover faster. Viral loads tend to decrease in the vaccinated after five days or so, Prof Toh noted.

But this is no reason to let down one's guard, Prof Leo stressed.

"A lot of people feel that after vaccination, it is time to relax. Our message today is for people to know: There is no time for complacency, even if they are vaccinated."

Prof Toh added: "We hope that people's behaviour does not change because they are vaccinated with two doses and think that they are Superman or Superwoman."

For one thing, the virus is still evolving. Other countries have reported cases of a new "Delta plus" variant, while some genetic differences have been observed in the local Delta strain. These are classified as belonging to the Delta variant as they share similar characteristics.

But Singaporeans cannot assume that this will always remain the case, Prof Leo said.

Every two weeks, NCID carries out a survey to find out Singaporeans' attitudes towards seeking treatment for COVID-19.

It has garnered responses from more than 1,000 people so far, and only half of them said they would see a doctor and get tested if they developed symptoms of an acute respiratory infection.

This needs to change, especially when people can still get COVID-19 and spread the virus to others even if they are vaccinated and only mildly ill, Prof Leo said.

Prof Toh stressed: "Go and see a doctor so we can put out the small fires - do not wait for the fire to become big."

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