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Singapore's COVID-19 reproduction number is above 1. What does this mean and should we be concerned?

Singapore's COVID-19 reproduction number is above 1. What does this mean and should we be concerned?

A transmission electron micrograph of SARS-CoV-2 virus particles, isolated from a patient, captured and colour-enhanced. (Photo: AFP)

SINGAPORE: On Sep 6, COVID-19 multi-ministry task force co-chair Lawrence Wong talked about a number rarely heard thus far in Singapore's fight against the coronavirus.

"What is of concern to us is not just the absolute number of cases, but the rate at which the virus is spreading. And that’s the reproduction rate, or R," he said, warning about the likely "exponential rise" in cases.

"Currently, the R is more than 1. Cases are doubling every week. And if we continue on this trajectory of infection, it means we could have 1,000 (daily) cases in two weeks, or possibly 2,000 (daily) cases in a month."

Mr Wong said it was important to bring the R number down and avoid a large number of cases, as this would translate to a "sizeable" number of people getting seriously ill and eventually dying.

"We will attempt to do so without going back to another heightened alert. And in particular, we will go for aggressive contact tracing and ringfencing of cases and clusters, and push for more pervasive testing," he added.


In epidemiology, R is the number of people that one infected person will, on average, pass on the coronavirus to.

There are two types of R numbers: R0 and Rt.

R0 (pronounced R-nought) refers to the basic reproduction number of COVID-19 when there is no immunity or interventions to stop it.

"This quantifies the fundamental transmissibility of the virus and how bad things will get if we stop control measures," said Associate Professor Alex Cook, who works on infectious disease modelling and statistics at the NUS Saw Swee Hock School of Public Health.

According to recent documents from the United States Centers for Disease Control and Prevention, the Delta variant's R0 is between 5 to 9. The R0 for measles in populations without immunity is 15.

But when politicians and scientists talk about R, they usually mean Rt, which is the effective, time-varying reproduction number of COVID-19.

"It goes up and down as immunity grows, as control measures change, and as the population’s social activities wax and wane. It represents how much transmission is happening today. Because the epidemic is currently growing, Rt is more than one," Assoc Prof Cook said.

Experts told CNA that Singapore's Rt is currently close to 1.5, which means that two people are infecting three others, explaining the exponential rise that Mr Wong warned about.

During the last period of heightened alert when cases fell, Assoc Prof Cook said Rt was below 1. This is the ideal situation in a pandemic as the number of new infections is below the level needed to sustain the outbreak.


While R gives some indication of how the virus is spreading, it has "no significant impact" once it is accepted that COVID-19 has become endemic in Singapore, said Professor Paul Tambyah, president of the Asia Pacific Society of Clinical Microbiology and Infection.

"For example, the R0 for seasonal influenza has been estimated at around 1.4, but that does not seem to bother us much," he said.

"We all know that during the flu season, if someone in the office starts sneezing and coughing, it is a matter of time before most people get infected."

Professor Dale Fisher, senior consultant at the National University Hospital's division of infectious disease, said R will eventually become "irrelevant" if Singapore stops mass testing and counting cases.

"For now, though, we don’t want a high R0; we don’t want to see a rapid and large escalation because we want to take some time to make sure the expectations are met," he said.

"We don’t want a sharp increase in cases because we want to exit the pandemic slowly having the opportunity to mitigate if things don’t go as expected."


Assoc Prof Cook suggested that the more infectious Delta variant "probably doubled" both R0 and Rt.

"Without that doubling, with the current measures and degree of protection from vaccination, the epidemic would be slowing," he said.

"I personally suspect that increased socialising after it was allowed again was (one of) the main reasons why the Rt went up, though it is also argued that the protection against infection due to vaccination is falling."

Prof Tambyah said the available data on mRNA vaccines, including the Pfizer-BioNTech mainstay used in countries like Singapore, Israel and the US, suggests they are more effective at preventing severe disease and hospitalisation than transmission of the virus. 

"As such, the R0 may paradoxically rise with a high level of vaccination as it protects against severe disease," he explained.

"So, those with mild disease may not seek medical attention and may end up spreading the infection to more people than would have happened if they were under a five-day MC with stay-home notice for respiratory infection or hospitalised for severe illness."


If Rt rose as high as 2, Assoc Prof Cook predicted that the majority of the Singapore population would get infected even with a high vaccination rate, though most infections would be mild.

"If cases go up, while 10 per cent of older individuals remain unvaccinated, the number of seriously sick people will start to climb," he said.

"At some point, it may be that the Government is unwilling to let it climb further, in which case more interventions would be needed to bring the number back down."

Prof Tambyah suggested that a high R number could convince authorities "that they were right weeks ago when they said that we should start to learn to live with the virus".

"We can then gradually lift all the restrictions and focus on protecting the vulnerable, vaccinating those who are not yet vaccinated and ensuring that the healthcare system is functioning well despite the increase in numbers," he said.


Measures to reduce Rt without going back to a state of heightened alert include rolling out booster vaccinations, Assoc Prof Cook said.

"There’s debate about the ethics of this while so many people worldwide are unvaccinated. But if we do do it, it should arrest any decline in the vaccine effectiveness," he said.

Another measure put forward is greater use of self-testing.

"Our models suggest this is pretty effective in reducing the Rt if the positive cases are isolated, and the experience of the dorms in mitigating transmission for a year using rostered routine testing provides empirical support for the policy," Assoc Prof Cook added.

"Other measures to reduce Rt are mask wearing, reducing social contacts and so on – the measures we are hoping to leave behind."

Prof Tambyah feels people need not do anything extra to reduce R beyond following the current advice and rules.

"We just need to do what we are doing right now – learn to live with the virus, get our seniors vaccinated, ensure that those who are not well get medical attention and follow the rules so they can gradually be lifted," he said.

Ultimately, Professor Ooi Eng Eong of the emerging infectious diseases programme at the Duke-NUS Medical School believes R will not go below 1.

"The idea of trying to keep R below 1.5, is it below 1.2, to me at some point is what the society is willing to tolerate," he told CNA's Heart of the Matter podcast.

"It (COVID-19) is not going to disappear. We might be able to get rid of it transiently in Singapore, but it will come back because we can't live by closing our borders. We're going to have to open up at some point.

"At the end of the day, this is a decision that has to be taken at the societal level."

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Source: CNA/hz