R0: The One COVID-19 Number That Singaporeans Should Watch

If there is one variable that might rule our lives in the coming months, it would be the R0 number of the Coronavirus pandemic that is taking over the world.

Pronounced as “R-naught,” you might recall it being mentioned in the 2011 film, Contagion. It is otherwise referred to as the basic reproduction number of a disease. R stands for reproduction, while 0 (naught) represents the zeroth generation, known as patient zero. Collectively, it calculates how contagious a disease truly is. The R0 value is the average number of people that an infected individual would spread the disease to.

For example, a R0 of 2 suggests that an infected individual would go on to spread the disease to 2 other people on average. These 2 individuals would then go on to spread it to 2 other individuals each. This replication process will continue unless there are intervention measures in place to stop it from occurring. Such measures include the use of vaccinations, the build-up of immunity against the disease among the population, and public health policies that are put in place to curb its spread.

It is important to understand that the R0 of a disease is not fixed. Instead, it is just an estimate, and as epidemiologists emphasise, a rather imperfect one at that. It varies across geographical locations and time, as different environmental and social factors affect the speed at which a disease is spread. Therefore, the same applies to the R0 of COVID-19, as can be seen from the constantly emerging numbers. Yet governments and public health experts use the R0 as the “closest thing to a compass in navigating the pandemic”.

At a glance, the R0 value offers government and public health officials key insights as to how effective their containment efforts have been and how to move forward in the future.

Why is R0 Important?

The R0 value is important because it helps to model the trajectory of an infectious outbreak and its responsiveness to public health policies. 

For starters, R0 has been useful in enabling Singapore to gauge the impacts of its now imposed circuit breaker measures. In a published study on March 24, the use of R0 helped to determine that a combined approach of physical distancing measures proved to be the most effective at reducing the number of COVID-19 cases in Singapore. These measures include mandatory quarantines for those infected with the Coronavirus and their close contacts, school closures, and workplace distancing.  

The R0 study showed that when the Coronavirus was allowed to spread under normal circumstances, the extent of community spread was significantly greater as compared to when a combined approach of physical distancing measures was imposed. The extent of spread worsened with an increase in the R0 value.

Moving forward, the R0 will likely continue to be of great importance in managing the COVID-19 pandemic in Singapore. It may be influential in determining whether the government will ease its circuit breaker measures to kickstart its economy come June 1.

For instance, if the R0 is less than 1, each existing COVID-19 infection will go on to infect less than one person. This suggests that there will be a decrease in the number of new Coronavirus cases in time to come. However, this does not mean that the virus is defeated. The virus is still being spread within the population, but at a slower rate. Over time, the detection of new COVID-19 cases will fall. In this scenario, the government would likely continue to ease the circuit breaker measures with great caution to prevent a second wave of COVID-19 infections.

However, if the R0 is 1 or greater than 1, that means that each existing COVID-19 infection will cause one or more new infections within the population. Thus, the Coronavirus’s rate of community spread may stabilise or become elevated. This, particularly the latter, poses a serious problem as it increases the burden on already strained healthcare capacities to care for the infected. 

Hence, the R0 gives the government a deeper insight as to whether its COVID-19 cases are decreasing as much as they would like or growing at a more alarming rate. It demonstrates the effectiveness of their public health policies targeted at fighting the Coronavirus, therefore determining whether these policies are worth their massive social and economic costs.

How is R0 Calculated?

The spread of COVID-19 droplets from a cough of an infected individual (Source).

Unfortunately, there is no hard and fast rule to calculating the R0 of a disease. It is built upon a complex mathematical model that involves the use of hard science, forensic investigation, and a good amount of guesswork that revolves around two main factors: the intrinsic features of the disease and factors pertaining to human behaviour.

One key consideration as part of the intrinsic feature of the disease is its infectious period. Some diseases are contagious for a longer duration compared to others, and this affects the window of opportunity that the virus has to spread. When a disease has a longer infectious period, it has a greater likelihood of transmitting the disease to others. Accordingly, this results in the disease being accorded a higher R0 value. The infectious period for COVID-19 is typically around 8 to 10 days, but this could be longer in some cases. Thus, this contributes towards the higher R0 values attributed to the Coronavirus.

Another crucial consideration under the intrinsic feature of the disease is its mode of transmission. Diseases that spread through the air are the quickest and the easiest, and direct physical contact is not required. Merely breathing near someone who is infected with that disease may allow one to become infected. Therefore, airborne diseases tend to have a higher R0 value as compared to those spread via direct contact. According to the World Health Organization, the Coronavirus spreads when a “COVID-19 case coughs or exhales, producing droplets that reach the nose, mouth or eyes of another person”. While large droplets are too heavy to be carried in the air for prolonged periods and fall onto surfaces within one-minute, micro-droplets continue to be suspended in the air and drift in an enclosed space, potentially infecting many others. As a result, the R0 value of the Coronavirus can be elevated depending on how its mode of transmission is calculated.

When it comes to human behaviour, it is critical to consider the contact rate of individuals during an outbreak of an infectious disease. If an infected individual comes into contact with many others who are susceptible to the disease, the disease will be spread more quickly. Hence, a higher contact rate will count towards a higher R0 value of a disease. This is where the various physical distancing measures of governments come into play. In Singapore, where the circuit breaker has been in place since April 8 and quarantine measures are strictly enforced, social interaction has been greatly limited. Such measures help to reduce the overall contact rate between infected individuals and the general population, slowing the overall rate of transmission within the community. While this has likely driven down the Coronavirus’s R0 value in the Singaporean community, the same cannot be said for its migrant population, where cramped living conditions facilitate the rapid transmission of the virus.

What is Singapore’s R0 Value?

Currently, the National Centre for Infectious Diseases does not have any ongoing studies to establish the R0 value for COVID-19 in Singapore. That being said, as of May 6, the Centre for Mathematical Modelling of Infectious Diseases estimated that Singapore has an R0 value of 1 to 1.2. This is significantly lower than the estimated global R0 value for COVID-19, which stands between 2 to 3. Yet this number varies widely. A study published by the Centres for Disease Control and Prevention claims that the global median R0 value for COVID-19 is as high as 5.7.  

A graph showing the effective reproduction number of COVID-19 in Singapore. In the Singapore context, the effective reproduction number and the R0 value are deemed interchangeable. Light ribbon = 90% credible interval; dark ribbon = 50% credible interval. (Source)

As can be seen from the graph, Singapore’s R0 value is constantly fluctuating. At the start of February 2020, Singapore’s R0 value for COVID-19 was 0.5 to 2. Currently, the R0 value has tapered to around 1, suggesting that the virus is becoming more contained. This is likely due to the international travel restrictions imposed in Singapore, the circuit breaker that has been put into motion, as well as the strict quarantine measures for those already infected and their close contacts. Yet its R0 value still remains above 1 likely due to the Coronavirus’s rampant spread within the migrant community, which needs to be further controlled.

Moving Forward: Further Reducing the R0 in Singapore

Indeed, the R0 of the Coronavirus allows us to formulate a clearer and bigger picture of the extent of COVID-19 infection in Singapore. Clearly, it is in our best interests to reduce our R0 value. However, we need to recognise that it may take a considerable period of time for the R0 to drop significantly, such that it no longer poses a major public health concern to Singaporeans. Simply put, it takes time for the effects of the government’s containment measures to be fully felt.

Singapore should continue to follow the advice of Associate Professor Alex Cook, vice-dean of research at the National University of Singapore’s (NUS) Saw Swee Hock School of Public Health. For one, Singaporeans can help to reduce its R0 by continuing to observe social distancing measures. The government can further drive down the R0 by keeping up its rigorous testing to detect, isolate, and conduct contact tracing of infected COVID-19 patients. 

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