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Everything You Need To Know About COVID-19 Reproduction Number Simplified For You

2021-09-17 , 12:00 pm

Thanks to the COVID-19 pandemic, many of us laypeople have become familiar with scientific terms and phrases we might never have known otherwise.

Examples include “polymerase chain reaction”, “doomscrolling”, “social distancing”, “endemic”, “flattening the curve”, and “sneeze”.

Reader: Wait… you didn’t know what the word “sneeze” meant?

I could have sworn it was a luxury shoe brand, but that’s not important right now.

There is still one term – or letter, to be specific – that lies beyond our comprehension; behold, the letter R.

Recently, our Finance Minister Lawrence Wong said that our R is more than 1.

Some people panicked, and others like me, who saw these people panicking, panicked as well even though we didn’t know what the letter meant.

Want to learn more about this mysterious letter that everyone’s talking about at the moment? Well, you’ve come to the right place.

Here’s everything you need to know about R, the second most-used letter in the alphabet behind “I”.

A Rating of Transmissibility

Introduced nearly 100 years ago, R was initially a metric for the reproduction of people. Now, it measures something a little different – the spread of infection in a population.

Simply put, R is the number of people that one infected person will spread a virus to, on average.

Take measles, for example. Measles has an R number of 15, meaning a person infected with the virus will spread it to 15 others on average.

Two Types of R Numbers

Okay, here’s the part where you might get traumatic flashbacks to secondary school; there are actually two types of R numbers – R0 and Rt.

R0 is the initial reproduction number of the virus at the beginning of the pandemic when there are no immunity or safety measures taken against it.

According to the BBC, if no preventive or safety measures are taken, COVID-19 has an R0 number of 3.

Rt refers to the reproduction number at a particular point in time, which is the metric experts are usually referring to when they use the letter R.

Reader: My head hurts

So do all our heads, dear reader.

How is R Calculated

Now, since we can’t know for sure when a person gets infected with COVID-19, we have to use data from previous cases to come up with estimates.

This data can be:

the number of people who had tested positive over a certain period of time
the number of COVID-19 cases who had been hospitalised over a certain period of time
the number of COVID-19 fatalities over a certain period of time

The R Number in Singapore is Above 1

Now that we have an idea of what R is, we can dissect what Mr Wong said earlier this month:

“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.

“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.”

When an R number is more than one, it means that the number of cases will keep increasing.

That’s because each case is likely to spread to disease to at least one more case, and that case will spread it to another, and so on.

When the R number is below 1, this means that the virus will stop spreading at some point, as there aren’t enough people getting infected to transmit the disease further.

Let’s say, Rt = 0.5; that means every two infectees will spread to only 1 other person. This means, as time passes and Rt remains the same, the number of infections divides by two.

That’s why Mr Wong said it’s important to bring down the number of cases, as it could lead to a “sizeable” number of people falling seriously ill and eventually dying.

Drawbacks of the R Metric

One problem with R is that its number can be skewed by isolated clusters.

For instance, if our R number spikes because of a huge cluster in a nursing home, but there’s no evidence of spread to the community, the new R number won’t be an accurate measurement of how quickly the disease is spreading in the country.

Not as Important When a Disease Becomes Endemic

What’s more, this metric may not be as important if the virus becomes endemic.

Speaking to CNA, Professor Paul Tambyah, an infectious disease expert, said the R number has “no significant impact” once COVID-19 is treated as endemic in Singapore.

He noted that although the R0 for seasonal influenza is around 1.4, no one is that worried about it.

“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,” he added.

The R Number May Rise With Vaccinations

Yes, this may not sound like it makes sense, but read on.

See, when a country has a high rate of vaccinations, it has the ability to open up more, meaning more people will socialise and inevitably, spread the coronavirus to one another.

Consequently, our R0 will go up, but this isn’t anything to be alarmed about as those who are vaccinated are far less likely to fall seriously ill from COVID-19 if infected.

What’s the Magic Number?

So, what R number should we aim to get?

If the R0 were to reach 2, the majority of residents here would likely get infected, though most cases would be mild.

The number itself may lose its significance if the authorities stop counting cases and doing mass testing, however, as we try to exit the pandemic and live with this disease.

So, as for what R number is ideal for our idiosyncratic situation, we’ll just have to wait and see.