Scared by Surging Coronavirus Numbers? Let's Explain Reproduction and Growth Factor

With some help from the film Contagion!

There is no sugar-coating the situation. The numbers coming out of Victoria are bad.

Today, 30 July 2020, marks a new high for cases of coronavirus (723) and COVID-19 deaths (13) for the state. There is a chance that if you find this article on a different date, that record will have been broken.

Victorian Premier Daniel Andrews has said at a press conference he’s "obviously concerned" about the surging numbers, especially as it comes several weeks after parts of the state went into lockdown. Health experts have cited the deteriorating situation in aged care facilities, transmission through home visits and people going to workplaces while sick as the main drivers of new cases.

However, Mr Andrews has noted that community transmission (which is when cases are passed along with no immediately understood point of origin) comprise "a much smaller number".

"I can understand people are concerned," he said.

"We are all concerned as well. And that's why the central point, the most important point: You cannot go to work if you are sick. But at the same time we know what's driving at least a very large part, and a controllable part of this. Something we can actually influence. And that is patterns of behaviour, and people continuing to go to work with symptoms."

To that, he added an alarming addendum: "If we were to reopen, across metropolitan Melbourne, and Mitchell shire, today, then it will not be 700 cases a day. You can add a zero to that. And our hospitals will be overrun and we will be conducting more funerals."

With new clusters emerging in Queensland and New South Wales connected to Victorian cases, there is reason for Australians in all states to feel worried that the second wave of COVID-19 will go nationwide. But for those who want to understand whether or not coronavirus is out of control, there are really just two numbers you need to know: the reproduction (R0) number and the growth factor.

Reproduction number

The R0 (pronounced "R naught") is a figure that conveys how contagious an infectious disease is. Each contagious disease has a reproduction number, and that number takes the place of the naught. If a disease has an R0 of 1, it means that a contagious person will transmit it to one other person. If the reproduction number is 2, a contagious person will transmit it to two other people, and so on. You can imagine that the higher the number, the more exponential the spread. This was explained concisely by none other than Kate Winslet in the year’s most surprisingly relevant movie, 2011’s Contagion. It also, unfortunately, explains the shortcomings of the R number, which is ultimately an estimate that isn’t fully accurate unless you know everything about the disease (infectious period included). COVID-19 remains, still, a bit of a mystery.

When additional control methods are in place—such as social distancing—we measure the 'effective reproduction rate' (or R-eff), which conveys the spread of the disease in special circumstances. Keeping the R-eff to 1 is imperative, to ensure we don’t experience a full blown outbreak.

"When you have so many cases, even a R-eff at 1, that will continue to see your numbers ... relatively stable but not coming down," Premier Andrews explained today.

"The key point here is until we can get only every second or every third person infecting someone else, we will not see these numbers come down. It is relatively stable but still far too high for us to see these numbers fall."

Growth factor

There is another, similar-sounding number that the ABC has relied on in its reporting: growth factor.

As of this writing, Australia’s growth factor is 1.02, having peaked at 1.28 on 18 March. But this is not the same as the reproduction number.

As the ABC explains, growth factor measures how fast the number of new cases is going up or down. You literally just take the day's new reported cases and divide it by yesterday's new cases. If the number is above 1, that means new cases are increasing day by day. You want to keep that number below 1 if you want to slow the spread. Use the link above to keep an eye on our progress.

And be sure to check your state’s advice to stay safe. Mask up where relevant and don’t be a Karen!

Header Image: Darrian Traynor via GettyImages