he vast array of data on the coronavirus pandemic can feel overwhelming, as we all try to make sense of how fast COVID-19 is spreading around the world and the relentless daily count of new cases and deaths.

Helpfully, there’s one number that can tell us quickly and clearly whether the coronavirus outbreak is getting better or worse. It’s called the growth factor.

The main thing to understand is this — to be sure we’re staying on top of the outbreak, we must keep this number below one.

And here is how the growth factor for Australia’s outbreak currently looks.

What is ‘growth factor’ and why is it important?

Put simply, the growth factor measures how fast the number of new cases is going up or down.

And here’s the key point:

• If the growth factor is above one, the number of new cases each day is going up. If it stays above one consistently, alarm bells should be ringing.
• If it’s below one, we’re getting the outbreak under control.

Calculating the daily growth factor is as simple as taking today’s new reported cases and dividing it by yesterday’s new cases. Because these numbers can be a little volatile, we’re using a five-day moving average of new cases for our calculations.

INFOGRAPHIC: How to calculate growth factor

Of course, the growth factor will change over time. Even though Australia’s current growth factor is below one — meaning there were fewer new cases today than there were yesterday — that could change.

But for now at least, things are headed in the right direction.

Australia looks to be successfully flattening the curve but as public health officials keep reminding us, the fight against coronavirus is likely to be a long one.

Even if the number of people sick with COVID-19 gets quite low, the virus could start spreading quickly again without ongoing vigilance.

[Want to see how Australia’s growth factor compares to the US, the UK and other countries around the world? Keep reading, or skip ahead.]

What else should I know?

A figure like this is only ever as good as the data being collected. So when reading the growth factor, there are two extra parameters to keep in mind: community transmission and testing.

The number of coronavirus cases originating overseas is starting to decrease drastically at the moment, and there’s a risk those falling numbers could hide something more worrying — a smaller but rising number of cases caused by spread by community transmission.

Experts say the rate of that local spread may be a better indicator of just how well our physical distancing measures are working.

It’s been difficult to compile data on community transmission. So far, the data we do have only starts in April.

It shows little difference in the growth factors for imported cases and local spread. However, there are about 89 cases still ‘under investigation’, meaning their origin remains unclear.

Another question mark around the data is testing. We will only find cases where we look for them and much of Australia’s testing has been focused on people arriving from overseas and their close contacts.

Even though Australia has tested a high proportion of the population by international standards, Catherine Bennett, the chair of epidemiology at Deakin University, says our high testing rate and low positive rate are a “red herring”.

“If we can look at that in more detail as we do the breakdown [of community transmission] within Australia I think that’s going to be helpful, because I do think it ties to the lockdown. The lockdown is all about community acquisition, not about the boats that came in four weeks ago.”

The unknown number of undetected cases currently in the community may prove crucial to keeping Australia’s outbreak manageable.

Several states have announced they’re now expanding testing criteria to include a broader cross-section of the community. So it’s important to remember that any rise in the growth factor could be, in part, a result of more widespread testing.

“As we expand our testing we expect it to go up but that doesn’t necessarily mean community cases are going up, it means that testing is broadening to capture them,” Professor Bennett said.

Finally, it’s worth remembering that even though the growth factor is a great litmus test for how well we’re keeping Australia’s outbreak in check, other numbers are important too.

How are other countries doing?

Now that we understand what growth factor means, it can also provide a useful insight into outbreaks around the world — and how different countries are faring in their efforts to bring coronavirus under control.

The following charts focus on the countries with the largest overall coronavirus outbreaks around the world, as well as some of Australia’s nearest neighbours.

As you compare those global figures, it’s even more important to factor testing regimes into your thinking. If one country is testing a lot and another is testing very little, it’s going to have a major impact on how their growth factors stack up.

See charts at this website https://www.abc.net.au/news/2020-04-10/coronavirus-data-australia-growth-factor-covid-19/12132478

About the data

• Australian case numbers are sourced from federal, state and territory health department media releases and press conferences and compiled by ABC News. For countries other than Australia, the number of cases comes from data compiled by Johns Hopkins University.
• Health authorities update their figures at different times of day, so the numbers shown do not reflect the same point in time in each jurisdiction.
• Growth factors have been calculated using a five-day moving average of new cases.
• Where there are fewer than five cases per day on average over the past five days, no growth factor is calculated. This is represented by a grey dotted line on the charts.
• It’s important to note that all data in this story represents confirmed cases, which includes presumptive positive cases actually identified by authorities. The actual number of cases in each country is likely to be higher, as an unknown proportion of people with the virus would not have been tested. Therefore, the numbers’ accuracy will also vary depending on how much testing each country is doing.

Credits

• Reporter & Developer: Simon Elvery
• Designer: Ben Spraggon
• Additional Reporting: Matt Martino
• Editor: Matt Liddy