One coronavirus number will let us know if Australia's outbreak is getting worse

With new daily coronavirus infections approaching the highest seen in Australia, the confidence many were starting to feel has been crushed by the outbreak in Victoria and growing clusters in NSW.

So with things changing quickly, is there a good way to track how Australia is doing that isn't so overwhelming?

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 getting the outbreak under control, we must get this number below one and keep it there.

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

Note: The chart below is no longer being updated. See the ABC News Coronavirus page for the latest Australian and international COVID-19 news and data.

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 (or keeping) 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 calculating a daily growth factor based on a comparison of the past week's new cases to the week before.

Of course, the growth factor will change over time and even though Australia has had early success, the fight against coronavirus is likely to be a long one, as the current resurgence in Victoria highlights.

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

Should I be worried when it goes above one?

Catherine Bennett, the chair of epidemiology at Deakin University, says how to interpret growth factor changes depending on how many new cases we're currently seeing.

When a country's outbreak is generally under control and we start seeing a fairly low number of new cases, "the importance of this number is really as a signal that something's changed over the last seven days and what we expect that to be now [in Australia] is probably clusters of cases."

When we see the growth factor go above one, we need to consider the details of the specific cases that pushed it up.

"When it's above one, it means that we've got work to do — we've got to make sure that the reason it's above one is well under control," Professor Bennett explains.

"I think it'll go above one really easily if we go and find another little cluster, and so that's something people have to be prepared for."

[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.]

So while a growth factor briefly above one isn't necessarily cause for immediate alarm in Australia, it is worth being alert to, and there are a couple of scenarios which would make it more worrying.

The first is if it stays above one for too long.

The rapid spread of cases in Victoria illustrates this point perfectly — while the number of new cases started low, the growth factor has stayed above 1 for a month.

That's a full month where, on average, the number of new cases one day was higher than the day before.

While how far it is above one is important (this indicates more rapid growth), the nature of the spread of the virus means a few cases can turn into a crisis in weeks.

The second main reason to worry is the type of cases.

If lots of the cases being found are of an unknown origin — that is they're not traced to an existing case or imported as people continue to return from overseas — it indicates that there may be more disease circulating in the community than authorities are aware of.

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, another important factor to keep in mind is testing.

It's possible more cases will be found in the community simply through the expansion of testing.

Professor Bennett notes that there is a correlation between the number of tests conducted and the number of new cases being found. However, she is also quick to point out that this correlation doesn't imply cause and effect.

"It could be you're doing a bit more testing, because you've discovered a case associated with [for example] an aged care facility, and then you go and screen everybody. So it could actually be linked to a specific case that triggered a bit more testing, but also it could just be that every time we look harder, we just pick up those extra few," she 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.

Is growth factor the same as reproduction rate?

You might have heard state and territory medical officers or epidemiologists use terms like 'R', 'R0' or 'reproduction rate'.

Like growth factor, these are measures of how quickly a virus spreads through a community, however they are slightly different and the difference can be important.

Growth factor is a direct measure of the change in new cases being reported.

Reproduction rate is an estimate of how many new people will be infected, on average, by each person who has a disease. The most relevant measure is effective reproduction rate, sometimes referred to as Reff.

This is the estimated reproduction rate at a given point in time, taking into account the various control measures, like physical distancing, that are in place.

Estimates of reproduction rate are just that, estimates. They're based on modelling which takes a number of factors into account and often involves assumptions. These estimates can be very useful for epidemiologists and health authorities, however experts can often disagree about the true number.

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.

However, when comparing countries' growth factors, it's important to remember that the same number can mean different things depending on the severity of the outbreak and what stage it's at.

A growth factor of 0.99 in a country with thousands, or even tens of thousands of new cases each day represents a huge problem and a health system in ongoing crisis.

On the other hand, a growth factor of 0.99 in a country where new daily cases is quite low represents a much more manageable situation.

As an example, Australia's growth rate has been higher than the US for a month, but because we're coming from a much lower base we're in a completely different situation.

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.

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 by taking a seven day growth factor and converting it to daily terms.
• Where there are no new cases in the past seven days or the seven days prior to that, 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
• Australian data compiled by the ABC News Digital Story Innovations team

Editor's note: The methodology used to smooth out daily fluctuations in new cases was changed on Wednesday April 29 to more accurately represent growth factor changes, and to prevent too much weight from being given to some days in the smoothing window. The length of the smoothing window was also increased to seven days from five in order to eliminate the possibility that fewer tests being conducted on weekends might have an effect on reported case numbers in the following days. The period of time shown in the graphs was also increased.