Now that the world has largely returned to normal and relaxed Covid-19 precautions, the flu, which has been mostly silent for the past two years, is making a comeback.
Although influenza can be transmitted at any time of the year, it typically peaks during the winter. This means that peak flu season alternates between the Southern Hemisphere (where June, July, and August are the winter months) and the Northern Hemisphere (where December, January, and February are the winter months).
This hemispheric alternation has recently led some experts to suggest that the severity of the flu season in the southern hemisphere could be a leading indicator of what should be expected in the northern hemisphere.
Melinda Wenner Moyer, a highly regarded science journalist, was quoted in the New York Times:
“We can see what happened in the southern hemisphere: Their flu season usually ends when ours starts. And it’s been a pretty bad flu season there, which doesn’t bode well for us.”
Katherine Wu, a microbiologist with a Ph.D., takes this line of argument even further in a recent article in the atlantic. The article, titled “The Strongest Sign Americans Should Worry About the Flu This Winter,” reports that in Australia, the number of flu cases made this season “one of the country’s worst in several years.” Which, he argues, “doesn’t bode well for those of us in the north” since “the same viruses that cause outbreaks in the south tend to be the ones that cause epidemics here when the seasons make their annual change.”
I’m not so sure, and here’s why.
The argument contains two claims, both debatable. The first claim is that the southern hemisphere actually experienced a “pretty bad flu season” in 2022, that is, “the country’s worst in several years.” The second claim is that there is, in fact, a connection between the hemispheres, which we might call the leading indicator hypothesis. In fact, the evidence for both claims is quite scant.
Data of the Global Influenza Program of the World Health Organization are freely available to anyone. So we can take a look.
It is true that the number of laboratory-diagnosed cases in Australia in 2022 was higher than ever before.
However, more flu is only one possible explanation for this fact. An alternative explanation is that there was simply more testing. That is, the more you look for something, the more you find. Influenza is a nationally notifiable disease in Australia. That means that every time a test is performed, the result must be recorded in a log. As a form of passive surveillance, these data suffer from variations in the frequency of testing, making interpretation difficult. Given the presence of Covid-19, it stands to reason that Australian healthcare providers have been testing more aggressively for the flu in 2022 than ever before.
Is there evidence for this hypothesis from further testing? Yes. The red line in the figure above is the percentage of tests that came back positive, and with the exception of the two years of Covid-19 (when there was almost no flu), this “positivity rate” was actually considerably lower. in 2022. than is normal for Australia. The combination of a low positivity rate and a large number of positive tests means that the total number of tests must have been higher than usual, perhaps much higher.
A potentially more accurate measure of flu intensity comes from Australia’s surveillance of influenza-like illnesses (also known as ILI). The figure below shows the number of ILI cases (in green) found in two “sentinel” networks of general practitioners known as the Australian Sentinel Practice Research Network (ASPREN) and the Victorian Sentinel Influenza Practice Network ( VicSPIN).
While the 2022 season was definitely more severe than the 2021 and 2020 seasons, that’s not surprising. The reason is the Covid-19 pandemic. But otherwise, the 2022 flu season in Australia is hardly noteworthy, and in fact appears to have been mild compared to years like 2012, 2014, 2015 and 2017. I don’t see the evidence to say that the flu season 2022 in Australia was “pretty bad”.
We can use the same data to analyze the second claim, namely that the number of cases in the southern hemisphere is a leading indicator of cases in the northern hemisphere.
The US also reports ILI data.
If Australia were to serve as a leading indicator of ILI in the Northern Hemisphere, then we would expect the number of cases at the peak of each season to be highly positively correlated. Drawing against each other should generally fall along an upward sloping diagonal line. The diagram below shows that this is not the case. (In fact, the correlation is even slightly negative!)
So there is no evidence here that Australia is a leading indicator of flu in the US This is not to say that the US will experience a mild flu season. I think we don’t know yet.