The southern hemisphere skipped flu season in 2020

EVERY WINTER, from May to October, tens of thousands of Australians and New Zealanders are asked how they feel. More precisely, they are asked by their governments in weekly surveys if they have a cough or a fever. Although 2020 has been a difficult year in many ways for Aussies and Kiwis, it has not necessarily been bad for their physical health. This winter only around 0.4% of people in the two countries said they were suffering from flu-like symptoms, down by four-fifths compared with last year. Other countries in the southern hemisphere have reported similar slowdowns in the spread of influenza.

The cause for this steep decline in infections is clear. Governments all around the world have enacted costly lockdowns to fight the novel coronavirus. In doing so, not only have countries in the southern hemisphere slowed the spread of covid-19, but they also appear inadvertently to have stopped the proliferation of another deadly disease: the flu.

Since 1952 the World Health Organisation (WHO) has tracked influenza in member countries, relying on local partner laboratories to report both the number and types of viruses they detect. In the first two weeks of August, the WHO processed nearly 200,000 influenza tests, and found just 46 were positive. In a typical year, the number would be closer to 3,500.

One might worry that because health-care systems are strained, the declines in reported flu cases reflect reduced testing capacity, rather than a genuine reduction in infections. Fortunately, this is not so. WHO data are readily available in six countries in the southern hemisphere: Australia, Argentina, South Africa, Paraguay, New Zealand and Chile. There the total number of influenza tests has fallen by just 20%, while the share of tests that have come up positive has plummeted to record lows.

Data from Australia tell a remarkable tale. From May to mid-August of 2015-19, an average of 86,000 Australians tested positive for the flu each year, and around 130 died of it. This winter the government has registered only 627 influenza infections and just a single death.

The reduction in flu cases helps explain at least one puzzle in covid-19 data: some countries have seen a smaller increase in overall mortality than their covid-19 deaths would suggest. For instance, Chile has recorded around 9,800 covid-19 deaths from June to August 25th, but an increase of only about 8,800 deaths overall compared with the same period in 2015-19. It is possible that Chile is undercounting how many of its residents have died. But the near elimination of the flu has helped too.

Influenza cases may yet rise in the south both this year and next, since fewer people have developed immunities. Meanwhile, countries in the northern hemisphere should expect fewer flu cases since fewer will be imported from abroad, and most people are social distancing. Seasonal influenza kills an estimated 300,000-650,000 people annually. In a year filled with terrible news, a victory against the flu is a welcome respite. ■

Sources: “Estimates of global seasonal influenza-associated respiratory mortality” by Danielle Iuliano et al. (2018);WHO; Johns Hopkins University