Southern hemisphere has record low flu cases amid Covid lockdowns

Health systems across the southern hemisphere were bracing a few months ago for their annual surge in influenza cases, which alongside Covid-19 could have overwhelmed hospitals. They never came.

Many countries in the southern half of the globe have instead experienced either record low levels of flu or none at all, public health specialists in Australia, New Zealand and South America have said, sparing potentially tens of thousands of lives and offering a glimmer of hope as winter approaches in the northern hemisphere.

General practitioners in New Zealand have not detected a single influenza case since they started screening patients in June, health data shows; last year about 57% of the samples they collected were positive.

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The last flu cases detected by major hospitals in Auckland, the country’s largest city, were in April. “It’s amazing. There’s just nothing there at all. No influenza,” said Michael Baker, professor of public health at the University of Otago in Wellington.

New Zealand’s Covid-19 rates are among the lowest in the world, but even notwithstanding the pandemic, people in the country have experienced their healthiest cold months on record. “Our excess winter mortality peak has largely disappeared,” Baker said.

A tracking system that monitors a cohort of at least 30,000 people for influenza-like symptoms shows as few as 0.3% of New Zealanders reported coughs or fevers some weeks during their winter, a tenfold decrease on some previous years.

The trend holds true across the Tasman Sea in Australia, where Covid-19 restrictions have also deeply dented rates of flu and other respiratory illnesses. The country recorded more than 131,000 influenza cases in the peak months of July and August last year, according to government data. Over the same period this year, there were 315.

“Cases have fallen off a cliff since March,” said Prof Ian Barr, deputy director of the World Health Organization’s collaborating centre for reference and research on influenza, in Melbourne.

Fewer than 40 Australians have died from influenza this year, compared to more than 950 last year, “and there haven’t been any deaths for the past three to four months”, Barr added.

Even across South America and in South Africa, where lockdowns have been patchy or harder to enforce, and Covid-19 has spread widely and killed tens of thousands of people, flu rates have been well below historical rates or nonexistent – despite increased testing for it in the Americas, according to the Pan American Health Organisation.

This apparent contradiction – Sars-CoV-2 growing exponentially while influenza virtually disappears – illustrates a key difference between the two viruses. The seasonal flu is not just less deadly, but significantly less virulent, Baker said.

Populations have higher immunity to seasonal influenza, whether acquired naturally or through vaccines, while travel bans instituted from March interrupted the normal migration of the virus from the northern hemisphere to the south.

As an unprecedented live experiment on a massive population, it could offer some good news for northern hemisphere countries heading into their own flu season, just as drier air and more time indoors are expected to drive up Covid-19 rates. Even relatively less stringent quarantines appear to be surprisingly potent at suppressing influenza and other common respiratory illnesses.

“You would still see a flu season, but I expect it would be much less intense,” Baker said. “Northern hemisphere countries that are actively suppressing Covid-19 to some degree should get a lot of protection [from influenza] by doing that.”

It could mean that more people are susceptible to flu strains in the years ahead, having not acquired any kind of immunity this year, Barr said, though he added the threat would significantly decrease if people kept washing their hands thoroughly and wore masks in crowded areas even after the pandemic subsides.

In a year that will be studied by public health specialists for decades, it also points to new ways of fighting the future influenza pandemics that some scientists regard as inevitable.

“[Before,] it was thought that when a new influenza pandemic virus arrives, all you can do is dampen it down, you can’t stop it,” he said. “We now know that if you had a pandemic flu virus of sufficient severity, you could take the elimination approach, or even the exclusion approach, as Taiwan has done with Covid-19.”