The Southern Hemisphere skipped flu season this year, likely because of social distancing

The run-up to flu season in the US has been fraught with fear. Health experts worry that fighting Covid-19 and influenza simultaneously could burden the health care system's ability to treat both infections.

Flu season in the Southern Hemisphere, though, came and went with so few cases that there was "virtually no influenza circulation," according to the Centers for Disease Control and Prevention.

CDC experts believe that efforts like social distancing, mask-wearing and school closures might have critically crippled flu season in countries in the Southern Hemisphere.

The Northern Hemisphere -- particularly the US, where as many as 56 million people might have been infected with the flu last season -- can learn from the Southern Hemisphere's response to Covid-19, which might have prevented a massive flu outbreak, CDC experts wrote in a report last month.

Continuing to stay home, wearing masks and maintaining social distance in public, they said, could protect us from more viruses than one.

A virtually nonexistent flu season

To measure the potential impact of the coronavirus on flu infections, the CDC evaluated flu activity recorded by the World Health Organization in three "robust sentinel sites" in the Southern Hemisphere -- Australia, Chile and South Africa -- between June and August, typically an active period for flu activity in the hemisphere.

All three sites showed "very low" flu activity, the CDC reported. In Australia, among the 60,031 people tested for the flu, only 33 test results were positive. In Chile, 12 out of 21,178 tests were positive for flu, and in South Africa, only 6 out of almost 2,100 people had the flu.

That's a total of 51 people who tested positive for flu among 83,307 tested, or a 0.06% positivity rate. Previous flu tests from April to June in 2017 through 2019 showed about over 13% flu positivity rate overall in those same countries, the CDC said.

The findings suggest that methods like social distancing, wearing masks and closing schools reduced the number of flu infections in the Southern Hemisphere -- and could do the same in the Northern Hemisphere.

"In countries or jurisdictions where extensive community mitigation measures are maintained (e.g., face masks, social distancing, school closures, and teleworking), those locations might have little influenza circulation during the upcoming 2020-21 Northern Hemisphere influenza season," CDC researchers wrote.

The flu and Covid-19

In the Southern Hemisphere, flu season typically ends this month, while it begins in earnest in the Northern Hemisphere.

But the kickoff for flu season in the Southern Hemisphere in May coincided with mass lockdowns and other mitigation efforts to prevent transmission of the coronavirus. While CDC researchers can't confirm that those mitigation measures caused the extremely low number of flu cases, they said the trends are "compelling and biologically plausible."

As flu season beings in the Northern Hemisphere, many of those early mitigation efforts have been dismantled. Particularly in the US, lockdowns have been lifted, masks aren't mandated nationwide and new coronavirus hot spots crop up in different parts of the country regularly.

The CDC's September report says that flu activity remains fairly low in the US and worldwide for now.

But an average flu season in the US can still be devastating. During the 2018-2019 flu season, over 35 million people got the flu, over 490,000 people were hospitalized and 34,200 people died from it -- and that season was considered to have had "moderate severity," per the CDC.

Then throw Covid-19 in the mix -- there are tens of thousands of new cases every day. Health experts fear that an average flu season, compounded with the coronavirus, could overwhelm the health care system, limiting hospitals' ability to treat patients, and our immune systems, inhibiting our body's ability to fend off infection.

That's why health experts are touting this year's flu vaccine as the most important one Americans will ever get -- protecting oneself from the flu is one way to prevent at least one highly infectious respiratory infection. Getting vaccinated, along with staying home, masking up and keeping one's distance, could dampen flu season and prevent a dual outbreak.