Does a Mild Flu Season in Australia Mean a Milder Season for America?

Ever since the pandemic took hold earlier this year, we’ve been hit with one piece of bad public health news after the other. But as the heat of summer cools into fall and flu season lurks around the corner, scientists have noted a possible (possible) glimmer of hope from the other half of the globe: Countries in the Southern Hemisphere have just had its mildest flu season on record, according to the World Health Organization (WHO).

“I don’t think anyone quite knew what to expect, so it was quite surprising,” Stephen Leeder, MD, PhD, editor-in-chief of the International Journal of Epidemiology and professor emeritus of public health and community medicine at The University of Sydney, tells Verywell.

In August 2020 (that’s peak flu season for the Southern Hemisphere), labs in Australia confirmed 121 cases of flu. In August 2019, the number was just over 61,000. That’s pretty heartening in a country of about 25 million people.

Chile and South Africa reported similarly mild flu seasons, according to a recent Centers for Disease Control and Prevention (CDC) report. 

But does this mean Americans can expect to have a mild flu season, too? Maybe. Or maybe not. “It’s hard to predict,” David M. Morens, MD, senior advisor to the director, National Institute of Allergy and Infectious Diseases, National Institutes of Health, tells Verywell. “No one can answer that.”

What flu season in the U.S. will look like depends on what, in fact, caused the Australian flu season to ease to begin with.

Timing May Not Be Everything, but It’s Important

Mid- to late fall, the start of flu season in the Northern Hemisphere, promises to look quite different from what was happening in Australia back in April, the start of flu season for that part of the world. At the time, Australians (as with Americans) were just coming to grips with the extent of the danger of coronavirus, and focused on keeping the virus under control. That meant sheltering-in-place and practicing social distancing. Mask recommendations also went into effect, particularly in the most populated states.

All of these precautions make for a much more rigorous means of virus prevention than the typical flu-season measure of washing hands. By the time Southern Hemisphere flu season began in April, restaurants, pubs, gyms, churches, and casinos in Australia had been closed for a month. Schools eventually shut down. As their flu season comes to a close, social distancing measures are still in effect

For these strange times, the restrictions were actually fortuitous. Both the virus that causes COVID-19 (SARS-CoV-2) and the flu virus are respiratory viruses. “So the measures taken to reduce the likelihood of catching COVID-19 are the same as we would recommend to avoid catching the flu,” Leeder says.

Whereas a typical year would often simply involve more-diligent hand sanitizing, this season, the stringent tactics of masks and social distancing also kept the flu virus at bay. In fact, a recent CDC statement cites these measures as the likeliest reason for the strikingly mild flu season in the Southern Hemisphere. 

The timing of America’s flu season is set to occur at a very different stage of the pandemic. Come October, it will be about seven months since the pandemic’s sheltering-in-place measures began. Depending on the state, many restaurants, colleges, gyms, churches, and more have already opened up, albeit with social distancing and mask requirements. And even the most conscientious among us are feeling at least a pinch of pandemic fatigue.

For many parts of the U.S., the winters won’t be as mild as those in Australia, so opening windows or spending socially distanced time outdoors—aspects that seem to control virus spread—won’t be as appealing either.

Does COVID-19 Affect the Flu Virus? 

The idea of “viral interference” is the notion that the infection of one virus protects against infection by other viruses. The notion has been around for decades, but only in the past 10 years has technology improved enough to allow a closer look at how the phenomenon plays out with respiratory viruses in humans. 

Viral interference may help explain why, in a typical flu season, if you become infected by one strain, and then you shortly thereafter get dinged again by another, the second infection might possibly result in milder symptoms, Morens says. 

A Lancet Microbe paper, published in September, provides some insight into the underlying mechanisms how this might happen between different types of viruses, too. By looking at patient data collected over the course of three flu seasons (November 1-March 1, 2016-2019) at Yale-New Haven Hospital, immunologist Ellen Foxman, MD, PhD, and her team investigated how often the cold virus occurred with flu virus. Out of 13,707 cases analyzed, about 900 people had either the cold or the flu virus, but only 12 had both the cold and flu viruses.

Foxman’s team also created cultures of human-airway epithelial tissue and found that inoculating these cultures with rhinovirus (the common cold) kept them from being infected by the flu virus. The rhinovirus elicited the production of an antiviral agent called interferon, which, in turn, protected against the flu virus. 

The thought that COVID-19 might also prevent the flu (or vice versa) is enticing. And while the study might someday lead to clues as to how other viral infections may react with COVID-19, Foxman says it’s unlikely that viral interference played a significant role in Australia’s mild flu season this year.

“If you think about masks and social distancing, it’s a dramatic change in the way people have been going about their business, and it specifically helps prevent the transmission of respiratory viruses,” Foxman, who is also an assistant professor of laboratory medicine and immunobiology at Yale School of Medicine, tells Verywell.

Maybe We’ll Get Lucky This Year

As with any year, we can expect the flu season to hit in fall and winter, with the peak between December and February. For the past 36 years of record-keeping, the peak month has been far and away February, followed by December. You can monitor the flu situation in the U.S. in the Weekly US Influenza Summary Update, which updates each week between October through May.

Each year, there may be up to three different flu viruses circulating—two or more influenza A and B viruses, and within these there may be different subtypes and strains. The influenza vaccine is updated every year based on circulating virus types. Some years are better than others in terms of how good a match the vaccine is for the viruses actually circulating in the thick of flu season. Last February, the CDC reported that overall the vaccine was 45% effective—falling in line with previous years, which have ranged anywhere between 40 and 60% This prevented an estimated 4.4 million flu cases, 58,000 hospitalizations, and 3,500 flu-related deaths. 

Why not 100% efficacy? A lot can change between the time a vaccine production starts and when it’s used. “Day to day, the viruses are in flux,” Morens says. “There are millions and billions changing all the time.” 

Over the course of six months, some flu viruses might continue to exist, but other minor ones can come up. As the population grows immune to certain strains, new viruses have a chance to overtake old ones. “As one is coming, another is going,” Morens says. "There’s a tendency for bad strains to persist for a year or so. So it’s likely that if you have a bad flu season in Australia, you’ll have a bad one here. But there are exceptions. My own suspicion is that it’s just chance."

While Leeder believes that anti-COVID strategies were the main reason for the milder flu season in Australia, he concedes that "maybe this year’s flu vaccine was especially good."

Whatever the case may be, Leeder says Australia’s experience is not a guaranteed indication of what Americans will experience this year.

What You Can Do

Rather than waiting and hoping for the best, scientists strongly advise taking action to prevent the spread of flu—for the benefit of ourselves and our healthcare workers.

Stay the course with social distancing, mask wearing, and hand washing.

It very well may be that COVID-19 kept some people from going to the doctor for the flu, resulting in fewer lab tests and making flu rates look lower in the Southern Hemisphere. But the prevalence of anti-COVID measures probably exerted far more of an effect. “Without these measures, we would have seen far more COVID-19 and far more flu,” Leeder says.

Get the flu vaccine.

Make sure everyone in your family gets it, too. Unlike wearing a mask or practicing social distancing, the vaccine won’t protect you from COVID-19, of course, but it will considerably lower your chances of getting a respiratory illness like the flu. The CDC recommends the shot for all people ages six months and older (with exceptions for those who are allergic to it). Even if you end up falling ill with the flu, being vaccinated will very likely reduce its severity and duration. 

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.