While the United States has so far avoided a "twindemic" of both Covid-19 and flu cases, this year could be different. Australia is currently experiencing its most severe flu season in five years—an indicator that the United States may face a similar surge of flu cases amid continued Covid-19 spread.

Australia is experiencing a 'perfect storm' of viral infections 

Since the Covid-19 pandemic began, many experts have feared that a "twindemic" of surging flu cases coupled with a Covid-19 spike would occur—but the flu season has been relatively mild in recent years. Now, Australia is experiencing its worst flu season in five years, serving as a potential indicator for the United States.

According to Andy Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health, who has been tracking Australia's flu and Covid-19 rates for months, three factors have contributed to a "perfect storm" of viral infections that could be an indicator of what the United States may face this flu season.

First, Pekosz noted that the Australian flu season, which typically occurs from June to September, started in April this year and has already peaked months ahead of its typical pattern.

In addition, Australia's surge suggests this year's flu strains are spreading widely across the Southern Hemisphere, infecting those who do not have natural or vaccine-acquired immunity. On average, Australia's weekly cases since mid-April have surpassed its average over the past five years.

According to Pekosz, Australian health officials have also reported a rise in Covid-19 cases, largely driven by the omicron variants, with over 47,000 new cases every day—a 62% increase since February.

"The early nature of the flu season in Australia is important," Pekosz explained. "If there's not a lot of immunity in the population, we often see influenza earlier in the flu season. This is one of the earliest seasons on record in Australia, so it may indicate that there's a lot of people who are susceptible to influenza."

Notably, health officials have not yet determined how effective Australia's flu shots are against the current strain.

"We haven't seen those numbers yet coming out of Australia, but they should be coming sometime soon," Pekosz noted. "It will be important to see how well the vaccine is matched to those flu strains that are circulating."

Will the US experience a 'twindemic' this year?

The trends in the Southern Hemisphere, along with declines in prevention measures in the United States, have sparked concern that Americans may face a "twindemic" this year.

"We've been predicting that the U.S. could be hit with this twindemic of influenza and COVID for the last couple of years, but it did not materialize before, in large part because influenza was relatively under control," said Leana Wen, an emergency doctor and public health policy professor at George Washington University. "But now, with people returning to pre-pandemic normal [activities] and with less immunity to influenza because of the lack of recent infection, we could see that twindemic this year."

Separately, William Schaffner, a professor of infectious diseases at Vanderbilt University School of Medicine, said he is concerned that many people have what he calls "vaccine fatigue," which could result in millions of Americans skipping their annual flu shot or the updated Covid-19 boosters that are set to be released this fall.

"Once I heard, just a few days ago, about what is happening in Australia my concern for what might happen here this fall increased," said Schaffner, who also serves as the medical director of the National Foundation for Infectious Diseases and the foundation's liaison to the CDC's Advisory Committee on Immunization Practices.

"We've had a falloff of the acceptance of the influenza vaccine. This fall, the anticipation is that we will have updated COVID vaccine boosters. And that's great, but we'll also have to persuade people to get their influenza vaccine, so they'll have to roll up both sleeves. And this is not going to be easy," Schaffner added.

Alicia Fry, chief of the epidemiology and prevention branch at CDC, said the agency will remain "very alert," searching for signs of an early, aggressive flu season.

"We watch … all of the Southern Hemisphere countries very closely, hoping that we get some sort of insight, but it's not perfect by any means," Fry added. (Edwards, NBC News, 8/4; Bean, Becker's Hospital Review, 8/4; Tate, WebMD Health News, 7/20)