What Australia's flu season could foreshadow in the US this fall

Australia is nearing the end of its worst flu season in at least five years, which could be a sign of what's to come in the United States heading into the fall and winter.

According to data from Australia's Department of Health and Aged Care, as of Aug. 28, there have been nearly 218,000 laboratory-confirmed cases of influenza reported to the country's National Notifiable Diseases Surveillance System.

The season came on earlier than usual and, during its height in June, more than 30,000 cases were being reported to the NNDDS per week, according to the latest surveillance report. Comparatively, at the height of the season in 2017 there were 25,000 cases being reported every week.

Additionally, there have been 1,708 influenza-related hospitalizations -- 6.5% of which were admitted to intensive care units -- and 288 deaths associated with the virus in Australia to date during this season.

Meanwhile, there was just one flu-linked hospitalization and no deaths last year, health department data shows.

Researchers and modelers often look to the southern hemisphere, which experiences its flu season first -- typically from May to October -- to predict how the season will look in the U.S., and experts tell ABC News we should take warning from Australia.

"We often look to Australia and the southern hemisphere as a signal of what we may expect," said Dr. John Brownstein, an epidemiologist at Boston Children's Hospital and an ABC News contributor. "Obviously, it's not a perfect 1-to-1 match but, more often than not, the severity of the flu season in Australia is a good correlate of what we might expect, and it helps us prepare."

During the last two years, not as many cases were reported in the U.S. compared to previous years due to COVID-19-related mitigation measures in place, such as mask-wearing and social distancing, as well as school and business closures.

One study from Wayne State University looking at Detroit Medical Center found there were no positive tests for influenza A or B among adults or children during the 2020-21 flu season. However, during the 2019-20 flu season, 13% of adults' tests and 20% of children's tests were positive for the virus.

Another study examining Akron Children's Hospital in Ohio found no cases of influenza A and just two cases of influenza B were detected during the 2020-21 season -- a 99% decrease from the prior season.

But with COVID-19 expected to peak again in December 2022 or January 2023 and with less flu immunity among the population and fewer mitigation measures, this could be the first time Americans have to grapple with two respiratory viruses at the same time, which could put a further strain on hospital systems.

"Given the concerns we have about health care capacity and health care burnout, the last thing we want is to have parallel epidemics at a time when our health systems are stretched thin," Brownstein said.

Health experts said they are highly recommending Americans get their flu shots by the end of October to get the best protection but say it is never too late, even if people get the shot later into the season.

"One of the potential things that could make flu come back with a vengeance is low immunity," Dr. Shira Doron, an infectious disease physician and hospital epidemiologist at Tufts Medical Center in Boston, told ABC News. "It's definitely important now more than ever because there's a potential for a bad flu season and a bad COVID season."

She added, "In fact, [Tufts was] still requiring [flu vaccination] of any new employees or vendors until June 1 because last flu season went so long, so it's never too late."

The experts also add that flu can lead to severe disease and death, so it's important to lower the risk of infection as much as possible.

"While COVID has dominated headlines for years now, we have to remember that flu is a serious infection and while most who get the flu recover, we have to recognize that flu leads to tens of thousands of hospitalizations and thousands of deaths," Brownstein said. "A portion of flu infections could result in severe illness and death. Just like we try to mitigate risk of COVID infection, we have to try to provide a similar effort against flu infection."