A bad flu season in Australia is serving as a warning sign for the U.S.

There have been more than two and a half times more flu cases reported to Australia's National Notifiable Diseases Surveillance System this year compared with the same period last year, according to Australia's Department of Health .

Most the cases have been in people over the age of 80 and in between the ages of 5 and 9. They have largely involved a strain of influenza virus known as H3N2, which reportedly can cause more severe issues for older people and those with weakened immune systems.

The outbreak in Australia has health officials in America watching for a particularly bad flu season.

"In general, we get in our season what the Southern Hemisphere got in the season immediately preceding us," Dr. Anthony Fauci, director of the United States' National Institute of Allergy and Infectious Diseases, told CNN . An "intelligent guess," he said, is the north will most likely have a bad flu season.

Dr. Steven Lawrence, an infectious disease specialist at Washington University in St. Louis, told the St. Louis Post-Dispatch that flu season typically begins when temperatures start to drop, usually around Halloween. It generally peaks around the Super Bowl and is finished by Easter.

A key thing to watch is the strain of the virus hitting Australia, Adolfo García-Sastre, director of the Center for Research on Influenza Pathogenesis at the Icahn School of Medicine at Mt. Sinai, told CNN .

The current available flu shot protects against what is circulating in Australia, according to the Post-Dispatch.

But if the strain has changed, García-Sastre notes that a mismatch between it and the vaccine "could explain larger incidence" in Australia. A vaccine guarding against four virus strains is reportedly expected to be more widely available this year.

Influenza viruses are believed to be spread from person-to-person contact through saliva from sneezing, talking or coughing, and symptoms from the ensuing respiratory illness include fever, cough, muscle aches and fatigue.