Mario Ancalmo receives an influenza vaccination from Raphael Lynne, Pharm. D., MBA, at the CVS/pharmacy on October 4, 2018 in Miami, Florida.

A surge in influenza cases in Australia and at least three deaths in the U.S. in recent weeks have some physicians worried that this year's flu season could be a bad one for Americans.

Though public health officials say they don't know yet whether the 2019-2020 season will be worse than usual, they are still urging people to get their flu vaccines before it kicks into high gear. Australia, which is just getting over its flu season, saw a surge in cases with the H3N2 strain.

Health agencies in Riverside County, California, Los Angeles County and Marion County, Indiana, have reported their first flu-related deaths of the 2019-2020 season. Nevada, Kentucky and Louisiana have also reported flu activity, Centers for Disease Control and Prevention spokesman Scott Pauley said.

"A death so early in the flu season suggests this year may be worse than usual," Dr. Cameron Kaiser, Riverside County public health officer, warned in a September news release announcing the fatality there.

The U.S. flu season is just beginning and can last through May. Peak infections generally hit between December and February, according to the CDC. It takes about two weeks for the vaccine to build an immunity to the disease.

Predicting the duration and size of the outbreak isn't an exact science.

"Flu season is never totally predictable," said Dr. Michael Ben-Aderet, associate director of Hospital Epidemiology at Los Angeles' Cedars-Sinai hospital. Changes in the flu virus, environment and even "some factors we honestly don't understand" can contribute to how the flu season hits nations, he added.

Pauley agreed.

"It's still too early to tell what this flu season might be like," he said in an email. "The Australian season was bad, but the numbers were somewhat inflated by the fact that the season started earlier than normal and that caused a significant increase in testing."