Ahead of an unpredictable flu season, public health experts urge nation to sign up for flu shot

The flu season is notorious for being difficult to predict. However, flu trends from last year and from other parts of the globe can help us make informed estimates.

So far, experts are on the fence if this year's flu season may be mild like last year's, or if it may take a turn for the worse. But experts do agree on one thing; we can do our part by getting vaccinated.

And according to newly released U.S. Centers for Disease Control and Prevention data, about 52% of the U.S. population got a flu vaccination last flu season, which was similar to the prior season. The CDC and other public health agencies are trying to get even more people vaccinated this year because experts are worried about a worse flu season this year because population immunity is low due to a mild flu season last year.

"We are preparing for the return of the flu this season. The low level of flu activity last season could set us up for a severe season this year," said CDC Director Dr. Rochelle Walensky, speaking during a press event hosted by the National Foundation for Infectious Diseases.

With many Americans staying home, washing hands and practicing social distancing, last year's flu season saw the lowest rates of positive tests, hospitalizations and deaths from the flu since the CDC started recording this data in 2005.

"It was the lowest influenza season we'd had in memory. It was really virtually no influenza," Dr. William Schaffner, an infectious disease and preventative medicine specialist at Vanderbilt University, told ABC News.

Dr. Richard Webby, director of a World Health Organization Influenza Collaborating Center and infectious disease specialist, adds that it may be a "global phenomenon."

"I think international travel has been low so, you know, just infected people moving around the globe and seeding other geographic spaces has been reduced," Webby told ABC News.

And these changes have meant that there have been lower levels of the flu virus globally.

"The flu hasn't really circulated for three successive seasons: southern hemisphere, northern hemisphere, southern hemisphere," Dr. Amesh Adalja, senior scholar at Johns Hopkins Center for Health Security, said.

The southern hemisphere - South America, Africa, Australia, New Zealand - experiences its flu season during our summer. Their flu patterns can give us an idea of what to expect come fall in the U.S. And this past summer, the southern hemisphere experienced another low flu season.

"The flu is often in a cycle between the northern and southern hemispheres," Adalja told ABC News. "There's a high likelihood that it could also still be a mild flu season just because there's less flu circulating on the planet, in general."

But as COVID restrictions continue to evolve, southern hemisphere flu patterns may be less helpful for knowing what lies ahead. For example, COVID-19 restrictions are easing up in many places in the United States, but remain in place in parts of Australia, which is often a litmus test for the flu.

"I think that, generally speaking, we turned to, you know, other parts of the globe to make better predictions, and the unfortunate situation now is, given the complexity of COVID restrictions, travel pattern changes, it makes it a little bit more difficult to have a complete predictive lens on what might happen," said Dr. John Brownstein, infectious disease epidemiologist at Harvard Medical School and ABC News contributor.

And as more people get vaccinated and return to work and social gatherings, along with children returning to school, there is a real possibility that this year's flu season may be worse than last year's.

"When they are infected with flu, they shed very large amounts of virus, more than adults and for longer periods of time," Schaffner said. "They are a real distribution mechanism for the virus."

Along with the potential for more spread, we may have slightly lower immunity to the flu.

"We have been through one and a half seasons, with no real flu circulation. So, it's also possible that, as a population, our immunity to flu is a little bit lower," Webby said.

Faced with the possibility of a worse flu season, experts said the flu shot is crucial.

"Go out, get your flu shot. This year, continue being protected," said Dr. Jay Bhatt, an internist in Chicago and ABC News contributor.

Newly released CDC data highlighted some alarming new trends. Only 59% received the flu shot last season compared with 64% the prior season. And racial and ethnic disparities widened, with 56% of white Americans getting the flu shot, compared to 43% of Black Americans and 45% of Hispanic Americans.

The flu shot is not a 100% guarantee that you won't get the flu, but it will reduce your symptoms, and the likelihood of winding up in the hospital.

"We want to prevent severe hospitalizations and death, but we want to prevent symptomatic infection, too," Bhatt told ABC News. "Symptomatic infections can keep people out of work, can make you feel miserable."

If you are eligible for a COVID vaccine booster, that is a great time to get your flu vaccine as well, health experts like Dr. Paul Goepfert, professor of medicine and microbiology at the University of Alabama and Director of the Alabama Vaccine Research Clinic said.

"It's perfectly fine to get them both at the same time," Goepfert told ABC News.

And for those who worry they may get sick from the vaccine, Goepfert added, "I know a lot of people say they get sick and they got, you know, flu from the flu vaccine and that's just not possible."

Experts also encourage pregnant women to get the flu vaccine.

"That's a group that, you know, if they get any of these viral infections, they can do poorly if they don't have adequate protection," said Dr. Simone Wildes, associate director of infectious disease at South Shore Health and ABC News contributor.

And with the uncertainty of the coming flu season looming, the CDC and other government health officials are now encouraging all eligible Americans to sign up for a flu shot, and help prevent a possible "twindemic" - a bad flu season in the midst of the COVID-19 pandemic.

This year, Walensky said, "it's doubly important this year to build up community immunity."

Sara Yumeen, M.D., is a dermatology resident at Brown University's Warren Alpert Medical School and is a contributor to the ABC News Medical Unit.

Sony Salzman contributed to this report.