Following a severe flu season in the Southern Hemisphere, experts are urging Americans to prepare for an increase in flu infections, hospitalizations, and deaths, Joseph Choi writes for The Hill.

What the Northern Hemisphere can learn from this year's Australasian flu season

Why people may be more vulnerable to the flu this year

According to Choi, there are two primary reasons people may be more vulnerable to the flu this year. First, people are more likely to encounter the flu this year because of relaxed Covid-19 safety precautions. In addition, people are less likely to have immunity due to the low flu transmission rates over the past two years.

Richard Webby, a virologist at St. Jude Children's Research Hospital's infectious diseases department, noted that the past two flu seasons have not seen normal levels flu transmission.

"As a population, our immunity to the flu is down a bit," Webby said. "When the virus comes back, it's probably going to have a little bit more room to spread, a little bit more room to potentially cause disease." 

Typically, the overall population's exposure to the influenza virus produces some community immunity—with roughly 10% to 30% of the population exposed in a normal season. However, in 2020 and 2021, not as many people were exposed, leading to a drop in the population's natural immunity.  

For instance, there were typically more than 100 pediatric flu deaths every year before the pandemic. Over the past two flu seasons, there were fewer than 40 reported pediatric flu deaths, with just one confirmed pediatric death in 2020. 

The lower level of population immunity puts people at a higher risk of flu infection this year, Webby noted.

Separately, Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security at the Bloomberg School of Public Health, said the flu season over the past two years has been virtually "nonexistent," adding that this pattern was always going to change once social distancing practices ended.

Ultimately, increased flu transmission signals that people are returning to "some semblance of their life pre-COVID," Adalja added.  

The US flu season may mirror the Southern Hemisphere's

Since this year's flu season began in the Southern Hemisphere, the region has experienced "a tough flu season," Choi writes.

For example, Australia experienced its worst flu season in five years, serving as a potential indicator for the United States. Flu cases peaked earlier than usual in Australia this year, with significantly higher infection rates than 2020 and 2021.   

"There were nearly 600 cases of laboratory-confirmed influenza in Australia in 2021," Choi notes. "During this year's flu season, the country has reported more than 217,000 cases, though this is still lower than in 2019, when Australia reported more than 300,000 cases, the highest number of cases on record for the country."

However, flu deaths and hospitalizations in the country remained relatively low this year. Typically, flu deaths and hospitalizations are primarily driven by the elderly, and Australia took precautions to protect vulnerable demographics. 

According to Webby, if the United States takes similar precautions this year, a significant increase in flu hospitalizations and deaths could be avoided.

Experts encourage vaccination

Many experts agree that the Southern Hemisphere's flu season seemed to be a return to a normal flu season. Webby and Adalja also expressed doubts that a "twindemic" of flu and coronavirus cases would occur this year.  

"I don't think that these two viruses can sort of go gangbusters at the same time," Webby said.   

Still, the White House is recommending that people receive both their updated Covid-19 booster shots and flu shots at the same time to avoid surges of the viruses, Choi writes. CDC has said that September and October are ideal times to get the vaccines. 

"With many COVID-19–conscious people likely to get their booster shots sooner than later, some have questioned whether September, when COVID-19 boosters became available, is too soon to get immunized against the flu and if it would be better to get the two shots at different times," Choi writes.

According to Adalja, it is important to get a flu shot at the right time to ensure that it will be effective throughout the entire flu season.  

"If you get it too early, there's clear evidence that it wanes off by the end of the season," he said. "Traditionally, it peaked around February. So if you're getting a flu shot now in early September, you can't expect it to be that effective at the tail end of the flu season. So I've always recommended people get their flu vaccine sometime in late October." (Choi, The Hill, 9/15)