Epidemiologists this summer are closely watching the sharp rise in flu activity in the Southern Hemisphere, where it is currently winter, to see if it’s a sign of what’s to come in the U.S.
The Australian Department of Health and Aged Care reported an increase in influenza-like illness activity in almost all jurisdictions in the final two weeks of May.
The illness activity was highest in young children ages 5 to 9, followed by children 4 years and younger, and then those ages 10 to 14.
"There are several countries experiencing higher or earlier activity compared with years prior to the COVID-19 pandemic, while others are experiencing typical activity for this time of year or activity that is lower than historical trends," Dr. Carrie Reed, chief of CDC’s Influenza Division Epidemiology and Prevention branch, told Fox News Digital.
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A review of both the World Health Organization’s surveillance data and the latest Australian Surveillance Report shows the increasing flu activity in Australia is currently within historic ranges, she added.
"However," she said, the CDC "continues to monitor influenza trends across the globe."
The flu season in the Southern Hemisphere usually occurs between April and September — while the flu season occurs between October through May in the Northern Hemisphere, Reed said.
"What goes on in the Southern Hemisphere is frequently an important predictor of things to come regarding the flu season," Dr. Aaron Glatt, chief of infectious diseases at Mount Sinai South Nassau Hospital in Long Island, New York, told Fox News Digital.
The activity often portends what will happen in the United States in the following winter, he said — while also noting, "There are no hard and fast rules."
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He added, "We just have to wait and see whether it is accurate or not every year."
Reed also emphasized that Southern Hemisphere flu activity does not necessarily predict what will happen in the U.S.
"This is because different influenza viruses may predominate in different parts of the world and population immunity may be different between populations in the Northern and Southern Hemispheres," Reed said.
Most areas across the country report "minimal" or "low" influenza-like activity, which is typical for this time of year, according to the CDC’s Weekly U.S. Influenza Surveillance Report.
Between 2010 and 2020, flu resulted in 9 million to 41 million illnesses every year, 140,000 to 710,000 annual hospitalizations, and 12,000 to 52,000 annual deaths — yet the exact number is unknown, according to the CDC.
Many people recover from the flu without seeking care, while some may seek care later in their illness when the flu can no longer be detected on a respiratory sample.
"Sensitive flu tests are only likely to detect flu if performed within a week after onset of illness," the agency noted on its website.
"In addition, some commonly used tests to diagnose flu in clinical settings are not highly sensitive and can provide false negative results (i.e. they miss true flu infections)."
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States are only required to report deaths from the flu for children less than 18 years of age, so many cases of adults are not reported.
Because surveillance data may "drastically under-represent the true burden of flu in the U.S.," the agency uses statistical models to estimate the annual number of seasonal flu-related cases, hospitalizations and deaths, per its website.
Generally, people start getting sick from the flu around October, with clinics starting to get busy between December and February, according to recent report.
Experts point to predictable human activities that spread flu every year — children going back to school and people traveling to see loved ones over the holidays.
But it’s the unknown variables, such as the weather and how many people in the population will be immune to the flu during the winter, that make it difficult to predict the upcoming flu season.
Human behavior and social interactions also play a role, so masking by a large number people could help make a difference in blunting the spread of flu.
"There is no way to know what flu activity in the United States will look like this season," Reed said.
"Each flu season is different, and the best way to protect yourself is by getting an annual flu vaccine."
September and October are generally good times to be vaccinated against flu, she said.
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People accustomed to normally getting their annual flu vaccine will have the option to also get vaccinated for an updated COVID-19 vaccine as well as potentially the new respiratory syncytial virus (RSV) vaccine, two of which are now FDA-approved for adults over the age of 60, according to recent reports.
The CDC’s Advisory Committee on Immunization Practices will meet on June 21-23 to provide recommendations for specific patient populations to get vaccinated for RSV.
