When flu season begins in the Northern Hemisphere, the Southern Hemisphere is close to wrapping up its own flu season, which usually runs from April to October each year. Flu activity in the Southern Hemisphere can give public health experts, such as those at the Centers for Disease Control and Prevention (CDC), an indication of how the upcoming flu season may develop in the Northern Hemisphere.
Flu season in the Northern Hemisphere does not always reflect the flu season in the Southern Hemisphere. Different flu viruses may circulate in the two hemispheres, and the people who live in those areas may not have the same immunity to those viruses.
However, with the timing, peak, and severity of flu seasons in greater flux than before the COVID-19 pandemic, monitoring flu activity in the Southern Hemisphere can still be helpful to experts in signaling what could happen in the Northern Hemisphere flu season. It can also provide a sense of how well flu vaccines may protect against flu viruses in the Northern Hemisphere during flu season, if those viruses are the same as those that circulated in the Southern Hemisphere.
Southern Hemisphere flu season as an indicator for Northern Hemisphere flu activity
For example, in the 2023 Southern Hemisphere flu season, researchers gathered preliminary data on flu vaccine effectiveness from five countries in South America. Researchers found that flu vaccines in those countries gave significant protection against the flu viruses that were in high circulation. Flu hospitalizations due to Influenza A(H1N1) were reduced by 55%; hospitalizations due to Influenza B viruses were reduced by 46%.
Subsequently, the CDC reported in September 2023 that the majority of flu viruses circulating in the early weeks of the 2023–2024 flu season in the United States were the same ones as those that circulated in South America. Because the composition of flu vaccines in the U.S. was similar to those in the Southern Hemisphere, the CDC noted that possible that the 2023–2024 U.S. flu vaccines could have similar protective effects as the Southern Hemisphere flu vaccines did in 2023. (As it happened, preliminary estimates of U.S. flu vaccine effectiveness in the 2023-2024 flu season showed that they provided protection similar to or even better than flu vaccines in past seasons.)
Looking at the most recent Southern Hemisphere flu season, flu trends in 2024 showed that the different continents and regions in that hemisphere experienced high levels of different flu strains. For example, Influenza A(H3N2) predominated in the continents of South America and Australia. In contrast, the highest source of flu activity in the African continent came from Influenza A(H1N1) viruses. Similarly, levels of flu activity and when flu activity started in the Southern Hemisphere varied by country and region in 2024.
Regardless of the Southern Hemisphere flu season that precedes it, the start, progress and impact of the U.S. flu season depend on three things. These include the flu strains that circulate; variations in population immunity to those circulating flu strains; and the effectiveness of flu vaccines in each season.
So how this recent Southern Hemisphere flu season will influence the flu season here remains to be seen. Nevertheless, the seasonal flu has demonstrated its high potential for causing illness. In the 2023–2024 U.S. flu season, preliminary estimates from the CDC suggest that the flu was responsible for between 380,000 and 900,000 hospitalizations, and between 17,000 and 100,000 deaths, in the United States.
Getting vaccinated for the flu at the beginning of the flu season is the best way to protect yourself against the flu—and from severe flu-related illness as described above. People who are vaccinated but get sick from the flu can still benefit from the protection that the flu shot provides against more severe illness and hospitalization.
The CDC provides weekly updates on U.S. flu activity, as well as flu-related illnesses, hospitalizations and deaths. You can track these updates on the CDC website as the flu season progresses.
Read the most current CDC report on flu activity in the United States
