CDC Tracks Ongoing Flu Activity in the Southern Hemisphere

As flu activity in the United States continues at low levels that are typical for this time of year, CDC also is tracking flu activity in the Southern Hemisphere, which generally has its “flu season” at this time. Surveillance data show that several Southern Hemispheric countries are currently experiencing higher or earlier flu activity compared to what was seen prior to the COVID-19 pandemic; however, activity varies by country or region.

According to flu activity data submitted to the World Health Organization:

• Influenza A(H1N1) viruses have been most commonly reported.
• Chile has reported an early start to their flu season.
• Argentina is experiencing typical levels of flu activity.
• South Africa has been experiencing high flu activity for this time of year.
• Australia is experiencing typical levels of flu activity.
• While not in the Southern Hemisphere, Mexico has been experiencing abnormally high flu activity in May and June. Typically, Mexico has low flu activity during this time of the year.

Limited data are available on flu-related hospitalizations in Southern Hemisphere countries at this time, but South Africa has reported a “moderate” level of flu related hospitalizations, while Australia has reported a “normal” level of flu-related hospitalizations compared with historic trends.

Flu season in the Southern Hemisphere usually occurs between April and September, compared with October through May in the Northern Hemisphere. Experts often look at Southern Hemispheric flu activity for clues as to what could happen in the United States. This 2023 Southern Hemisphere flu season, some Central and South America countries have early or intense influenza A(H1N1) activity, while others have typically timed seasons with a mix of influenza A(H3N2), influenza A(H1N1), and influenza B/Victoria.

What happens in the Southern Hemisphere does not necessarily predict what will happen next in the Northern Hemisphere because different influenza viruses may predominate in different parts of the world and immunity may be different between populations. Nevertheless, flu seasons in the Southern Hemisphere remind us of what could happen in the United States and help us think of scenarios to better prepare for our flu season.

While flu activity is always challenging to predict, it has been especially unpredictable since the COVID-19 pandemic. The continued impact of COVID-19 on the circulation of respiratory viruses like influenza, remains unclear, but ongoing global flu surveillance remains important for trying to understand flu virus seasonality going forward. CDC will continue to monitor ongoing flu activity in the Southern Hemisphere as their season progresses.