Southern Hemisphere Flu Season Could Provide Clues on Upcoming US Flu Season

CDC Southern Hemisphere Flu Update

Flu activity is waning in much of the Southern Hemisphere, signaling that the 2024 flu season is nearing its end in many countries. Typically, flu follows a seasonal pattern in Southern Hemisphere countries, with activity usually occurring between April and September and sometimes lasting until October or November. Data from this year's Southern Hemisphere flu season show that seasonal flu continues to pose a significant threat, causing many illnesses, hospitalizations, and deaths across the globe.

During the 2024 Southern Hemisphere flu season, most countries experienced similar levels of flu activity compared to trends observed in prior seasons (2017-2019 and 2022-2023 flu seasons). Two countries in South America and Southern Africa experienced very high influenza virus detection levels, and three South American countries experienced earlier activity compared with before the COVID-19 pandemic.

Flu trends in different regions of the Southern Hemisphere have been characterized as follows:

South America: Flu activity has been primarily attributed to influenza A(H3N2) viruses, though influenza A(H1N1) and B viruses have also been reported. Compared with flu trends observed before the COVID-19 pandemic, Argentina, Chile, and Uruguay reported data suggesting an earlier start to their 2024 flu seasons. However, compared to the 2023 flu season no countries experienced earlier starts during the 2024 season. Notably, Chile experienced very high levels of flu activity and Chile, Ecuador, and Uruguay also saw high levels of severe flu-related disease, including very high levels of flu-related hospitalizations. Most other South American countries reported moderate or low flu activity levels similar to levels observed in prior seasons. The 2024 flu season has ended earlier as compared to pre-COVID-19 pandemic trends in Argentina, Chile, Paraguay, Uruguay, and Bolivia.

Africa: Influenza A(H1N1) viruses have predominated in Africa, though influenza B detections have increased in recent weeks. Zambia experienced an earlier start to their season compared with the previous flu season. So far, most African countries have remained at low and moderate levels of influenza detections during the 2024 season. Zambia 1 experienced very high levels of flu activity and severe flu-related disease, including very high levels of flu-related hospitalizations, and South Africa 2 had high levels of flu-related hospitalizations. Activity in South Africa and Zambia continues, with both countries having B/Victoria viruses make up a larger proportion of detections in recent weeks.

Oceania: Data from Australia, which provides most of the data for the Oceania region, indicate that influenza A(H3N2) viruses were predominant among subtyped viruses. Australia's 2024 flu season had a similar start week compared to both pre-pandemic trends and the previous 2023 season. Influenza detections briefly reached moderate levels but have been decreasing in recent weeks, though several jurisdictions continue to show increasing flu activity 3.

These findings are based on data that have been retrieved from countries with flu data uploaded to the World Health Organization's (WHO) FluNet data reporting system and the Pan American Health Organization's Influenza, SARS-CoV-2, RSV, and other Respiratory Viruses Regional Situation report. Over the past several years, an increasing number of countries have reported flu data to WHO FluNet, as a result of continued investments in global flu surveillance. These improved data help CDC better understand which influenza viruses are circulating globally, and how flu activity is impacting different populations.

It is important to consider Southern Hemisphere flu activity before the start of the flu season in the United States; however, it should be noted that what happens in the Southern Hemisphere does not always predict outcomes in the United States. Factors such as different predominant influenza viruses and variations in population immunity can contribute to these differences.

While activity has differed across the Southern Hemisphere during the 2024 flu season, what remains consistent across regions is the significant illness and hospitalizations associated with flu.

Even though we cannot predict what will happen in the United States this upcoming season, we know that flu has the potential to cause significant illness, hospitalizations, and deaths, all of which underscore the importance of annual flu vaccination. Vaccination remains the best defense against flu and even if vaccination does not prevent the risk of flu entirely, it can help reduce the severity of flu illness in people who get flu despite being vaccinated. In the United States, September and October are generally good times to be vaccinated against flu.