What We Can Learn from Flu in the Southern Hemisphere

September 8, 2023, 5:30 PM EDT

Updates on respiratory illness and vaccine-preventable diseases.

Activity for respiratory viruses like the flu is currently low across most of the United States.

This is a map of the United States that shows outpatient respiratory illness activity by state and territory determined by data reported to ILINET. This system monitors visits for respiratory illness that includes fever plus a cough or sore throat, also referred to as ILI, and not laboratory confirmed influenza. ILINET may capture patient visits due to other respiratory pathogens that cause similar symptoms. For Week 34, ending August 26, 2003, six states reported low activity, 44 states and territories reported minimal activity, and one state reported moderate activity.

The level of flu activity can vary from year to year. In past years, the flu has caused millions of people in the United States to get sick, with hundreds of thousands of hospitalizations and tens of thousands of deaths.

This is a picture of a pyramid that shows the estimated range of annual burden of flu in the U.S. from 2010-2020. CDC estimates that flu has resulted in 9 million – 41 million illnesses, 140,000 – 710,000 hospitalizations and 12,000 – 52,000 deaths annually between 2010 and 2020.

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CDC expects flu activity in the United States to increase in the coming weeks. Flu in the Northern Hemisphere is often caused by viruses similar to those that spread in the Southern Hemisphere during their winter flu season.

This year’s Southern Hemisphere flu season had an earlier peak during April to May, compared to typical peaks in June to July. Of the specimens that tested positive for Influenza, approximately 90% of the viruses were Influenza Type A and 10% were Influenza Type B. What happens in the Southern Hemisphere does not necessarily predict what will happen in the Northern Hemisphere. This is because different flu viruses may spread more commonly in different parts of the world and immunity may vary between populations.

This is a bar graph that shows the weekly number of rtPCR-confirmed hospitalizations in 5 South American countries (Argentina, Brazil, Chile, Paraguay, and Uruguay) resulting from influenza-associated severe acute respiratory infections (SARI), including by influenza type and subtype, between MMWR weeks 13 through 27. Data was collected through SARINET/REVELAC-I Sentinel site this 2023 Southern Hemisphere flu season. The graph shows that SARI hospitalizations began to increase during week 17 and peaked during week 20, then continued to decline through week 27. The predominant subtype detected was influenza A(H1N1)pdm09, followed by influenza B(Victoria). The graphic also shows unsubtyped influenza A viruses. Very few influenza A(H3N2) viruses were detected.

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The U.S. 2023-2024 flu vaccines have a similar vaccine virus composition as the 2023 Southern Hemisphere flu vaccines. A new CDC study, which looked at children and adults in five South American countries during the 2023 winter flu season (our summertime), shows flu vaccine performed well against those viruses. This study found that people who had received a flu vaccine were half as likely to be hospitalized with flu compared to people who had not been vaccinated. This finding shows that flu vaccine provides protection against serious outcomes.

This season’s U.S. flu vaccines could provide similar benefit against serious flu outcomes during the upcoming season if the same viruses continue to spread here.

Last flu season started early in the U.S., with activity increasing nationally at the beginning of October 2022 and peaking in early-December 2022, causing an estimated 27 million illnesses and 300,000 hospitalizations. To prepare for this flu season, Americans should begin getting vaccinated against flu, and ideally, be vaccinated before the end of October. Check vaccines.gov to find a flu vaccine near you.

Learn more about flu in the Southern Hemisphere.

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