PAHO updates influenza situation in the Americas as the winter season advances in the Northern Hemisphere

Following a 29% increase in severe cases in the Southern Hemisphere in 2025 and the onset of winter in the north, PAHO calls for strengthened preparedness to reduce the impact of respiratory viruses.

Washington, D.C., December 8, 2025 (PAHO) - The Pan American Health Organization (PAHO) is urging countries in the Americas to adjust their health service preparedness and response plans as the Northern Hemisphere enters the season of increased influenza and other respiratory virus circulation.

According to a recent PAHO epidemiological alert, the combination of vaccination, surveillance, hospital preparedness, timely access to diagnosis and treatment, and personal preventive measures are crucial to reducing disease burden, protecting vulnerable populations, and avoiding potential strain on health systems.

According to data reported through the end of November, global influenza activity, driven by influenza A viruses, continues to rise. A sustained increase is being observed, particularly in North America, while activity remains relatively low in the rest of the Americas.

In the Caribbean and Central America, influenza A(H1N1)pdm09 predominates, while in Canada and the United States, there is greater circulation of the A(H3N2) subtype, including a progressive increase in subclade K, already widely detected in Europe and Asia. Although no significant increase in severity has been observed so far, A(H3N2)-dominated seasons tend to affect older adults more severely.

Preliminary evidence suggests that the current vaccine maintains protection levels similar to previous seasons, particularly in preventing severe cases and hospitalizations.

End of the 2025 season in the Southern Hemisphere

The Southern Hemisphere concluded the season with a 29% increase in reported severe acute respiratory infection cases compared with 2024. Respiratory syncytial virus (RSV) primarily affected infants under six months, while influenza mainly affected older adults.

Influenza circulation was initially dominated by A(H1N1)pdm09 and later by A(H3N2), with no evidence of increased severity. By early November, subclade K had not been detected in South America.

RSV circulated earlier than usual, reaching higher levels than in 2023 and 2024, particularly in some South American countries. SARS-CoV-2 detection was higher early in the year, followed by the predominance of the XFG variant, with no signs of increased severity.

A study in eight Southern Hemisphere countries showed that the 2025 vaccine provided moderate protection against influenza A-related hospitalizations and higher protection against influenza B.

PAHO recommendations

PAHO urged countries to strengthen surveillance for influenza, RSV, and SARS-CoV-2, ensure continuous data reporting, and maintain regular sample submission for sequencing to detect variants that could alter virus transmission or severity.

The Organization also reminded countries that unusual respiratory events-such as severe cases with no identified cause or outbreaks outside expected circulation periods-should be promptly investigated and reported under the International Health Regulations (IHR).

PAHO recommended reviewing and updating national clinical management guidelines as needed, strengthening access to early diagnosis, and ensuring the availability of antivirals for those at higher risk of complications.

Health services should prepare for a potential increase in hospitalizations during the winter months when multiple respiratory viruses circulate simultaneously.

The Organization reiterated the importance of vaccination against influenza, COVID-19, and RSV, especially for older adults, people with chronic conditions, pregnant women, young children, and healthcare workers. Vaccination during pregnancy and the use of monoclonal antibodies in newborns have been shown to significantly reduce the risk of severe RSV disease. For COVID-19, priority groups are recommended to receive booster doses every six to twelve months.

Simple measures such as handwashing, respiratory etiquette, and staying home when experiencing fever or symptoms are essential to limit transmission.