Portugal is currently facing a surge in influenza cases, with health officials confirming the 2025-26 flu season has reached epidemic levels weeks earlier than typical. the National Institute of Health Doctor Ricardo Jorge (INSA) reports rising hospitalizations, including admissions to intensive care, linked to both influenza A and B strains. Health authorities are notably concerned about a newly identified subgroup of the AH3N2 virus, designated K, which exhibits concerning mutations and may reduce vaccine effectiveness.
Portugal is currently experiencing a growing influenza epidemic, with confirmed cases – including hospitalizations in intensive care units – on the rise, according to a recent alert from the National Institute of Health Doctor Ricardo Jorge (INSA).
The country’s flu activity for the 2025-26 season has reached epidemic levels as of week 48, beginning November 24, confirmed Raquel Guiomar, head of the National Reference Laboratory for Influenza and Other Respiratory Viruses at INSA.
Over the past two to three weeks, laboratory-confirmed cases of influenza infection, as reported by the Sentinel Physicians Network, have increased, Guiomar explained.
The latest epidemiological surveillance bulletin from INSA, released today, reveals that the incidence rate of acute respiratory infections rose between November 24 and 30, reaching 10.5 cases per 100,000 inhabitants. This increase represents a rise compared to previous weeks.
The most significant increases in cases were observed among children aged zero to four years and adults 65 years and older, with the latter group experiencing the highest rates. This highlights the vulnerability of these age groups to severe illness.
During the week of November 24-30, 82 severe cases of acute respiratory infection were admitted to Local Health Units (ULS) that reported data. Of those, 10 patients required admission to intensive care units (ICU).
According to the bulletin, all ICU patients had underlying chronic conditions and were recommended to receive seasonal flu vaccination, with three having already been vaccinated. The proportion of flu cases in ICUs rose to 6.0% this week, up from 1.6% the previous week.
Guiomar noted that the severity of the illness can be linked to the circulating virus strain, but is often also influenced by individual patient health factors.
Currently, both type A and type B influenza viruses are circulating in the population, with type A viruses being predominant during this season.
“We have been detecting both subtypes of influenza A virus, both AH1N1 and AH3N2,” Guiomar said, adding that AH1N1 currently has a slight edge in prevalence.
However, she cautioned that the co-circulation of both influenza A subtypes could shift in the coming weeks.
The AH3N2 subtype has been receiving increased attention both nationally and internationally, Guiomar explained. This virus, which circulated in the Northern Hemisphere last season and also in the Southern Hemisphere, has evolved and acquired mutations that may enhance its transmissibility and infection rate.
These mutations differentiate it from viruses circulating in previous seasons and from those included in the 2025-2026 vaccine.
According to the researcher, the new subgroup of the AH3N2 virus, designated K, has already been detected in Portugal and accounts for approximately 45% of the AH3N2 viruses characterized so far.
When asked if the prevalence of the AH3N2 subtype is expected to increase in the coming weeks, Guiomar said it is a possibility, given its characteristics. Citing European data, she noted that countries with an earlier flu season than Portugal saw an increase in circulation of these viruses once flu activity began.
The specialist also noted that the epidemic arrived “three to four weeks earlier” than usual in Portugal, mirroring the pattern observed in 2023-24.
“However, we have not yet reached peak activity. Currently, we are experiencing an epidemic of flu activity with a growing trend,” she emphasized.
Regarding other respiratory viruses, circulation of SARS-CoV-2 is currently low, and respiratory syncytial virus (RSV) remains at low activity levels, although it may increase in the coming weeks.
