Flu Epidemic Declares Early Arrival in Portugal, New Viral Strain a Concern
Portugal is currently experiencing an influenza epidemic with a concerning upward trend, according to the National Institute of Health Doctor Ricardo Jorge (INSA). Data indicates the epidemic status began in week 48,starting November 24th,with a noticeable rise in confirmed cases and hospitalizations,including admissions to intensive care units.
Raquel Guiomar, head of the National Reference Laboratory for the Flu Virus and Other Respiratory Viruses at INSA, reported a notable increase in laboratory-confirmed influenza cases reported through the Sentinel Doctors Network over the past two to three weeks. The latest epidemiological bulletin reveals a surge in severe acute respiratory infections, reaching 10.5 cases per 100,000 inhabitants between November 24th and 30th.
The most affected age groups are young children (0-4 years) and the elderly (65+), with the latter demonstrating the highest incidence rate. Currently, 82 severe respiratory infection cases are hospitalized, with 10 requiring intensive care. A significant portion of those hospitalized had underlying chronic conditions and were recommended to receive the seasonal flu vaccine, though only three had been vaccinated. the proportion of ICU cases attributed to influenza has risen from 1.6% to 6.0% in the last week.
While the severity of illness is frequently enough linked to the circulating virus and individual patient health,researchers are particularly focused on the evolving influenza A strains. Currently, both AH1N1 and AH3N2 subtypes are circulating, with a slight prevalence of AH1N1. Though, attention is growing regarding the AH3N2 subtype, which has undergone mutations after circulating in both the northern and southern hemispheres.
These mutations differentiate the current AH3N2 strain – designated K – from previous iterations and from the strains included in the 2025-2026 flu vaccine. The K subtype already accounts for approximately 45% of characterized AH3N2 viruses in portugal, and experts suggest it could become more dominant in the coming weeks, mirroring trends observed in other European countries.
Notably, this year’s epidemic arrived three to four weeks earlier then usual, mirroring the pattern seen in the 2023-24 season. Guiomar emphasized that while the epidemic is currently increasing, peak activity has not yet been reached.
Other respiratory viruses, such as SARS-CoV-2, are currently circulating at low levels, and respiratory syncytial virus (RSV) remains at low activity, though a potential increase is anticipated in the near future.
