“The flu activity data for the 2025-26 season (…) showed that in week 48, which began on November 24, flu activity is epidemic,” stated Raquel Guiomar, head of the National Influenza and Other Respiratory Viruses Reference Laboratory at the Insa, to Lusa.
Over the past two to three weeks, there has been “an increase in the number of laboratory-confirmed flu virus infections,” reported by the Sentinel Physicians Network, according to the researcher.
The Flu and Other Respiratory Viruses Epidemiological Surveillance Bulletin, released today by Insa, indicates that in the week from November 24 to November 30, there was a rise in the incidence rate of severe acute respiratory infections compared to previous weeks, reaching 10.5 cases per 100,000 inhabitants.
Age groups from zero to four years and 65 years and older showed the largest increase, with the latter having the highest value.
During this week, 82 severe acute respiratory infection cases were admitted to reporting Local Health Units (ULS), with 10 intensive care unit (ICU) admissions reported.
“All the cases had underlying chronic illness and were recommended for seasonal flu vaccination, three of whom were vaccinated. This week, the proportion of flu in ICU was 6.0%, up from the previous week’s 1.6%,” noted the bulletin.
Guiomar explained that disease severity can typically be linked to the circulating virus, but it is often also due to the individual conditions of each patient.
Regarding the circulating viruses, the researcher noted that the viruses infecting humans belong to two types, A and B, with predominantly influenza A viruses circulating this season.
“We have been detecting both subtypes of type A flu virus, namely the AH1N1 virus and the AH3N2 virus,” with a current “slight predominance” of AH1N1.
However, she emphasized that the co-circulation of the two influenza A virus subtypes might change in the coming weeks.
Guiomar indicated that, recently, the AH3N2 subtype has attracted more attention both nationally and internationally, explaining that this virus, which circulated in the northern hemisphere last season and also in the southern hemisphere, has mutated, potentially increasing its transmission and infection capability.
These mutations differentiate it from previous season viruses and those covered by the 2025-2026 vaccine.
According to the researcher, the new subgroup of the AH3N2 subtype, designated K, has already been detected in Portugal and accounts for about 45% of the characterized AH3N2 viruses so far.
When asked if it is likely to grow in the coming weeks, she stated that it is a possibility due to its characteristics, noting, by citing European data, that in countries that experienced an earlier season than Portugal, these were the viruses that saw increased circulation when flu activity began.
The specialist also mentioned that the epidemic arrived “three to four weeks earlier” than usual in Portugal, approaching the pattern observed in 2023-24.
“But we have not yet reached peak activity. Essentially, we are currently at epidemic flu activity with an upward trend,” she asserted.
Regarding other respiratory viruses, the circulation of SARS-CoV-2 is currently low, and respiratory syncytial virus (RSV) activity remains low, although it may increase in the coming weeks.
