Influenza with an increasing trend and higher-than-expected mortality in those over 45 years of age

Data from the Epidemiological Surveillance Bulletin for Influenza and other Respiratory Viruses

Hospitals reported 38.771 cases of respiratory infection and 6.262 cases of influenza in the 2023/2024 season, according to the Ricardo Jorge Institute, which points to higher-than-expected mortality from all causes in people over 45 years of age.

According to the latest Epidemiological Surveillance Bulletin for Influenza and other Respiratory Viruses, from the National Institute of Health Doutor Ricardo Jorge (INSA) and which refers to the period between December 25th and 31st (week 52), 36 cases of influenza were reported by the 21 Intensive Care Units (ICU) that sent information.

Of these 36 cases, the influenza A(H1N1) virus was identified in one case, the influenza A virus was not subtyped in 34 cases and it was not possible to identify the type of virus in one of the cases. Of the patients, 32 had a chronic illness, 33 had a recommendation for vaccination against seasonal flu, but only nine were vaccinated (in six cases the information is unknown).

Since the beginning of the surveillance season (October 2), 85 cases of influenza have been reported by the ICUs that collaborate in surveillance. Of the total cases, 85,9% (73) have an underlying chronic disease and 91,8% (78) have a recommendation for vaccination against seasonal flu, but only 36,1% (22) were vaccinated.

In the last week of the year, 1.472 positive cases for the flu virus were identified, of which 1.372 were type A and 49 were type B. In 147 of the cases subtype A(H1) was identified and in 14 subtype A(H3).

The bulletin also indicates that, of the samples analyzed since the beginning of surveillance in the 2023/2024 season (2 October), the influenza A(H1) virus was detected in 88,7% of influenza cases.

Since October, 118 cases of co-infection with the flu virus and SARS-CoV-2, which causes Covid-19, have also been detected.

According to INSA, since the beginning of the surveillance season, other respiratory viruses have been detected in 106 cases of acute respiratory infection/flu syndrome (ARI/SG), of which three are cases of co-infections. The viruses detected were rhinovirus (67), respiratory syncytial virus (17), coronavirus (6), parainfluenza (13), adenovirus (1) and enterovirus (5).

As for the Respiratory Syncytial Virus (RSV), INSA reports an epidemic activity “with a probable decreasing trend” in the number of new hospitalizations in children under two years of age. However, the institute states that this trend “must be interpreted with caution” given possible delays in notification and the effect of the festive period.

INSA also states that the majority of subtype A(H1) viruses characterized to date (73%) have similar genetic characteristics to the virus included in the flu vaccine for the 2023/2024 season, but emphasizes that subtype A viruses (H3N2) characterized so far have «a set of genetic mutations in relation to the vaccine virus of this season», being considered «genetically different» from the strain included in the flu vaccine.

According to the bulletin, the proportion of influenza in ICU has been increasing in the last two weeks, reaching 17,1% in the last week of the year, a value above that recorded in similar periods.

The Ricardo Jorge Institute also recalls that the European Center for Disease Prevention and Control (ECDC) states that "non-pharmacological measures had an impact on the low circulation of respiratory pathogens and the reduction of the population's immunity, which could worsen the impact of respiratory diseases this winter”.

«Thus, the increase in the proportion of influenza cases in ICU observed in week 52/2023 may reflect the increase in the circulation of the influenza virus, in the context of increased social interaction, without individual protection measures, after a period of reduced circulation during the covid-19 pandemic», he adds.