As winter continues its grip across Europe, concerns are rising about the ongoing influenza season. While the season was described as low-to-moderate in magnitude, with influenza A(H1N1) as the dominant strain, experts are now closely monitoring developments, with some suggesting an earlier-than-usual peak this year.
A Protracted 2023-2024 Season
The influenza season, as reported by the Norwegian Institute of Public Health (Folkehelseinstituttet), was notable for its length. Influenza A(H1N1) predominated, but was accompanied by cocirculating A(H3N2) viruses and a smaller proportion of influenza B/Victoria-lineage viruses. This suggests a complex interplay of strains, potentially contributing to the protracted nature of the season.
Prior to the season, population immunity levels were assessed. Following the A(H1N1)-dominated season, there was an increase in antibodies against A/Victoria/2570/2019(H1N1) of the pdm09 5a.2 clade. However, immunity was lower against the subclade 5a.2a.1. This is particularly relevant in the observed immunity gap in the 0-4 year age group, a consequence of reduced influenza circulation during the COVID-19 pandemic. Seroprevalence against A(H3N2) remained stable and there was no indication of immune evasion by the new A/Thailand/8/2022 strain.
Early Peak Possible This Winter
Norway is preparing for another influenza season, and experts are warning that the peak could arrive as early as Christmas. This contrasts with the typical pattern of epidemics beginning in December, peaking in February, and subsiding by late March. In a normal season, up to 20% of the population may become infected, although most experience mild or no symptoms.
Last winter, Norway saw over 100,000 influenza-related consultations, more than 10,000 hospital-confirmed cases, nearly 300 intensive care admissions, and around 400 deaths. Additional fatalities linked to complications like pneumonia and heart attacks were also attributed to influenza. These figures underscore the significant burden influenza can place on healthcare systems and the population.
Unpredictability and a New Variant
The current outlook is described as particularly unpredictable. Early data suggest the epidemic may start sooner than usual, driven by a dominant A(H3N2) variant that differs significantly from previous strains and those included in current vaccines. This mismatch between circulating strains and vaccine composition raises concerns about potentially lower vaccine effectiveness and a higher risk of severe illness, especially among young children and older adults.
Reports from England indicate reduced vaccine effectiveness in seniors, increasing hospitalization risks. However, experiences from Hong Kong suggest the epidemic with this strain is already waning there, with a normal disease burden. This highlights the regional variability of influenza and the challenges in predicting its course.
European Surveillance and Monitoring
Surveillance efforts are ongoing across Europe to monitor the spread of influenza and other respiratory viruses. The European Respiratory Virus Surveillance Summary (ERVISS) provides a weekly integrated epidemiological summary for influenza, respiratory syncytial virus (RSV), and SARS-CoV-2. This comprehensive surveillance system helps track the evolution of these viruses and inform public health responses.
The European Centre for Disease Prevention and Control (ECDC) complements weekly surveillance with epidemic intelligence activities, monitoring both official and unofficial sources for outbreaks and other influenza-related developments. This proactive approach is crucial for early detection and response to emerging threats.
The Importance of Vigilance
Given the unpredictability of this year’s influenza season and the potential for a mismatch between circulating strains and vaccines, experts are urging vigilance. Close monitoring of developments and maintaining preparedness are essential to mitigate the impact of the epidemic. While influenza epidemics are an annual occurrence, their timing and severity remain uncertain, necessitating a continued focus on surveillance, vaccination, and public health measures.
The Global Influenza Surveillance and Response System (GISRS) plays a critical role in tracking influenza viruses globally, including through sentinel and non-sentinel surveillance systems and the FluNet data platform. This international collaboration is vital for understanding the evolution of influenza viruses and informing vaccine development.
