Season influenza
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Seasonal Influenza: Incidence, Seasonality, and Prevention
Incidence of Seasonal Influenza in the United States
Seasonal influenza, a common respiratory illness caused by influenza viruses, affects a significant portion of the population annually. Research indicates that the incidence of symptomatic influenza in the United States varies between 3% and 11% each season, with a median value of approximately 8% . This variation is observed across different age groups, with children under 18 years experiencing a slightly higher incidence compared to adults . These findings are consistent across different methodologies, including statistical estimates from hospitalization rates and meta-analyses of cohort studies .
Seasonality of Influenza and Other Respiratory Viruses
Influenza exhibits a distinct seasonal pattern, particularly in temperate climates where outbreaks predominantly occur during the winter months . This seasonality is less pronounced in tropical regions, where influenza viruses circulate year-round, often peaking during the rainy season . The seasonality of influenza is influenced by various factors, including climatic conditions and population immunity, and tends to be more marked with increasing distance from the equator .
Clinical Presentation and Transmission
Influenza presents with a range of symptoms from mild respiratory illness to severe systemic manifestations such as fever, myalgia, and acute respiratory distress Paules2017Abdullahi2021. The virus spreads through respiratory droplets, making close contact and crowded environments significant risk factors for transmission . The incubation period is short, and the disease can lead to complications like pneumonia, especially in high-risk groups such as pregnant women and the elderly .
Prevention and Control with Vaccines
Annual vaccination remains the cornerstone of influenza prevention. The Advisory Committee on Immunization Practices (ACIP) recommends routine annual influenza vaccination for all individuals aged six months and older, barring contraindications Grohskopf2022Grohskopf2020Grohskopf2021. The vaccines available for the 2022-2023 season are quadrivalent, designed to protect against four different influenza virus strains . Vaccination not only reduces the incidence of influenza but also mitigates the severity of the disease and helps alleviate the burden on healthcare systems, especially during concurrent outbreaks of other respiratory viruses like COVID-19 Grohskopf2020Grohskopf2021.
Challenges in Vaccine Effectiveness
The effectiveness of influenza vaccines can vary from season to season due to antigenic drift, where small changes in the virus's surface proteins reduce the match between the vaccine and circulating strains . This was notably observed during the 2014/15 influenza season in Europe, where a drifted A(H3N2) virus led to reduced vaccine effectiveness . Continuous monitoring and updating of vaccine compositions are essential to address these challenges Grohskopf2022Grohskopf2021.
Forecasting and Managing Seasonal Outbreaks
Advancements in forecasting techniques have improved our ability to predict the timing, duration, and magnitude of influenza outbreaks. Real-time data assimilation methods, similar to those used in weather prediction, have shown promise in providing accurate forecasts several weeks in advance . These predictive models are crucial for public health planning and response, enabling timely interventions to mitigate the impact of seasonal influenza .
Conclusion
Seasonal influenza remains a significant public health challenge due to its variable incidence, distinct seasonality, and potential for severe outcomes. Annual vaccination is the most effective strategy for prevention, supported by continuous surveillance and updates to vaccine formulations. Advances in forecasting and real-time data analysis further enhance our ability to manage and respond to influenza outbreaks, ultimately reducing the disease burden on populations worldwide.
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Seasonal Incidence of Symptomatic Influenza in the United States
During 2010-2016, the incidence of symptomatic influenza among vaccinated and unvaccinated US residents was approximately 8%, with a range of 3% to 11% across seasons.
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Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2022–23 Influenza Season
The 2022-23 seasonal influenza vaccines should be administered during September or October, with a preference for those aged 65 years and those with contraindications.
Start of the 2014/15 influenza season in Europe: drifted influenza A(H3N2) viruses circulate as dominant subtype.
This season's influenza season in Europe begins with influenza A(H3N2) viruses dominating, potentially reducing vaccine effectiveness and highlighting the importance of treating influenza with antivirals in addition to vaccination.
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