Influenza: annual seasonal severity

Abstract

Purpose of review Influenza remains a major cause of morbidity and mortality. The 2017–2018 season was one of the most severe in the past decade. The exact factors determining the severity of a particular influenza season are complex and often poorly understood. Recent findings Factors impacting annual influenza severity include characteristics of the specific virus, influenza vaccination, and antiviral use. Although viral virulence factors are important in this context and our knowledge of these is growing, there is a complex interplay between expression of these factors and their impact on a particular patient population. Vaccination has demonstrated efficacy in preventing disease, but vaccination rates remain sub-optimal and vaccine effectiveness can vary significantly between influenza strains and patient populations. Finally, while antiviral treatment is available and has shown benefits, many patients with influenza do not receive treatment. Summary Strides have been made in recent years towards understanding the many factors that contribute to the severity of any particular influenza season. Obvious areas for improvement include improved vaccination rates and antiviral use. Additionally, a more complete understanding of reasons for poor strain and population-specific vaccine effectiveness may help reduce the severity of future influenza seasons.

A mosaic hemagglutinin-based influenza virus vaccine candidate protects mice from challenge with divergent H3N2 strains

“…4,5 For example, vaccine effectiveness for H3N2-caused disease in the 2017–2018 season in the US was only 25%, possibly due to circulating H3N2 strains demonstrating antigenic drift compared to the vaccine strain. 6 In general, H3N2-dominated seasons are associated with more severe illness and higher mortality than H1N1- or B-predominant seasons, especially among the elderly population. 7–10 A universal influenza virus vaccine that confers protection against drifted virus strains, especially for H3N2, would abolish the need for annual reformulation and could afford long-term protection.…”

Influenza Vaccination: Effectiveness, Indications, and Limits in the Pediatric Population

“…Type A virus can be classified using surface antigens hemagglutinin and neuraminidase. In the last years, the main circulating A strains have been the H1N1 pandemic strain and the H3N2 strain (1). The two major antigenically B viruslineages are B/Victoria and B/Yamagata (2).…”

“…Each year both B virus strains co-circulate and are responsible for about 25% of influenza disease cases (3). However, the proportion of circulating influenza B strains varies by season and countries (4–6) Different strains have different virulence and prefer to infect different age clusters, probably based on previous exposure to antigenically similar viruses (1). In particular, influenza B virus is likely to affect children, and young adults.…”

Influenza “Trains” the Host for Enhanced Susceptibility to Secondary Bacterial Infection

“…Influenza is a major global health concern with seasonal outbreaks and pandemics that result in significant morbidity and mortality (1, 2). The 2017-2018 influenza season showed significant increases in hospitalizations confirmed to be due to influenza for both adults and children in the United States alone (3). While vaccination provides significant protection, the ability to predict the specific influenza virus strains to be incorporated into the following year’s vaccine sometimes fails (4, 5), which can lead to a virus-vaccine mismatch and reduced vaccine efficacy (3).…”

“…The 2017-2018 influenza season showed significant increases in hospitalizations confirmed to be due to influenza for both adults and children in the United States alone (3). While vaccination provides significant protection, the ability to predict the specific influenza virus strains to be incorporated into the following year’s vaccine sometimes fails (4, 5), which can lead to a virus-vaccine mismatch and reduced vaccine efficacy (3). In addition to having to treat patients early in infection, increasing resistance to neuraminidase (NA) inhibitors (oseltamivir, zanamivir) and M2 channel inhibitors (amantadine, rimantadine) has limited the utility of antiviral drugs (6, 7).…”