Clinical characteristics and severity of influenza infections by virus type, subtype, and lineage: A systematic literature review

Abstract

AimStudies carried out in the early 2000s found that the number of influenza‐associated hospitalizations and deaths was highest in seasons dominated by A(H3N2), suggesting that the clinical presentation and severity of influenza may differ across virus types, subtypes, and lineages. We aimed to review the studies that examined this hypothesis.MethodWe conducted a literature review of studies published until January 2017 that compared the clinical presentation, disease severity, and case‐fatality ratio of influenza patients infected with different virus types (A, B), subtypes (pre‐pandemic A(H1N1), A(H1N1)p, A(H3N2)), and lineages (Victoria, Yamagata).ResultsThe literature search resulted in over 1700 entries: After applying in‐ and exclusion criteria, 47 studies were included in the literature review. Studies showed a wide diversity in setting and populations. Only a minority of studies provided results adjusted by patient's age and other potential confounders. There were very few differences in the clinical presentation of patients infected with different influenza viruses. We found weak evidence that the A(H1N1)p subtype in the post‐pandemic period was more often associated with secondary bacterial pneumonia, ICU admission, and death, than the other influenza virus (sub)types.ConclusionContrary to what is commonly assumed, the causal virus subtype does not seem to be a major determinant of clinical presentation and severity of influenza illness. However, drawing conclusions was made difficult by the low comparability and methodological shortcomings of included studies, and more well‐designed studies are warranted.

The epidemiology and severity of respiratory viral infections in a tropical country: Ecuador, 2009–2016

“…After adjusting by age, patients infected with A(H1N1)p had a 73% higher risk of death compared to test-negative patients. This is consistent with recent literature reports and contrasts the common belief that the most clinically severe influenza virus subtype is A(H3N2) [38]. In contrast, RSV patients had a significantly reduced risk of death compared to test-negative patients.…”

Severity and outcomes of influenza-related pneumonia in type A and B strains in China, 2013–2019

“…Several studies have compared the mortality rates between patients according to virus type, but many failed to control for confounders [9,10,[30][31][32]. Recently, a systematic literature review suggested the A (H1N1) pdm09 during the post-pandemic period was more related to poor outcomes (secondary bacterial pneumonia, ICU admission, and death) than influenza B viruses [33].…”

Influenza B virus infections in Western Saxony, Germany in three consecutive seasons between 2015 and 2018: Analysis of molecular and clinical features

“…However, the adult population that was seen had a high proportion of comorbidities and risk factors that might have contributed to a Fig. 3 (continued) severe outcome of the influenza disease, as it is known for example for cardiac insufficiency [26], additionally to age itself [2,27,28]. A shift towards the adult population thus increases the impact on the public health system and may explain the severity of this season with a high number of excess consultations.…”

“…Influenza B virus of the Orthomyxoviridae family [1] has a segmented single stranded, negative sense RNA genome. First isolated in 1940, influenza B virus diverged into two lineages, Victoria and Yamagata, in the late 1970s with similar clinical [2] but different phylodynamic properties [3]. Although infections in pigs and seals were observed, there is no known animal reservoir for influenza B [4].…”