Abstract
IntroductionWhen influenza vaccination is ineffective in preventing influenza virus infection, it may still reduce the severity of influenza-associated disease. Here, we estimate the effect of influenza vaccination in preventing severe outcomes e.g. intensive care unit (ICU) admission and death, even though it did not prevent influenza virus infection and subsequent hospitalisation.MethodsAn observational case–case epidemiological study was carried out in 12 sentinel hospitals in Catalonia (Spain) over six influenza seasons 2010/11–2015/16. Cases were individuals with severe laboratory-confirmed influenza virus infection and aged 18 years and older. For each reported case we collected demographic, virological and clinical characteristics. Logistic regression was used to estimate the crude, adjusted odd ratios (aOR) and 95% confidence intervals (CI).ResultsOf 1,727 hospitalised patients included in the study, 799 were female (46.7%), 591 (34.2%) were admitted to the ICU and 223 (12.9%) died. Influenza vaccination uptake was lower in cases that required ICU admission or died (21.2% vs 29.7%, p < 0.001). The adjusted influenza vaccination effectiveness in preventing ICU admission or death was 23% (95% CI: 1 to 40). In an analysis restricted to sex, age group and antiviral treatment, influenza vaccination had a positive effect on disease severity in all age groups and categories.ConclusionsWe found that influenza vaccination reduced the severity of disease even in cases where it did not prevent infection and influenza-associated hospitalisation. Therefore, increased vaccination uptake may reduce complications, ICU admission and death.
Highlights
When influenza vaccination is ineffective in preventing influenza virus infection, it may still reduce the severity of influenza-associated disease
When influenza vaccination is ineffective in preventing influenza virus infection it may still have an additional effect by reducing the severity of influenza [11]
Our results are consistent with other studies, for example, a study conducted in the United States (US) by Catania et al found that patients requiring intensive care unit (ICU) admission had a lower influenza vaccination coverage [19]; A Spanish study by Casado et al found that influenza vaccination was associated with a reduction in the odds of in-hospital death and ICU admission in adults hospitalised with laboratory-confirmed influenza [20]and a French study by Loubet et al reported a reduction in the risk of ICU admission but not death in patients hospitalised with laboratory-confirmed influenza [21]
Summary
When influenza vaccination is ineffective in preventing influenza virus infection, it may still reduce the severity of influenza-associated disease. We estimate the effect of influenza vaccination in preventing severe outcomes e.g. intensive care unit (ICU) admission and death, even though it did not prevent influenza virus infection and subsequent hospitalisation. Cases were individuals with severe laboratory-confirmed influenza virus infection and aged 18 years and older. Conclusions: We found that influenza vaccination reduced the severity of disease even in cases where it did not prevent infection and influenza-associated hospitalisation. Sentinel surveillance of patients hospitalised due to severe laboratory-confirmed influenza has shown that influenza is an important cause of severe illness and death, mainly among those aged 65 years and older and patients with underlying chronic diseases [3]. The influenza vaccine is the best tool for the prevention of influenza and its complications, in patients with underlying chronic diseases and those aged 65 years and older [4,5,6,7]. It has been suggested that while protection against influenza virus infection is primarily mediated through the antibody response, protection against severe outcomes is mediated through cellular immune responses affecting
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