Effect of influenza vaccination on the prognosis of hospitalized influenza patients

ABSTRACT

Objectives: This study aimed to assess whether influenza vaccination reduces the risk of severe and fatal outcomes in influenza inpatients aged ≥65 years.

Methods: During the 2013–2014 influenza season persons aged ≥65 years hospitalized with laboratory-confirmed influenza were selected in 19 Spanish hospitals. A severe influenza case was defined as admission to the intensive care unit, death in hospital or within 30 days after admission. Logistic regression was used to compare the influenza vaccination status between severe and non-severe influenza inpatients.

Results: Of 433 influenza confirmed patients, 81 (19%) were severe cases. Vaccination reduced the risk of severe illness (odds ratio: 0.57; 95%CI: 0.33–0.98). The cumulative number of influenza vaccine doses received since the 2010–2011 season was associated with a lower risk of severe influenza (odds ratio: 0.78; 95% CI 0.66–0.91).

Conclusion: Adherence to seasonal influenza vaccination in the elderly may reduce the risk of severe influenza outcomes.

KEYWORDS:

• Influenza virus
• seasonal influenza vaccine
• vaccine effectiveness
• severe influenza
• mortality

Financial & competing interests disclosure

This study was funded by the Instituto de Salud Carlos III (Projects PI12/02079 and PI12/00087) cofounded by the European Union (Fondo Europeo de Desarrollo Regional, FEDER, una manera de hacer Europa), and the Catalan Agency for the Management of Grants for University Research (AGAUR Grant number 2014/ SGR 1403). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Key issues

• Whether vaccination can reduce the severity of influenza in cases where vaccination does not prevent influenza illness remained unclear.

• In a multicenter study, we compared the influenza vaccination status between severe and non-severe influenza inpatients.

• A severe influenza case was defined as admission to the intensive care unit, death in hospital or within 30 days after admission.

• Vaccination was associated with a 43% lower risk of severe influenza.

• Pneumonia and death seem to be less likely in patients who had received the influenza vaccine, although these analyses did not reach statistical significance.

• The cumulative number of seasonal influenza vaccine doses was associated with a decline in the risk of severe influenza.

• Even when influenza vaccination fails to prevent influenza infection in the elderly, it may reduce the severity and prevent deaths.

• Influenza vaccination in more than one season seems to improve the risk reduction in severe outcomes; therefore, adherence to influenza vaccination in successive seasons should be recommended.

• Further studies are needed to corroborate these findings in other influenza seasons.