Effects of influenza vaccination on seasonal influenza symptoms: A prospective observational study in elementary schoolchildren in Japan

Although influenza vaccine has been shown to prevent influenza symptom onset, its further beneficial effects after vaccinated individuals become symptomatic remain undetermined. This epidemiological survey compared influenza symptoms in subjects diagnosed with influenza who were and were not vaccinated. A prospective survey was performed among the 13,217 schoolchildren who attended all 29 public elementary schools in Matsumoto City, Nagano Prefecture, Japan, during the 2014/2015 influenza season. Information about symptoms and background demographic and clinical factors were obtained from a questionnaire. Of these schoolchildren, 2,548 were diagnosed with influenza and 1,122 were previously vaccinated and 1,426 were unvaccinated. Fever duration and frequency of symptoms and hospitalization were compared in vaccinated and unvaccinated children. The hospitalization rate was lower in vaccinated children, whereas symptom frequency and fever duration were similar in the two groups. This study showed that hospitalization was less in vaccinated children. Vaccination may attenuate symptom intensity after symptom onset.