Annual report: surveillance of influenza and other respiratory infections in the Netherlands: winter 2016/2017.

During the 2016/2017 winter season, the influenza epidemic in the Netherlands lasted for 15 weeks. This was longer than the nine-week average duration of epidemics in the twenty previous seasons. Influenza subtype A(H3N2) was the dominant influenza virus throughout the season. In general, baseline natural immunity against A(H3N2) is relatively low among the elderly. Indeed, the number of patients older than 65 years, who visited a general practitioner (GP) for influenza-like symptoms, was higher than last year when influenza A(H1N1)pdm09 predominated. In nursing homes, the number of patients with influenza-like symptoms was also high. In total, an estimated 500,000 patients had symptomatic influenza in the period between the beginning of October 2016 and the end of May 2017 and 6,500 patients were admitted to hospital for influenza-related symptoms. During the epidemic, there were 7,500 more deaths than expected in this 15-week period.The effectiveness of the influenza vaccine against the A(H3N2) virus was 47 per cent. The circulating Dutch A(H3N2) viruses displayed a good to moderate match with the strain that was used in the 2016 vaccine. The WHO has recommended that the same strain be used for the trivalent vaccine for the 2017/2018 season in the northern hemisphere. The B component in the 2017 trivalent vaccine also remains the same as it was in 2016, but the A(H1N1)pdm09 component will be replaced with a more recent virus. The effectiveness of the vaccine varies every season because it is never known which influenza virus(es) will dominate in the next influenza season. Also, the circulating influenza viruses can evolve over time and deviate from the chosen vaccine viruses.There were more reports of the notifiable respiratory infectious diseases made in the 2016 calendar year than in previous years: tuberculosis (889 notifications), psittacosis (60 notifications) and legionellosis (454 notifications). The increase in legionellosis notifications may be associated with the warm, wet weather conditions in 2016. However, several geographic clusters were observed whose existence could not be explained by heavy rainfall or other weather conditions and for none of these clusters could the source of infection be found. The number of notifications for Q fever (14 notifications) is still decreasing. However, the notifiable infectious diseases that present as pneumonia are notoriously underreported because most cases of community-acquired pneumonia are managed in primary care without specific diagnostic laboratory tests being made. (aut. ref.