Annual report: surveillance of influenza and other respiratory infections in the Netherlands: winter 2017/2018.

Influenza In the winter of 2017/2018 the influenza epidemic lasted 18 weeks. This is longer than the average over the last 20 years (nine weeks). Between October 2017 and May 2018, an estimated 900,000 people had symptomatic influenza and 340,000 people consulted their general practitioner with influenza-like symptoms. Hospitals were also temporarily overstretched as many patients had to be admitted due to complications of flu (usually pneumonia); this number is estimated to have been over 16,000. Also, during the epidemic, 9,500 more people died than would normally be the case in the influenza season (October to May). Throughout the entire epidemic, people mostly became ill due to an influenza type B virus of the Yamagata lineage. This is the first time that an influenza type B virus has been dominant right from the start of the epidemic. Influenza vaccine effectiveness In the current season, vaccination prevented 44% of the vaccinated people from getting the influenza B virus. This is despite the fact that the Yamagata lineage of influenza virus type B was not included in the vaccine. Apparently, the other B virus in the vaccine provided a reasonable level of cross-protection. The long duration of the flu epidemic can therefore not be explained by a low effectiveness of the vaccine.The effectiveness of the vaccine can differ greatly from season to season. This is because the composition of the vaccine is decided upon six months in advance and is determined based on the viruses that dominated in the previous season all over the world. However, influenza viruses can change and when the influenza season breaks out in the Netherlands other viruses may dominate. This is why it is not possible to predict exactly which viruses will be dominant. Notifiable respiratory infections Some respiratory infections have to be notified to the Public Health Services in order to prevent any further spread. In 2017, there was a striking increase in the number of notifications of legionella; at 561 this was the highest number ever reported. The number of reports of tuberculosis dropped to 787. The number of reports of Q fever (23) and psittacosis (52) remained stable. Q fever, psittacosis and legionella generally manifest themselves in the form of pneumonia. The number of reported cases is an underestimation of the real number as these diseases are normally not tested for when people have pneumonia. (aut. ref.)