Mortality surveillance in the Netherlands: winter 2015/2016 of moderate severity
Authors: De Lange Marit , Teirlinck Anne , Dijkstra Frederika , Stoeldraijer Lenny , Harmsen Carel , Van der Hoek Wim 
Published: May 2017

ObjectiveWeekly numbers of deaths are monitored to increase the capacityto deal with both expected and unusual (disease) events such aspandemic influenza, other infections and non-infectious incidents.The monitoring information can potentially be used to detect, trackand estimate the impact of an outbreak or incident on all-causemortality.IntroductionThe mortality monitoring system (initiated in 2009 during theinfluenza A(H1N1) pandemic) is a collaboration between the Centrefor Infectious Disease Control (CIb) and Statistics Netherlands.The system monitors nation-wide reported number of deaths(population size 2014: 16.8 million) from all causes, as cause ofdeath information is not available real-time. Data is received fromStatistics Netherlands by weekly emails.MethodsOnce a week the number of reported deaths is checked for excessabove expected levels at 2 different time-lags: within 1 and 2 weeksafter date of death (covering a median 43% and 96% of all deathsrespectively). A weekly email bulletin reporting the findings is sentto the Infectious Disease Early Warning Unit (at CIb) and a summaryof results is posted on the RIVM website (National Institute for PublicHealth and the Environment). Any known concurrent and possiblyrelated events are also reported. When excess deaths coincide withhot temperatures, the bulletin is sent to the Heat Plan Team (also atRIVM). Data are also sent to EuroMOMO which monitors excessmortality at a European level. For the Dutch system baselines andprediction limits are calculated using a 5 year historical period(updated each July). A serfling-like algorithm based on regressionanalysis is used to produce baselines which includes cyclical seasonaltrends (models based on historical data in which weeks with extremeunderreporting have been removed. Also periods with high excessmortality in winter and summer were removed so as not to influencethe baseline with previous outbreaks).ResultsIncreased mortality occurred during the entire influenza epidemicand up to three weeks thereafter (weeks 1-14 of 2016), except for adrop in week 7 (figure1). Excess mortality was primarily observedin persons 75 or older. Additionally, in several weeks mortality wasincreased in 65-74 year olds, (weeknr 4-6; peaking in week 4 with564 deaths, when 468 baseline deaths were predicted). Also, inweek 4, mortality in the 25-34 year-old age group was significantlyincreased (25 deaths, while 14 were expected as baseline). Cumulativeexcess mortality was estimated at 3,900 deaths occurring duringthe 11 weeks of the 2015/2016 influenza epidemic and at 6,085during the total winter season (44 weeks running from week 40 up toweek 20).ConclusionsIn terms of number of deaths during the winter season (weeks40-20) and during the influenza epidemic (weeks 1-11), the 2015/2016season in the Netherlands was of moderate severity compared with theprevious five years (and was of similar magnitude as the 2011/2012winter). Notable was the short three-week time span with a higherpeak in mortality in 65-74 year olds than has been observed in recentyears. Although the influenza epidemic reached its peak in week7, the mortality data showed a dip in week 7. The reason for thetemporary decrease is unknown but there was a partial overlap witha public holiday.