An attempt at an epidemiological explanation - Epib.nl

ChapEe,. 7In this thesis several aspects of the epidemiology of meningococcal disease in theNetherlands have been addressed. The core theme was studying the spread of meningococcaldisease in the Netherlands in the past decade in an attempt to explain theincreased incidence in the 19805. This was done by observing several characteristics ofthe pathogen, which took a central position in this thesis, and the host. TIle ultimategoal was to provide information which may assist the (future) prevention of meningococcaldisease. In the following three sections the relevance of the results of thevarious chapters will be discussed in retrospect, and recommendations fo r furtherresearch will be given. The first section discusses the factors that are related to th eincreased incidence of meningococcal disease, the second section deals with aspectspertaining to the future vaccine, and the third witb chemoprophylaxis.Increased incidence of meningococcal disease in the Netherlands from 1980 to 1990Epidemic or endemic?Neisseria meningilidis is capable of causing ep idemics of meningococcal disease (MD),and epidemics have been reported world-wide, as well as in the Netherlands. l2During the 1980s the incidence of MD in the Netherlands increased gradually froml.l/l00,000/year in 1982 to 3.5/100,000/year in 1990. At the end of 1988, alarmingreports appeared in the lay press, which gave rise to much concern among the public,and the question was, whether we had to fear another epidemic of MD.It is difficult to give an exact definition of an epidemic. In a handbook on theepidemiology of bacterial infections the following definition is given: 3 "An epidemic issaid to exist when an unusual number of cases of a disease occur in a given tim eperiod and geographic area as compared with the previous experience with thatdisease in that area". It is also stated that this definition is arbitrary and will varyfrom disease to disease. The problem is bow to determine what should be consideredas an unusual number. For the designation "epidemic of MD in the Netherlands" anarbitrary cut-off value of 4.0 cases per 100,000 inhabitants per year has been applied.'According to tbis definition, the increased incidence of MD in the Netherlands in thelate 1980s can not be considered an epidemic, because it did not exceed this cut-offvalu e. However, if the estimated 80% coverage rate of the submissions of meningococcito tlle Netherlands Reference Laboratory for Bacterial Meningitis (RLBM) istaken into consideration,S the true incidence in 1990 would have been around4.3/l00,000/year, which would then meet this criterion.It can be argu ed that this nominal criterion for the definition of an epidemic ofMD alone will not suffice, and that other aspects should be considered as well. Oneof these aspects is the rate of the increase in frequency of the disease, which dependson the duration of the incubation period of the disease. Most epidemics of MD, asreported in the past, showed a sudden steep rise in the number of cases in a shortperiod of time, which is in agreement with the short incubation period of MD,)foll owed by a rapid decrease l 26 The 1966 epidemic in the Netherlands is a good86