Seroepidemiology of diphtheria, tetanus, poliomyelitis and pertussis: evaluation of the National Immunisation Programme in the Netherlands.

In view of the evaluation of the National Immunisation Programme in the Netherlands the main objectives were to obtain insight into the immunity to diphtheria, tetanus and poliomyelitis, into the occurrence of pertussis and to improve serodiagnosis of pertussis. In a population-based nationwide sampling, 8359 sera (response 55%) were collected, and to gain access to orthodox reformed individuals refusing vaccination, in a sample from municipalities with low vaccine coverage 1589 sera (response 52.5%). In the nationwide sample, the prevalence of diphtheria and tetanus antibodies (≥0.01 IU/ml in toxin inhibition assay) was 88% and 84%, resp. In at least 90% antibodies (titre≥1:8 in neutralisation assay) against poliovirus types 1, 2 and 3 were measured. For those born after mass vaccination was introduced (l45 years) the prevalence of antibodies to diphtheria, tetanus, poliovirus types 1 and 2 was at least 92.5% and for poliovirus type 3 at least 80%. Diphtheria and tetanus antibodies decreased with age for those born before vaccination was introduced (≥45 years). Only 40% of orthodox reformed individuals had diphtheria and 60% had tetanus antibodies. Less than 70% had poliovirus type 1, 2 and/or 3 antibodies. We concluded that the Dutch immunisation programme induced long-term diphtheria, tetanus and poliomyelitis immunity. While adults are very well protected against poliomyelitis, a great number of adults lack diphtheria or tetanus antitoxin antibodies. These adults might benefit from diphtheria (re)vaccination; however, offering a primary tetanus vaccination to cohorts born before the introduction of vaccination would probably be more effective than routine revaccination. Introduction of C. diphtheriae or poliovirus in socio-geographically clustered orthodox reformed groups might constitute a danger of spread of these pathogens. Pertussis surveillance data from notifications, positive serology and hospital admissions (1976-98) showed a sudden increase in the number of pertussis cases in 1996-97. According to notifications and serology data, the increase among, mostly unvaccinated, children less than 1 year was similar to the increase in hospital admissions. For older, mostly vaccinated, individuals the increase in hospital admissions was relatively small. The increase of reported vaccinated patients of all ages was higher than for unvaccinated patients. We postulated that the proportion of pertussis infections resulting in recognizable symptoms has increased among vaccinated individuals due to a mismatch of the vaccine strain and circulating B. pertussis strains. To investigate at which level IgG antibodies against pertussis toxin (IgG-PT) in a single serum sample are indicative for recent pertussis, IgG-PT was analysed in 7756 population-based sera, in sera of 3491 patients with at least a fourfold IgG-PT increase, in paired sera of 89 patients with positive cultures or polymerase chain reactions and in sera of 57 pertussis patients with a median follow-up of 1.4 years. IgG-PT levels of at least 100 U/ml were present in less than 1% of the population, are reached by most pertussis patients within 4 weeks after disease onset and persist only temporarily. We concluded that such levels are diagnostic for recent or actual infection with B. pertussis . Our results not only show that childhood vaccination should be sustained, but that adult vaccination could be considered. We have to anticipate long-term effects of mass vaccination, such as gaps in immunity as a result of decreased circulation of the pathogens and waning immunity. Epidemiological studies directed towards evaluation of vaccination should continue to provide a scientific basis for vaccination strategy.