1 Department of Infectious Diseases Epidemiology, National Institute of Public Health and the Environment, Bilthoven, the Netherlands.
2 Laboratory for Control of Biological Products, National Institute of Public Health and the Environment, Bilthoven, the Netherlands.
3 Research Laboratory for Infectious Diseases, National Institute of Public Health and the Environment, Bilthoven, the Netherlands.
4 Department of Virology, Eijkman Winkler Institute, University Medical Center Utrecht, Utrecht, the Netherlands.
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ABSTRACT |
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antibodies; immunity; poliomyelitis; polioviruses; seroepidemiologic studies; vaccination
Abbreviations: CI, confidence interval
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INTRODUCTION |
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Poliovirus vaccination was introduced in the Netherlands in 1957 and was offered to all those persons born in 1945 and thereafter. Children are vaccinated at age 3, 4, 5, and 11 months with diphtheria, tetanus, pertussis, and inactivated polio vaccine and at age 4 and 9 years with diphtheria, tetanus, and inactivated polio vaccine. The rate of coverage for receiving at least three vaccinations by age 12 months has been 97 percent in past decades (7, 8
). Major outbreaks similar to those that developed in the previous decades did not occur in the 1960s and early 1970s. Smaller outbreaks did occur in the first 1015 years after vaccination began in communities with a high percentage of Orthodox Reformed persons not vaccinated for religious reasons. However, despite high vaccination coverage, three larger poliomyelitis outbreaks occurred in recent decades (1971, 1978, 19921993), with consequent exportation to polio-free countries (Canada and the United States) (9
, 10
). The last two outbreaks were confined to Orthodox Reformed persons who refused vaccination. The last outbreaks occurred in 1978, with 110 reported cases of disease caused by poliovirus type 1, and in 19921993, with 71 cases caused by poliovirus type 3 (10
, 11
). Between these two outbreaks, three imported cases of poliomyelitis were reported in persons who acquired the disease abroad; no cases have been reported since the 19921993 outbreak.
The majority of unvaccinated persons in the Netherlands have not been vaccinated for various, mostly trivial, reasons but are protected against poliomyelitis because of natural or herd immunity. However, an Orthodox Reformed minority of approximately 275,000 persons is insufficiently protected by herd immunity since they form a sociogeographically closely knit network (12). Vaccination is accepted to some degree among even the Orthodox Reformed groups; about a third of the Orthodox Reformed participants in our study reported that they had been vaccinated. This proportion matches the prevalence of tetanus antitoxin that we detected in the sera from this cohort (13
).
We established a serum bank through population-based sampling in 19951996 to evaluate the effects of mass vaccination in the Netherlands (6), which offered an opportunity to study the prevalence of antibodies against polioviruses in the Dutch population and in groups refusing vaccination. The objective was to gain insight into the population's immunity. Such data could also provide insight into the possible waning of natural and vaccine-induced immunity in the absence of boosting opportunities. Furthermore, the study enabled us to detect evidence of poliovirus circulation by comparing the serologic profiles of cohorts born before and after recent outbreaks, both in the general population and in Orthodox Reformed groups.
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MATERIALS AND METHODS |
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Antibody assay
The sera were stored at -86°C. Neutralizing antibodies against poliovirus types 1, 2, and 3 were determined in a microneutralization assay with the Mahoney strain for poliovirus type 1, the MEF-1 strain for poliovirus type 2, and the Saukett strain for poliovirus type 3, as described previously (14). The sera were titrated in a twofold dilution range to 1:4096. The results were given as 2log reciprocal titers, expressed as the reciprocal of the greatest dilution showing complete neutralization of the cytopathic effect of 100-percent cell culture infection doses. A titer of 1:8 (2log titer = 3) was defined as an indication of protective immunity.
Statistical analysis
Frequencies of protective immunity and geometric mean titers in each municipality were weighted by the proportion of the age group in the population. To produce national estimates, the weighted frequencies and geometric mean titers were averaged over the 40 municipalities (15). For the low vaccine coverage sample, the geometric mean titers were averaged by weighting by the population size of the municipality. The effect of differential probabilities of response on both sample estimates was less than one standard error and therefore was ignored.
In the different analyses, the following groups were distinguished:
Linear regression analysis was used to determine the persistence of poliovirus type 1, 2, and 3 antibodies after complete participation in the national immunization program. The association between poliovirus type 1, 2, and 3 antibody titers and age was studied for persons from the nationwide sample who received the sixth documented vaccination at age 89 years and only for those without self-reported or documented revaccination or a military service history. Since the number of persons aged 34 years or more who met these criteria was very small, this analysis was restricted to those aged 1034 years.
Participants more than 16 years of age were asked by questionnaire whether they had received any additional vaccination, because of military service, travel, or professional activities, for example, for diphtheria, tetanus, and polio-myelitis after their childhood vaccination. Those who reported such vaccination were considered revaccinated. Documented revaccination was defined as any documented vaccination with inactivated polio vaccine given in addition to vaccinations documented on a certificate from the national immunization program.
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RESULTS |
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Persistence of poliovirus antibodies after vaccination in the nationwide sample
For persons aged 1034 years in the nationwide sample who had received the sixth and last documented vaccination at age 9 years, without any evidence of revaccinations (n = 969), the overall percentages of protective antibodies were 99.7 (95 percent confidence interval (CI): 99.2, 100) for poliovirus type 1, 98.7 (95 percent CI: 97.7, 99.6) for poliovirus type 2, and 94.5 (95 percent CI: 90.9, 98.2) for poliovirus type 3. For those aged 1014 years versus those aged 3034 years, the geometric mean titers for poliovirus types 1, 2, and 3 decreased from 10.5 (95 percent CI: 10.2, 10.7) to 8.0 (95 percent CI: 7.4, 8.6), from 8.6 (95 percent CI: 8.4, 8.8) to 5.2 (95 percent CI: 4.7, 5.7), and from 8.2 (95 percent CI: 8.0, 8.5) to 4.8 (95 percent CI: 4.2, 5.3), respectively (figure 2). In the linear regression analysis of the relation between antibody titers and age (figure 2), the intercepts for poliovirus types 1, 2, and 3 were, respectively, 11.9, 10.8, and 10.1 (in 2log titer), with slopes for age (in years) of -0.12, -0.18, and -0.16.
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Protective immunity in the Orthodox Reformed groups
Orthodox Reformed persons had a smaller percentage of protective antibodies for all three poliovirus types than participants in the nationwide sample (table 1). When Orthodox Reformed persons were excluded from the low vaccine coverage sample, no differences were observed in these percentages or the geometric mean titers for poliovirus types 1, 2, and 3 between the low vaccine coverage sample and the nationwide sample.
The age-specific percentages of protective antibodies for poliovirus types 1, 2, and 3 for the Orthodox Reformed persons are compared with those for all persons in the nationwide sample in figure 3. Because of the small numbers of Orthodox Reformed persons, the age-specific percentages were less precise; therefore, the fluctuations shown could have been a result of chance. For poliovirus type 1, the percentage of protective antibodies in Orthodox Reformed persons increased from 39.1 percent for those aged 1519 years to 88.7 percent for those aged 3034 years. This percentage was more or less stable at approximately 90 percent for older persons. An increase in the percentage of protective antibodies for poliovirus type 2, from 54.4 to approximately 80 percent, was observed after age 2529 years. For poliovirus type 3, the percentage increased after age 14 years (54.2 percent) and then fluctuated at an average of 75 percent.
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DISCUSSION |
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Although the prevalence of antibodies (1:8) against poliovirus was generally somewhat lower for type 3 than for types 1 and 2, it was still close to 90 percent. It is unclear at present whether all those persons with a low level of or no detectable antibodies are susceptible to infection; some, particularly the elderly, may be protected by memory immunity, an accelerated antibody response because the immune system has been primed previously. We are currently studying memory response in seronegative elderly persons after challenge with oral polio vaccine. Moreover, the potential for poliovirus circulation in the well-protected population vaccinated with inactivated polio vaccine is an important issue for polio eradication.
The age-specific profile of antibodies was mirrored in the age-specific geometric mean titers, as shown in figure 1. The geometric mean titers were consistently high. In the cohorts born before 1945 (i.e., in our study of those persons aged 50 years or more) with predominantly natural immunity, the geometric mean titers were stable, and there was no indication of waning immunity. A seroprevalence study that used a different sampling scheme and limited numbers was carried out in the Netherlands in 1980 and 1985 (16). Comparison of the present results with those from previous studies indicated no waning immunity in these naturally infected persons several decades after natural infection (16
, 17
). Persons naturally exposed to infection seem to retain their neutralizing antibodies, which implies that those without detectable antibodies have never been exposed to live, wild virus. In contrast, a small decrease in the geometric mean titer with age or time was observed for those persons aged 1040 years whose antibodies were probably induced predominantly by vaccine. It appears that the decrease in geometric mean titer is due to waning immunity after vaccination. No further decline was observed in the first cohorts to whom vaccination was offered (persons aged 4049 years). These cohorts are also likely to have been exposed to live polioviruses circulating during their childhood and therefore probably have a combination of natural and vaccine-induced immunity. It is remarkable that the differences in antibody prevalence between poliovirus type 3 and types 1 and 2 were stronger in persons aged 3044 years. This finding might be attributable to a lower potency of the poliovirus type 3 antigens. As such, assessment of poliovirus type 3 antibodies may provide the most sensitive tool to study possible waning immunity after vaccination.
For persons aged 1034 years who had been completely vaccinated according to the Dutch vaccination program and did not have evidence of revaccination, regression analysis showed a linear decrease in antibody levels for all types of poliovirus. Antibody levels were still high, however, even about 20 years after vaccination. We cannot completely rule out the possibility that the long persistence of high levels of vaccine-induced antibodies is partly influenced by boosting through circulating poliovirus. The explanation of these high antibody levels by the boosting phenomenon might correspond to the slightly smaller slope for poliovirus type 1 in the model, as poliovirus type 1 was most prevalent in the past. However, virus circulation probably stopped in the late 1960s or early 1970s (1719
). In a follow-up study of children vaccinated with inactivated polio vaccine in Sweden, Böttiger et al. showed that poliovirus neutralizing antibodies could persist for more than 18 years after vaccination (20
). A more marked decline in antibody titer was seen in the first few years, while a very slow decrease was observed afterwards (20
).
Epidemiologic data from the recent polio outbreaks in the Netherlands confirm the high level of protective immunity to poliomyelitis in the general population. In both the 1978 and 19921993 outbreaks, cases occurred among only unvaccinated persons, nearly all of whom belonged to Orthodox Reformed groups. Our results, however, show that despite a general vaccination coverage rate of 97 percent, the potential for outbreaks in these Orthodox Reformed groups is still high; only 65, 59, and 69 percent of these persons had protective antibodies for poliovirus types 1, 2, and 3, respectively. We found that the percentages of persons aged 119 years with protective antibodies were 48 percent for type 1, 47 percent for type 2, and 68 percent for type 3. As expected, persons without protective antibodies were found predominantly in the cohorts born after vaccination was introduced. Actually, this occurrence seems to be a paradoxical effect of mass vaccination, although these unvaccinated groups benefit from the interruption of virus circulation that results from widespread vaccination. The Ministry of Health, advised by the Health Council, has decided against making vaccination mandatory in the Netherlands. Therefore, mandatory vaccination is not considered socially and politically feasible; thus, poliomyelitis can ultimately be prevented only through global eradication of the causative agents, the polioviruses (21).
The effect of virus circulation during the 1978 (type 1) and 19921993 (type 3) outbreaks among Orthodox Reformed persons with no evidence of vaccination becomes evident when the serologic profiles of the cohorts born before and after the outbreaks are compared. For poliovirus type 1, the seroprevalence for those persons born between 1978 and 1992, that is, after the 1978 type 1 epidemic, was only 12 percent, but it was 64 percent for those born between 1945 and 1978; for type 3, the seroprevalence for those born after 1992, the year of the type 3 outbreak, was only 1 percent in contrast to 53 percent for those persons born between 1978 and 1992. This finding indicates that as many as half of the persons in these unvaccinated groups were infected during the epidemic.
Remarkably, the pattern of cohortwise estimates of seroprevalence for poliovirus type 2 is similar to that for type 1, although there have been no signs of wild poliovirus type 2 circulation in the Netherlands for decades. We cannot explain this finding satisfactorily. However, the numbers of Orthodox Reformed persons were small, particularly when stratified by cohort. The last polio patient with a poliovirus type 2 infection in the Netherlands was reported in 1966. A wild poliovirus type 2 occasionally has been isolated from adopted children and, once, from river water in the early 1980s (17). Since then, only a few vaccine-derived poliovirus type 2 strains have been isolated (17
). The apparently elevated poliovirus type 2 seroprevalence in those born after 1978 cannot be ascribed to circulation of oral-polio-vaccinederived poliovirus type 2 after the 1978 outbreak, as only monovalent type 1 oral polio vaccine was used then to control the outbreak. In the 19921993 outbreak, however, trivalent oral polio vaccine was applied, which may have resulted in poliovirus type 2 circulation (22
). There is some cross-reactivity between poliovirus serotypes 1 and 2, but it is improbable that this cross-reactivity can satisfactorily explain the comparable levels for serotypes 1 and 2 (23
). Using seroepidemiologic data from a study of gypsies, Aylward et al. concluded that wild virus circulation could have influenced the prevalence, although the possibility could not be ruled out that the high prevalence was caused by the spread of vaccine virus (24
). In our study, the differences in the prevalence of poliovirus type 3 antibodies in cohorts born before and after the 19921993 outbreak must be ascribed mainly to wild virus circulation.
In the nationwide sample and in the low vaccine coverage sample in which Orthodox Reformed persons were excluded, we found no differences in type 1 seroprevalence in the cohorts born before and after the 1978 outbreak, and only a small difference occurred in type 3 seroprevalence in cohorts born before and after the 19921993 outbreak. This finding supports the assumption that little or no virus has spread outside the Orthodox Reformed groups, either in the general population or among other inhabitants of the municipalities in which these groups live.
As we have discussed in this paper, there are no signs of waning immunity in cohorts that supposedly have predominantly natural immunity. The seroprevalence for none of the three types of poliovirus reached 100 percent in the oldest cohorts, indicating that the endemic virus in the prevaccination era never fully depleted the pool of susceptible persons. This conclusion agrees with the threshold value for the percentage of protected persons required to prevent poliovirus transmission, estimated as 8287 percent given the condition of homogeneous mixing (25). The 1978 and 19921993 outbreaks also seem not to have infected all susceptible persons in the Orthodox Reformed groups; only 64 percent of those born between 1945 and 1978 were found to have type 1 antibodies, and 53 percent of those born between 1978 and 19921993 had type 3 antibodies. The large percentage of children without protective antibodies clearly shows a potential for another polio outbreak in the Netherlands that will exist as long as polioviruses have not been eradicated worldwide.
In conclusion, routine childhood vaccination with inactivated polio vaccine has provided excellent protection against poliomyelitis in the general population of the Netherlands. Antibodies persist for very long periods of time, not only in naturally infected persons but also in those with vaccine-induced immunity. Our study, conducted in the era of polio eradication, provides additional evidence of the absence of poliovirus circulation in the general population during the outbreaks among Orthodox Reformed persons who refused vaccination in 1978 and 19921993. Since mandatory vaccination is politically and socially unacceptable in the Netherlands, pockets of susceptibility will remain because persons object to vaccination for religious reasons. Therefore, global eradication is the only means of protecting these persons against poliomyelitis.
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ACKNOWLEDGMENTS |
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NOTES |
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Editor's note: An invited commentary on this paper appears on page 215, and the authors' response is on page 217.
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REFERENCES |
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