Poliomyelitis in the Netherlands 1958-69: the influence of a vaccination programme with inactivated poliovaccine

Bull. Org. mond. SanM 92 6 3-4 Bull. Wld Hlth Org. 1972, 46, 735-745 Poliomyelitis in the Netherlands 1958-69: the influence of a vaccination programme with inactivated poliovaccine B. HOFMAN 1 The Netherlands is one of the few countries where poliomyelitis vaccination is practised with inactivated vaccine. Vaccinations started in 1957 and since 1962 have been carried out with a diphtheria-tetanus-pertussis-poliomyelitis vaccine according to a fixed schedule. The overall acceptance rate has been over 90 %. Since 1957 the poliomyelitis morbidity has decreased by more than 99 % in fully vaccinated persons and by 60-80 % among nonvaccinated persons. The latter reduction probably resulted from a decrease in the amount of virus circulating after the vaccination campaign had started. The rather unequal distribution of acceptance rates allowed for a more detailed examination, which showed a relation between acceptance rates and morbidity figures. The morbidity pattern is now characterized by a few local outbreaks among nonvaccinated subjects in areas with low vaccination rates, and by the virtual disappearance of the disease in the rest of the country. The results ofimmunization with inactivated polio- myelitis vaccine (IPV) in the 8-year period 1958-65 were presented in a previous paper (Hofman, 1967). The vaccination programme has been continued in the same way ever since. IPV has been used for the prevention of poliomyelitis by only a few countries and thus it is interesting to review the occurrence of the disease, and the influence of the vaccination programme over a 12-year period and to compare the poliomyelitis morbidity before and after vaccinations began. MATERIALS AND METHODS Survey population Figures for the incidence of poliomyelitis in the period 1924-69 were obtained from the reports of the Chief Medical Officer of Health, as were the details of the poliomyelitis cases occurring in the 1958-69 period. The number of vaccinated subjects in each age group on 1 September of the second calendar year after birth were also obtained from these reports for children born after 1959 as well as the corresponding 1 Head, Poliomyelitis Vaccine Laboratory, National Insti- tute of Public Health, Bilthoven, The Netherlands. numbers for those born between 1945 and 1958 based on the figures of 1 October 1960. From these figures and from population statistics (Central Bureau voor de Statistiek 1963, 1953-69) we calculated the number of vaccinated children in each age group in each of the 12 years considered, the different morbidity rates, and the percentage reduc- tion in morbidity. Vaccines and schedules The vaccines and vaccination schedules for those born in the period 1945-61 are given in Table 1. The plain vaccines used in the early years were bought from different manufacturers in the USA and from Belgium. Since 1962 the vaccination schedule has consisted of a primary series of three injections of diphtheria-tetanus-pertussis-poliomyelitis vaccine given at the ages of 3, 4, and 5 months, respectively, followed by a booster injection at 11 months of age. Since 1965 revaccination with diphtheria-tetanus-po- liomyelitis vaccine has been offered at the ages of 3 and 8 years, but sometimes quadruple vaccine has been preferred for children of 3 years. A 1-ml dose of quadruple vaccine prepared in the Netherlands by the National Institute of Public Health (Rijks Instituut voor de Volksgezondheid) contains poliovirus types 1 2852 -735- B. HOFMAN Table 1. Poliomyelitis vaccination schedule in the Netherlands, 1957-65 a Year of Year of birth vaccination 1945-49 1950-51 1952-54 1955 1956 1957 1958 1959 1960 1961 1957 Po Po 1958 Po Po Po Po Po Po Po Po Po 1959 Po Po Po Po Po Po Po Po 1960 PO Po Po Po Po Pa Po Po 1961 Po Po Po Po Po Po Po Po 1962 Po Po 1963 1964 DT-Po 1965 DT-Po DT-Po DT-Po DT-Po DT-Po DT(P)Po DT(P)Po a PO = poliomyelitis (plain) DTPPo = diphtheria-tetanus-pertussis-poliomyelitis (four components adsorbed on Al-phosphate) DT-Po = diphtheria-tetanus-poliomyelitis (three components adsorbed on Al-phosphate) DT(P) Po = DTPPo or DT-Po. (Mahoney), 2 (MEF1), and 3 (Saukett), 15 Lf of diphtheria toxoid, 5 Lf of tetanus toxoid, 16x109 Bordetella pertussis organisms, and 1.5 mg of alumi- nium phosphate as an adjuvant. The formula for the diphtheria-tetanus-poliomyelitis vaccine for revac- cination is: 2.5 Lfofdiphtheria toxoid, 5 Lfof tetanus toxoid, poliovirus types 1, 2 and 3, 1.5 mg of aluminium phosphate per 1-ml dose. From 1966 onwards the poliovirus components in these com- bined preparations have been concentrated 2-2.5- fold. Antibody titrations Twofold serum dilutions were mixed with equal volumes of a suspension containing 100 TCID50 of poliovirus. The mixtures were held at 37°C for 4 hours and overnight at 4-6°C. Two tubes per dilution of monkey kidney cell cultures were inocu- lated with these mixtures and the cytopathic effect (CPE) was read after 7 days. The lowest serum dilution tested was 1: 4. RESULTS Incidence ofpoliomyelitis in the Netherlands In 1924 poliomyelitis became a notifiable disease. Fig. 1 shows how it has developed since that date. A distinction between paralytic and nonparalytic cases was made after 1951: since then paralytic cases have comprised about 85% of the total number of notified cases. Since 1957, when the vaccination campaign began, the morbidity has fallen from a yearly average of 4.5 per 100 000 in the period 1924-57 to 0.20 per 100 000 inhabitants in 1958-69, an overall reduction of 95.5 %. The age distribution of paralytic poliomyelitis in five periods from 1954 to 1969 showed a gradual shift (Fig. 2): the proportion of cases among children 1-4 years old decreased, whereas that among the age groups of 5-9, 10-14, and 15-19 years increased. Between 1965 and 1969, 40% of the cases occurred in children 5-9 years old. 736 POLIOMYELMS IN THE NETHERLANDS 1958-69 737 Fig. 1. Number of notified cases of poliomyelitis anterior acuta in the Netherlands, 1924-69. Fig. 2. Numbers of cases of paralytic poliomyelitis and percentage distribution by age group before and after vaccinations began in 1957. V% 25 '.I_ zZ 24 '26 28 30 '32 '34 36 '38 40 42 '44 '46 '48 50 - YE AR B. HOFMAN Table 2. Number of notified cases of paralytic poliomyelitis in different periods by age group and vaccination history a Age 1954-57 1958-60 1961 1962-64 1965-69 Total 1958-69 (years) - - ± + ± + i + - ± + <1 183 5 0 0 8 1 0 2 0 0 1 0 0 16 1 0 1 - 4 1 324 12 8 12 18 3 5 21 1 5 7 1 0 58 13 22 5- 9 587 5 1 1 11 3 6 16 2 7 13 0 1 45 6 15 10- 14 90 6 2 1 2 0 2 4 0 1 5 0 0 17 2 4 15- 19 54 2 0 0 3 0 0 1 0 0 2 0 0 8 0 0 >20 227 12 0 0 5 0 0 11 0 0 2 0 0 30 0 0 total 42 11 14 47 7 13 55 3 13 30 1 1 174 22 41for each vaccination group total 67 67 71 32 237 no. not includedb 2 2 0 3 7 total reported 2465 69 69 71 35 244 a - = not vaccinated, ± = 1 or 2 doses, + = 3 or more doses. b 7 of the 244 reported cases were not included in subsequent calculations. In 2 cases the vaccination history was not known, 2 were vaccinated in another country, and 3 were already ill upon arrival in the Netherlands. The case-fatality rate remained at the same level. In the years 1954-57, 25 patients died from the disease, i.e., 4.2% of the notified cases and 5.1 % of the paralytic cases. In the period 1958-69, 13 deaths occurred (including 3 fully vaccinated children), the corresponding proportions being 4.5% and 5.3%. Effects ofpoliomyelitis vaccination We compared the poliomyelitis morbidity in a 4-year period (1954-57) before vaccination began and in the 12 years after the beginning of vaccination. Where possible these 12 years were subdivided into 4 periods (1958-60, 1961, 1962-64, and 1965-69), which roughly correlated with changes in vaccines and vaccination schedules. The number of cases that occurred in these periods are shown in Table 2 according to age group and to vaccination history. Of the 244 notified paralytic cases, 7 were not included in the calculation of the effect of vaccina- tion; in 2 of them the vaccination history was unknown, 2 more were vaccinated in other countries, and 3 showed signs of disease upon arrival in this country. The average yearly morbidity per 100 000 is shown in Table 3 according to period, age group, and vaccination history. The reference period 1954-57 Table 3. Morbidity per 100 000 cases of paralytic poliomyelitis in different periods by age group and vaccination history a Age 1954-57 1958-60 1961 1962-64 1965-69 1958-69 (years) _ ± + - + + - ± + - ± + - i + < 1 20.3 0.71 0 0 -3.76- 0 -0.27- 0 -0.08- 0 -0.60- 0 1 - 4 37.3 4.53 0.95 0.74 25.5 7.83 0.61 10.5 0.92 0.20 2.85 1.08 0 7.43 1.20 0.23 5 - 9 12.9 0.96 0.12 0.05 12.4 7.11 0.61 6.01 1.41 0.24 3.13 0 0.02 3.49 0.47 0.13 10 - 14 2.2 0.45 0.19 0.10 2.16 0 0.20 1.53 0 0.03 1.16 0 0 0.80 0.13 0.04 15 - 19 1.6 0.08 0 0 0.52 0 0 0.13 0 0 0.46 0 0 0.19 0 0 > 20 0.8 0.06 - - 0.071 - - 0.04 - - 0.005 0 0 0.03 0 0 a - = not vaccinated, ± = 1 or 2 doses, + = 3 or more doses. 738 POLIOMYELMTS IN THE NETHERLANDS 1958-69 Fig. 3. Reduction in paralytic poliomyelitis morbidity according to age: comparison of figures for 1958-69 with those for 1954-57. The small figure by each point indicates the age group. included a pre-epidemic year (1955), an epidemic year (1956), and a postepidemic year (1957). The reference period did not differ substantially from the 4-year period 1950-53. In a previous paper (Hofman, 1967) the same two periods were used for comparison. In Fig. 3 the average yearly morbidity rates in 1958-69 and 1954-57 are compared for different age groups. In the age group 1-6 years, in which morbidity in both periods was relatively high, the reduction reached almost 98 %. Older children showed a slightly different picture. Here the reduc- tion was about 95 %, with the exception of those aged 11, 13, and 14 years, for whom the reductions were between 85% and 90%. Using the total number of cases in the period 1958-69 it was possible to compare the protection afforded by partial and complete vaccination in three age groups between 1 and 14 years old. From the morbidity rates of the nonvaccinated, the partly vaccinated, and the fully vaccinated in this period (Table 3) it appears that the reduction was about 85% after 1 or 2 doses of vaccine. After completion of the vaccination schedule of three or more injections, the reduction in morbidity among the children 1-4 years of age, the group that includes the most recently vaccinated children, was 96.9%. Among children 5-9 years of age the reduction was 96.2 % and among the 10-14 year-olds it was 95%. Comparison of the four periods showed a gradual increase in protection (Table 4). In the last 5-year period (1965-69) this protection was 100, 99.4, and 100%, respectively, for the age groups of 1-4, 5-9, and 10-14 years. The morbidity rates presented in Table 2 also give information about the degree of protection attained in nonvaccinated persons as a result of the immunity of vaccinated individuals. For this purpose we compared the morbidity rates in such subjects over the years 1958-69 with those in the period 1954-57. There was a clear reduction in the incidence of poliomyelitis but the degree of reduction was different in the different age groups. In children under 15 years of age it was less (60-80%) than in older persons (88-97 %). In the fully vaccinated sub- jects the morbidity fell by about 99-99.5% compared with the 1954-57 figures. Whereas the national acceptance rate of vaccina- tion steadily increased to the high level of about 95% (Fig. 4) it was much lower in some municipalities. Low vaccination rates are still found in some rural communities where some groups of the population /. reduction 100 99 98 95 90 80 50- 8 3~~~~~~~~~~~2 C0. 10~~~~~~~~~~~~~ .0 9~~~~~~~~~ o U, -X 10 co o) 12 41 0~~~~~~1 m 1 o41 m~~~~~~~2 average yearly morbidity per 100000 1958-1969 739 0) 0) 0 C,, r- Cto 0N LO 0 CM4 0 0 co CD) 0 _ _~00 co co r. 0) 0 0D tD 0 N £a N £0 0:0 co O, r£ £0 0) - 00n N- La CD) N XD - 9 0 m C) CD M N 0 M N 0 0 CX 0 t o 0 0 N - co0 r- 0 _ 0 - _ 0 LO CO 0) 0£o 0 N CD 0 0 CX) u£ 00 0 r- 0 1- cD - co . - 0 0 o)WI' (D .0 0 c 0 a LO 0 0oa) 0 C 0 - CD Ci 4 c co 0 0 N £0 Lo0 I- 0 .0 C) 101- Lo 0 B. HOFMAN object to vaccination. It is in these regions that some of the outbreaks of poliomyelitis have occurred and Fig. 5 shows how morbidity was related to the acceptance rate in children in different municipalities. Only 3.5% of the total population of the Nether- lands live in the rather small number ofmunicipalities where less than 80% of the children are vaccinated. More than 80% of the people live in places where the coverage is between 90% and 100%. The decline of morbidity rates in relation to better vaccination status in the period 1958-69 is clear when the 12-year period is divided into 4 subperiods (Fig. 5). In the first 3 years after vaccination (1958-60) practically no relation between morbidity rate and acceptance rate could be seen, but in the two periods 1961 and 1962-64 this relation became stronger and in the last 5-year period was clearcut. Information about the development of the polio- myelitis situation can also be derived from antibody determinations in sera and some results with sera of nonvaccinated subjects are given in Table 5. The proportion of children (1-9 years of age) born after the beginning of vaccination in 1957 who were triple negative was about 50 %. In this age group there were few triple positives. Antibodies against type 2 pre- vailed in all age groups, while the frequency of type 1 and type 3 antibodies was about equal. Stool or throat samples were submitted for virus isolation from 244 paralytic poliomyelitis patients in the period 1958-69. From 62 patients the isolation attempts were negative and no samples were tested from 8 patients. The remaining 174 patients yielded polioviruses. The relation between types 1, 2, and 3 was 141:5:28, i.e., 81%, 3%, and 16%. The last isolation of type 3 from a patient was in 1965 and of type 2 in 1966. All the type 2 and type 3 isolates came from isolated cases. In nonvaccinated 1-15-year-old children the fre- quency of type 1 isolation was 87.1 % and that of type 3 isolation was 9.7%. Among fully vaccinated children relatively more type 3 poliovirus was iso- lated, the frequencies being 68.4% and 31.6%, respec- tively. Type 2 was isolated only from nonvaccinated subjects (Table 6). DISCUSSION The foregoing results depict the pattern of polio- myelitis in the Netherlands as it gradually developed in the 12 years after vaccination began in the autumn of 1957 and show that the disease has virtually disap- peared. In the period 1957-60 all the children born in 1945 and later were offered vaccination with vaccine 740 8 O N Lo 0 - 0 N 0 0s 0 0r-0). N 0 CD o N co Cw 0 co U 0o 0 cC. E 0 Q 0 o -o uz ut 0 ID 0 0. E 0 1- 0 0 La 0 - 0 cL c £0 C. 0 -o £0 *0 0 0 ._ 0 0 C 0 0Q £0 Q 0 ._ 0._ C C) 0 o.Sc Is0.0 .0 E _f 0.ZO C ~0 00 .0 0.. 00. D 6 C 00 o.0 zo C .20 o CO oCL 0 0.E zco 0.- 00E 0Co 0 *_ £- 1-Lo N N c C4 C4 . - 0 C POLIOMYELITIS IN THE NETHERLANDS 1958-69 Fig. 4. Vaccination status of infants and children in the Netherlands with diphtheria-tetanus-pertussis-poliomyelitis vaccine according to year of birth. from different manufacturers. In 1961 a vaccine produced in the Netherlands by the National Institute of Public Health became available. In 1962, when the combined vaccines containing diphtheria, tetanus, pertussis, and poliomyelitis anti- gens adsorbed on aluminium phosphate were intro- duced, the health authorities decided on a rigid schedule of vaccinations, starting at the age of 3 months and ending before the end of the first year. Although in regard to the poliomyelitis components the optimum age might have been a few months older (Perkins et al., 1958; Perkins & Gaisford, 1959) it was necessary to compromise, because the risk of pertus- sis was known to be high in the very young. Moreover, for reasons of organization, it was desir- able to complete the schedule within the first year of life. By this approach it was possible to achieve a high acceptance rate. The gradual change in the age distribution of cases, with the maximum prevalence shifting from the 1-4-year age group to older children (Fig. 3), was probably a result of the efforts made to obtain good coverage, particularly the decision to vaccinate the youngest children first. Two factors have been responsible for the reduc- tion in the number of cases of poliomyelitis since 1957. The first was the immunity conferred on the individual by vaccination, which was almost complete in the period 1965-69 (Table 4). The second factor was a herd immunity that probably resulted from the decreased capacity of the vaccinated persons to spread poliovirus (Henry et al., 1966). The combined influence of the two factors is shown by the 99% reduction in prevalence since 1954-57 in the three age groups under 14 years of age over the period 1958-69. Considering separately the 1965-69 period this reduc- tion was 99.9% in the 5-9-years-old children and 100% in the age groups 1-4, 10-14, and 15-19 years. Although a situation with decreased poliovirus circulation developed probably because the vacci- nated subjects shed less virus, the results of neutraliz- ing antibody titrations (Table 5) show that the 100 _ 98 _ 96 - 15t DTP-Po (age 3 months) 2 nd DTP-Po (age l months) 94 _ 3rd DTFP-Po (age 5 months) 92 - 90 f 4 DTP-Po (age 11-12 months) 88 _ 86 - 84- 82 - c 80 _ u 78 - $ 76 - 74 - 72 - - . I I- year ot birth *,1962 63 64 65 66 67 *559-63 situation on 1964 65 66 67 66 69September 1I _ _ _ _ __ _ _ _ _ __ _ _ _ _ 741 742 B. HOFMAN Fig. 5. Relation between average yearly morbidity from paralytic poliomyelitis in four periods between 1958 and 1969 and the percentage of persons vaccinated in different municipalities. (The X percentage vaccinated n indicates the percentage of those born in the period 1959-63 who on 1 September 1964 had had one or more vaccination. Table 5. Distribution of poliovirus neutralizing antibodies in 142 serum samples collected in the period September 1967-July 1969 from nonvaccinated subjects No. of No. of persons with antibodies No. of persons with anti- Age persons with against: bodies against poliovirus:(years) no poliovirus Total antibodiesa 1 type of 2 types of 3 types of type type typepoliovirus poliovirus poliovirus 1 2 3 1- 4 20 17 3 4 44 10 18 7 5- 9 28 11 7 7 53 11 23 12 10-19 1 3 5 8 17 10 16 11 >20 0 1 7 20 28 26 26 23 total 49 32 22 39 142 57 83 53 a Lowest serum dilution tested 1 : 4. 10.00 0 0 '4,0- '4 'U 1. vaccinated POLIOMYELITIS IN THE NETHERLANDS 1958-69 Table 6. Distribution of 112 paralytic poliomyelitis patients 1-14 years of age by vaccination history and poliovirus type isolated: 1958-69 Poliovirus type Vaccination Total history 1 2 3 No. % No. % No. % No. % nonvaccinated 81 87.1 3 3.2 9 9.7 93 100 fully vaccinated 13 68.4 0 0 6 31.6 19 100(3 or more doses) polioviruses did not disappear from the community. Antibodies were present more frequently in older children, but nevertheless 21, 41, and 19 of the sample of 97 children born since 1957 were positive for poliovirus types 1, 2, and 3, respectively. Moreover, the studies of Weiland (1968) and Wilterdink et al. (1970) in the periods 1964-65 and 1964-68, respectively, showed that polioviruses could be isolated from healthy infants in 5 day nurseries and from the effluent of 18 of 25 sewage treatment plants in the Netherlands. The observa- tions showed a continuing and fluctuating presence of all 3 types. The neurovirulence in monkeys of some of the isolates of each of the 3 polio types was studied and it was found that the isolated strains were different from typical " wild " strains. They were, in fact, characterized as " intermediate " by Wilterdink et al. (1969) and by Verlinde & Wilterdink (1970). These authors suggest that they belong to naturally occurring " wild " polioviruses with intermediate characteristics, but it is possible that they originated from oral poliomyelitis vaccines used in the adjacent countries Belgium and the Federal Republic of Germany. Closer observation of the changing pattern in the Netherlands showed that municipalities differed in regard to the degree of coverage. The morbidity was related to the acceptance rates. In the first 3 years (1958-60) of the vaccination compaign, when the vaccines and schedules varied, this relation was very weak, but in the last 5-year period (1965-69) it became clearcut (Fig. 5). If the morbidity figures for the unvaccinated persons in the communities featured in Fig. 5 were considered these trends would show even more clearly, and this makes it fairly clear that there was no natural decline in the prevalence of poliomyelitis independent of the vaccination cam- paign. We may conclude that, under the conditions prevailing in the Netherlands, at least 80% of the children, evenly distributed in a community, must be fully vaccinated to obtain a satisfactory reduction of poliomyelitis. The gradually increasing correlation between acceptance rates and morbidity (Fig. 5) does not favour the concept that dissemination of rela- tively avirulent poliovirus strains from adjacent coun- tries, if present, had any perceptible influence on the poliomyelitis situation. In the 12 years following 1957 the schedules and vaccines were gradually adjusted to make them more effective. The principal changes were the introduc- tion in 1962 of quadruple vaccines adsorbed on aluminium phosphate and in 1966 the 2-2.5-fold increase in the concentration of the poliovirus com- ponents in these vaccines. Revaccinations were offered from 1965 onwards at 3 and 8 years of age. Since the introduction of these procedures paralytic poliomyelitis has disappeared in fully vaccinated children. Only one child who had received one injection of diphtheria-tetanus-poliomyelitis vaccine caught the disease in 1968. The problem remains of how to reach the children whose parents refused to allow them to be vaccinated. Progress has been made in this respect but unvaccinated children in areas with low coverage remain particularly vulnerable. Results of antibody titrations in nonvaccinated subjects showed that many of these children lacked antibodies against one or more poliovirus types: about half were triple negative (Table 5). Type 1 poliovirus was, and is still, the prevailing type in paralytic poliomyelitis patients. In the period 1958-69 the percentages of types 1, 2, and 3 isolated were 81, 3, and 16, respectively. These percentages are different from those found in the USA and other European countries where vaccination is carried out with live oral poliomyelitis vaccine (Cockburn & Drozdov, 1970; Miller et al., 1970). Types 3 and 2 were isolated relatively less frequently in the Nether- lands than in these areas. The results of the present study appear to show that fully vaccinated patients yielded type 3 virus more frequently than nonvacci- nated patients. This could mean that the protection against paralytic poliomyelitis induced by type 3 virus is less than that induced by type 1, and this conclu- sion seems to be supported by the results of polio- virus antibody determinations in vaccinated children. These showed a less favourable picture for type 3 than for types 1 and 2 (Hofman, 1971). For a comparison we can look at the results in Scandinavian countries, where inactivated poliomye- litis vaccines have been used almost exclusively 743 B. HOFMAN (Radovanovic, 1966; Cockburn & Droldov, 1970). The results there look more favourable but probably there are no pockets with low coverage in these countries. In the municipalities with acceptance rates over 90% in the Netherlands, the situation has become comparable with that in the Scandinavian countries. Other reasons for the different results might include population density, family size, and housing conditions, which are probably more favour- able for virus dissemination in the Netherlands. The material presented extends the earlier conclusion (Hofman, 1967) that inactivated poliomyelitis vaccine can offer practically complete protection provided that a vaccine of good quality is used and a high rate of acceptance is achieved. The vaccination pro- gramme in the Netherlands as described above with combined preparations containing diphtheria, teta- nus, pertussis and poliomyelitis components offer a simple and effective system for immunization in the early years of life. POSTSCRIPT A serious outbreak of poliomyelitis, caused by poliovirus type 1, occurred in the town of Staphorst in the Netherlands (about 10 500 inhabitants) in February-March 1971 (Bijkerk et al., 1972). The rate of acceptance of vaccination in this town was less than 50%. Altogether 37 cases were recorded, including 26 paralytic cases: 5 of the patients with paralysis died. All the cases were in nonvaccinated subjects. ACKNOWLEDGEMENTS The author thanks Dr H. Bijkerk and his staff of the Office of the Chief Medical Officer for providing data about poliomyelitis cases, virus isolations and vaccine acceptance rates. RtSUMt LA POLIOMYELITE AUX PAYS-BAS DE 1958 A 1969: INFLUENCE D'UN PROGRAMME DE VACCINATION PAR UN VACCIN ANTIPOLIOMYILITIQUE INACTVE La vaccination antipoliomy6litique a ete introduite aux Pays-Bas en 1957. On a d'abord utilis6 des vaccins simples puis, en 1962, les poliovirus vaccinaux ont ete incorpor6s a une preparation mixte renfermant 6galement de l'ana- toxine dipht6rique, de l'anatoxine t6tanique et un com- posant anticoquelucheux, ainsi que du phosphate d'alu- minium comme adjuvant. Les vaccinations sont prati- quees suivant un sch6ma strict comportant 3 doses a l'age de 3, 4 et 5 mois respectivement, suivies d'une injection de rappel vers l'age de 11 mois. On procede a une revaccination a 3 et a 8 ans. Le taux annuel moyen de la morbidite est pass6 de 4,5 pour 100 000 en 1924-57 a 0,20 pour 100 000 en 1958-69, soit une reduction globale de 95,5%. La letalite, exprim6e par le nombre de d6c6s pour 100 cas paraly- tiques, ne s'est pas sensiblement modifi6e au cours des periodes 1954-57 et 1958-69, atteignant respectivement 5,1 et 5,3%. La protection conf6re'e par la vaccination s'est pro- gressivement renforc6e de 1958 a 1969. En 1958-60, la r6duction du nombre des cas de poliomyelite paralytique chez les sujets completement immunis6s etait de 83,6%, 95,0% et 78,9% dans les groupes d'age 1-4 ans, 5-9 ans et 10-14 ans. En 1965-69, les chiffres correspondants 6taient de 100%, 99,4% et 100%. On a constat6 simulta- n6ment une baisse de 60 a 80% de la morbidite chez les sujets non vaccin6s, due probablement a la diminution de la quantit6 de virus circulant. Selon I'auteur, la vaccination antipoliomy6litique pratiqu6e A l'aide d'un vaccin inactiv6 assure pratique- ment une protection complete, si l'on utilise un vaccin de bonne qualit6 et si l'on obtient un taux 6lev6 de partici- pation de la part de la population. La m6thode adopt6e aux Pays-Bas, bas6e sur l'emploi d'un vaccin mixte antipoliomy6litique, antidiphterique, antit6tanique et anticoquelucheux, repr6sente un moyen simple et efficace d'immunisation dans les premieres annees de la vie. 744 POLIOMYELMS IN THE NETHERLANDS 1958-69 745 REFERENCES Bijkerk, H. et al. (1972) Ned. T. Geneesk., 116, 549-558 Centraal Bureau voor de Statistiek (1953-69) Maandsta- tistiek van de bevolking en de volksgezondheid, Utrecht, Ed. de Haan Centraal Bureau voor de Statistiek (1963) Bevolking van Nederland: Leeftijd en geslacht 1900-1952, Utrecht, Ed. de Haan Cockbum, W. C. & Droldov, S. G. (1970) Bull. Wld Hlth Org., 42, 405-417 Henry, J. L. et al. (1966) J. Hyg. (Lond.), 64, 105-120 Hofman, B. (1967) J. Hyg. (Lond.), 65, 547-557 Hofman, B. (1971) Ned. T. Geneesk., 115, 1148-1151 Miller, D. L. et al. (1970) Publ. Hlth (Lond.), 84, 265- 285 Perkins, F. T. et al. (1958) Brit. med. J., 2, 68-71 Perkins, F. T. & Gaisford, W. (1959) Brit. med. J., 1, 680-682 Radovanovid, M. R. (1966) In: Proceedings of the Eleventh Symposium ofthe European Association against Poliomyelitis and Allied Diseases, Rome, 1966, Paris, Masson, p. 17 Verlinde, J. D. & Wilterdink, J. B. (1970) Arch. ges. Virusforsch., 32, 311-17 Weiland, H. T. (1968) Verspreiding van poliomyelitis en andere faecaal uitgescheiden virussen in Nederland gedurende de jaren 1964 en 1965, Thesis, Leiden, N.V Leidsche Drukkerij, p. 36-75 Wilterdink, J. B. et al. (1969) Arch. ges. Virusforsch., 28, 68-76 Wilterdink, J. B. et al. (1970) Arch. ges. Virusforsch., 32, 82-90 5