Update: Poliomyelitis Outbreak -- Finland, 1984-1985

From August 1984 through January 1985, nine cases of paralytic disease and one case of aseptic meningitis due to wild poliovirus type 3 were reported in Finland. The diagnoses were confirmed by either viral isolation or serology. The poliomyelitis outbreak was recognized in late October 1984, when a 6-year-old child from Vantaa, a neighboring town to Helsinki, developed aseptic meningitis; poliovirus type 3, subsequently characterized as "wild-like," was isolated from the child's stool. Further investigation of healthy family members and other close contacts revealed that 39 (45%) of 86 were shedding poliovirus type 3 in their stools and/or throats. During late 1984 and early 1985, wild-like poliovirus 3 was isolated from sewage samples from 10 locations in the Helsinki district and from 13 of 21 other cities or towns sampled in the country.

Concurrent with the recognition of widespread poliovirus circulation in the country, a case of paralytic poliomyelitis was recognized in a 17-year-old patient with flaccid paralysis who had been diagnosed in late October 1984 as having Guillain-Barre syndrome. The diagnosis was confirmed following the isolation of wild-like poliovirus type 3 in mid-November. Surveillance was intensified, and six more cases of paralytic poliomyelitis were diagnosed in various regions of Finland over the next 3-month period--one with onset in late October, three in November, one in December, and the last in January 1985.

A retrospective review was conducted of all patients in Finland with paralysis seen by a neurologist and/or an infectious-disease consultant since summer 1983. Two additional clinically compatible cases of paralytic poliomyelitis (i.e., acute onset of asymmetric flaccid paralysis without sensory involvement) were identified. Diagnoses of poliomyelitis were confirmed serologically. One patient had onset of illness in August 1984; the other, in September 1984 (Table 1).

Routine immunization with inactivated polio vaccine (IPV) was initiated in Finland in 1957. The recommended schedule for primary immunization consists of three doses of IPV given at 5 months, 6 months, and 2 years of age and booster doses at the ages of 6-7 years, 11-12 years, and 16-17 years. Additionally, conscripts in the army are vaccinated at about 20 years of age. Further booster doses are recommended at 5-year intervals, especially for those traveling abroad (1).

Before this outbreak, the last reported case of paralytic poliomyelitis occurred in Finland in 1964 in an unvaccinated 1-year-old girl from whom poliovirus type 3 was isolated. Surveillance for polioviruses by survey screening of stools from preschool-aged children, sewage testing, and isolation of viruses from individuals with aseptic meningitis was carried out routinely from 1964 through 1982, with consistently negative results (1,2). Sewage testing and survey screening of stools were not done between 1982 and 1984, but stool specimens from approximately 1,000 patients with aseptic meningitis and other neurologic diseases suspected to be of viral origin were tested annually. No additional wild-like polioviruses were found.

As soon as widespread circulation of poliovirus type 3 was recognized, the health authorities recommended an extra dose of the IPV used in Finland for all children under 18 years of age. Approximately 1.5 million doses were administered during the 4-month period November 1984-February 1985. From mid-February to mid-March 1985, a mass campaign with trivalent oral polio vaccine (OPV) was conducted. A single dose of OPV was offered to every person in the country, except infants under 6 months of age. Immune-deficient individuals and their household contacts also were excluded from the OPV campaign and were offered a more potent IPV that is more immunogenic than the usually available IPV. Overall, an estimated 94% of the target population of 4.8 million received a dose of OPV. Following the OPV campaign, routine childhood immunization was continued with the standard IPV. The more immunogenic IPV is expected to be in use in all areas of Finland by early 1986.

Regular screening for polioviruses in sewage and active surveillance for aseptic meningitis and other neurologic illnesses was reinstituted. No wild- or vaccine-like polioviruses have been isolated from the sewage specimens from various cities since May 1985. No cases of poliomyelitis were reported from neighboring countries, some of which use IPV exclusively.

All poliovirus type 3 isolates from the outbreak in Finland that were analyzed by oligonucleotide fingerprinting (3) were shown to be closely related wild strains. Sera from U.S. children in three age groups who had received at least three doses of OPV were tested for the presence of neutralizing antibodies to the Finland isolate (P3/Fin/84/23127) and three other genetically distinct poliovirus type 3 strains (Sabin 3, Saukett, and P3/USA/80/1565) (Table 2). Neutralizing antibodies to all four strains were detected in 97% of the sera from children aged 16-25 months and in 96% of the sera from children aged 5-7 years. A greater proportion of children in the 10- to 12-year age group had neutralizing antibodies to Sabin 3 (92%) and P3/USA/80/1565 (96%) than to Saukett (79%) or P3/Finland/84/23127 (75%). Reported by National Public Health Institute and National Board of Health, Helsinki, Finland; Div of Viral Diseases, Center for Infectious Diseases, Surveillance, Investigations, and Research Br, Div of Immunization, Center for Prevention Svcs, CDC.

Editorial Note

Editorial Note: The occurrence of poliomyelitis and the isolation of wild poliovirus type 3 in October 1984 were the first proof of indigenous wild poliovirus circulation in Finland during the last 20 years.

The IPV preparation used in Finland for the past 20 years induces low levels of antibodies to poliovirus type 3 but relatively higher levels to poliovirus types 1 and 2 (1). A seroprevalence survey in 1982 among 3-year-old children in Finland who had received full primary series of IPV showed that only 30% had neutralizing antibodies to type 3 poliovirus at a dilution of 1:4 (1). In some children, a secondary seroresponse followed revaccination, implying that they may have been immune before revaccination (1). As a consequence of this and other serosurveys, the health authorities in Finland decided to use a more immunogenic IPV beginning in 1986.

There also may have been a gradual decline in vaccination coverage in recent years in Finland. Coverage assessments for two doses of IPV in the early 1970s showed approximately 99% coverage. Coverage assessments for complete three-dose primary vaccination became available beginning in the late 1970s and suggested a decline among 3-year-old children, reaching a low of 78% in 1983. Vaccination coverage is much higher among school-aged children, since additional vaccination is given through the schools.

Among the younger age groups of U.S. children tested, significant differences were not demonstrated in the percent seropositive to the Finland isolate and to Sabin type 3 strain. The finding of slightly lower seropositivity rates against Saukett and P3/Fin/23127/84 isolates in the oldest group of U.S. children than against the Sabin 3 vaccine strain is consistent with well-documented antigenic differences among poliovirus strains within a serotype (5,6) and declines in neutralizing antibody titers with time since immunization. Lower neutralization titers have been found in serosurveys that use a heterologous strain of the immunizing vaccine virus as the test antigen than in those that used a homologous antigen, i.e., the actual vaccine strain (7,8). When neutralization titers to the immunizing strains are low ( less than or equal to 1:8), neutralizing antibodies to heterologous strains may be present but not detected at dilutions of 1:4, the lowest practical dilution in a standard neutralization test. When tested for neutralizing antibody to Sabin 3 virus, all three age groups showed 90% or higher seroprevalence, indicating that the U.S. population of children immunized with OPV would be protected if exposed to the Finland isolate.

The conditions responsible for the poliomyelitis outbreak and for wide dissemination of the wild poliovirus type 3 in Finland are not known definitively, but may have involved a combination of factors, including: (1) the immunogenicity of the type 3 component of the IPV used in Finland during the past 20 years was relatively low; (2) vaccine coverage may have declined in recent years; and (3) the antigenic differences between the Finland strain and the type 3 component of the Finland IPV may have allowed substantial replication and transmission of the outbreak virus among people with very low titers of neutralizing antibodies to type 3 polioviruses.

Since Finland is no longer endemic or epidemic for polio disease, CDC Advisory Memorandum No. 76 (issued January 2, 1985), which recommended that travelers to Finland be immune to poliomyelitis, was withdrawn on August 21, 1985 (see CDC Advisory Memorandum No. 82).

References

1. Lapinleimu K. Elimination of poliomyelitis in Finland. Rev Infect Dis 1984;6(suppl 2):S457-60.

2. Oker-Blom N, Penttinen K, Weckstrom P. Inactivated poliovirus vaccine in Finland. Rev Infect Dis 1984; 6(suppl 2):S461-2.

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