Sequential and combined use of inactivated and oral poliovirus vaccines: Dolj District, Romania, 1992-1994.

To determine the feasibility of a vaccination strategy that would reduce the risk of vaccine-associated paralysis while retaining a barrier against the spread of wild poliovirus, a 2-year project was undertaken using enhanced-potency inactivated poliovirus vaccine (IPV) administered at 2 and 3 months of age followed by doses of both IPV and oral poliovirus vaccine (OPV) administered at 4 and 9 months of age. Vaccination coverage by 12 months of age with three or more doses of IPV and two doses of OPV among 16,566 infants eligible for vaccination was > 95% and > 80%, respectively. Among 51 children from whom blood samples were obtained 45 days after their third dose of IPV and first dose of OPV, 100% had serum neutralizing antibodies (reciprocal titer > or = 10) to all three poliovirus types. No cases of paralytic poliomyelitis due to either wild or vaccine-related strains were reported. The project demonstrated the feasibility, safety, and high immunogenicity of sequential use of IPV followed by OPV in Romania.

Paralytic poliomyelitis in Romania, 1984-1992. Evidence for a high risk of vaccine-associated disease and reintroduction of wild-virus infection.

Although poliomyelitis due to wild-virus infection has virtually disappeared from Romania, with no cases having been documented between 1984 and 1989, vaccine-associated paralytic poliomyelitis has been reported at very high rates for over two decades. In November 1990, to decrease the risk of vaccine-associated paralytic poliomyelitis, oral poliovirus vaccine produced in Romania was replaced by imported oral vaccine made by a Western European manufacturer. To better quantify the risk of vaccine-associated paralytic poliomyelitis and the impact of the change in vaccine manufacturer, the authors reviewed clinical, epidemiologic, and laboratory data on poliomyelitis cases that occurred in Romania from 1984 to 1992. Poliovirus isolates were characterized at the US Centers for Disease Control and Prevention. During the period 1984-1992, 132 confirmed cases of paralytic poliomyelitis were reported in Romania, of which 13 were classified as wild-virus-associated, 93 as vaccine-associated, and 26 as "of unknown origin." Wild type 1 poliovirus was isolated during 1990-1992 from nine of 13 (69%) cases in an outbreak that occurred primarily among undervaccinated gypsy children. Vaccine-associated cases were epidemiologically and virologically distinct from wild-virus cases. Of the 93 vaccine-associated cases, 45 children were recipients and 48 were contacts. The overall risk of vaccine-associated paralytic poliomyelitis in Romania (1 case per 183,000 doses of oral poliovirus vaccine distributed) was 14-fold higher than the risk in the United States. The risks of recipient vaccine-associated paralytic poliomyelitis related to the first dose of oral vaccine were similar for Romanian and imported vaccine (1 case per 95,000 doses and 1 case per 65,000 doses, respectively), as were the total risks of vaccine-associated paralytic poliomyelitis. These findings definitively demonstrate a substantially elevated risk of vaccine-associated paralytic poliomyelitis in Romania which was not affected by a change in oral poliovirus vaccine manufacturer.

Serosurvey for polio antibodies.

From 1982 to 1988 a serological survey was performed on 573 subjects aged 3-80 years in order to evaluate the polio immunity level of the population. The presence of neutralizing antibodies was tested using the types 1, 2 and 3 Sabin vaccine strains as well as a wild strain of poliovirus type 1 isolated in France in 1978. According to their birth date, the subjects were assigned to 4 groups. The differences between the groups consisted in the different vaccination history of the subjects as well as in their different opportunities of having been in contact with the past epidemics of poliomyelitis. The results obtained indicate a satisfactory polio immunity level in all the 4 groups: seropositives, 96.7%-98.9% for type 2, 91.8%-98.2% for type 1 (Sabin vaccine strain), 89.3%-96.6% for type 3 and 84.2%-96.4% for type 1 wild strain. The highest immunity levels were found in group D (children with recorded history of complete polio vaccination) and in group A (unvaccinated people but contemporary with the past polio epidemics). A special comment is made with respect to 14 subjects showing satisfactory antibody titres for all the three types of Sabin-vaccine strains but who have proved to be seronegative (less than 4) for the wild type 1 poliovirus strain.

Optimum age for measles immunization in Romania.

From 1979 to 1985 a serological survey was carried out to evaluate the opportunity to start measles vaccination at the age of 9 months. The following aspects were investigated: the persistence of maternal measles antibodies in 188 infants aged 1-8 months: such antibodies could not be detected over 7 months of age; the antibody response to measles vaccination in 181 infants vaccinated at the age of 9-11 months versus 291 children vaccinated when aged more than 12 months: no significant differences were found between the two groups; the age-specific persistence of vaccine-induced hemagglutination-inhibiting antibodies during the 1-4 years following vaccination: no significant difference was found in terms of the children's age upon vaccination. The results lend support to the recommendation to consider 9 months as a satisfactory age to initiate measles immunization in Romania.

WHO collaborative studies on poliovirus type 3 strains isolated during the 1968 poliomyelitis epidemic in Poland.

In 1968 in Poland an extensive outbreak of poliomyelitis, caused by type 3 poliovirus, began about four months after small vaccine trials with the Leon 12a(1)b (Sabin) and USOL-D bac vaccine strains had been carried out. Because of the temporal association, and because the first cases appeared in the province in which the USOL-D vaccine trial was carried out, a detailed investigation of the strains isolated from cases in the epidemic was made in four laboratories in an attempt to determine whether they were related to the two vaccine strains or to a "wild" strain. All the studies were made under code. The rct marker was of no help in determining the relationship of the epidemic strains to the vaccine strains. The McBride test and the elution marker test clearly separated the Leon 12a(1)b strains from those from the cases, but were incapable of detecting whether the epidemic strains were related to the USOL-D bac strain or to wild type 3 strains. Thus the studies did not provide valid information on the origin of the epidemic.