The virological laboratory of the State Institution City Laboratory Centre of the State Sanitary and Epidemiological Surveillance Centre in Saint-Petersburg and laboratory of childs viral infections of Pasteur Institute developed serological investigation methods which became one of the main components of epidemiological surveillance. They revealed the target groups most susceptible to measles and tactics for specific prevention of the disease in the city on the basis of epidemiological surveillance (incidence, vaccination rate, humoral immune response, etc.) on different stages of vaccination.
How did we use the results of epidemiological surveillance? At the start of large-scale revaccination against measles in 1987 it was decided to administer it not at the age of six as stipulated by the Order of the Ministry of Health of the USSR, but to students of the 8th-9th forms, vaccinated earlier in 1973-1975. That decision was based on incidence data analysis among vaccinated in different years and postvaccinal immune response to measles. The results showed higher incidence among those vaccinated in 1973-1975 than among students vaccinated in other years. The prevalence among vaccinated was 1,5-2 times higher than the average rate among vaccinated per 100 000 population according to analysis data in 1985-1986. The data correlated with analysis data on postvaccinal immune response to measles. Students vaccinated in 1973-1975 had higher seronegative rate (close to 9,0) compared with the average rate for the city (6,0). The result justified the decision for routine revaccination of senior secondary school students. Already since 1989 the incidence rate at schools, colleges and technical schools was reduced to single occurrence, and since 1995 there were no outbreaks registered in higher education institutions.
One more example of the results of epidemiological surveillance: Immunologic structure of the population and the immunity of the urban population to the infection were calculated on the basis of data on the immune part of population, and the results of serological control of immune response to measles, in order to determine epidemiologically important risk groups. Immune protection was defined as combination of those who had been through the disease and vaccinated persons, where seronegative numbers (according to the records) were subtracted from the number of persons with immune response. Taking into consideration that scheduled vaccination in Saint-Petersburg had been implemented since 1964, the age margin of the population with postvaccinal immune response reached 45-48 years. Persons over that age limit, who had measles earlier, had 100% protection against the infection and did not participate in the epidemic spread. Protection of the population at the age from 1 to 45 years depended on vaccination and revaccination coverage by live measles vaccine, and share of seronegative ones among vaccinated.
Taking the above mentioned into account, it was concluded that the highest herd immunity to the infection was among population 7-14 years old (96%) and 15-30 years old (95%). Children at the age from 1 to 6 years (94%) and adults vaccinated once at the age of 30-40 years (93%) were less protected. Immunity of infants under 1 year depended on maternal immunity and was usually lost by the age of 10-12 months.
Immunologic protection analysis of different age groups of population was the basis for the decision in 1998 to carry out large-scale revaccination against measles of those who were not sick or were not revaccinated earlier, born in 1954 and later, worked in preschool child-care institutions, schools, colleges and higher educational institutions, and in health-care institutions of different profiles. Those measures contributed to prevention of the disease in all epidemiologically important institutions.
In 1978-1994 selective control of biological and immunogenic activity of live measles vaccine supplied to Saint-Petersburg provided considerable assistance in implementation of measures to decrease measles incidence. Among tested vaccine series only 73% had titre stipulated by technical documentation. As the result of the follow-up seven especially weak series of live measles vaccine were not used for vaccination because the virus content in vaccine was not high enough. It allowed to prevent introduction of low quality vaccine to over 40 000 children. Epidemiological surveillance and implementation of specific prevention measures against measles in Saint-Petersburg resulted in decrease of incidence to the level of single occurrence. In the recent five years only sporadic cases has been registered in the city, and prevalence did not exceed 2,0 per 100 000 population.
At present the World Health Organisation aims at elimination of measles by 2010 in specific geographic areas. The Russian Federation has developed a national programme aimed at elimination of the infection by 2007 and certification of measles-free territories by 2010.
On the basis of this programme Saint-Petersburg has developed the city programme for elimination of measles on its territory by 2010. It highlights the great importance of epidemiological surveillance under sporadic occurrence of the disease.