From 1976 to 1983, on average around 9000 cases of mumps were registered annually. Mass vaccination against mumps, which began in 1983, had a significant impact on the epidemic process of this infection. The number of mumps cases was reduced to less than 0.5% in comparison with the pre-vaccination period. The first rise in number of cases in the post-vaccination period was noted in 1997, when 469 persons had the disease. The second rise was registered in 2000 and 2001. In this article we describe the epidemiology of mumps in Latvia in 2000-2003.
Fig. 1. Mumps incidence in Latvia, 1992-2003
Materials and methods
We have used epidemiological investigation cards from the epidemic areas and data from official reports, applying descriptive and graphic methods for processing the data.
Vaccine effectiveness was calculated on the basis of data from the epidemiological investigation, considering children with reliable data on vaccination as vaccinated, using the statistical number of children in the given age-group (data from 2000-2003) and also data on the vaccination coverage. Vaccine effectiveness (VE) was calculated using the following formula:
where the attack rates (AR) were calculated by the following formulas:
Laboratory diagnostics of mumps cases and serological analysis (ELISA IgM and IgG) were carried out at the virological research laboratory at the State Agency "Public Health Agency". In 2000-2003, the diagnosis "mumps" was based on the clinical picture of the disease and laboratory confirmation in a small number of the patients.
The epidemiological analysis of the number of mumps cases in the observed period allows us to conclude that the periodic rise began in 2000, when 1949 cases or 80.4 per 100 000 of the population were registered. Mumps was registered in all territories of Latvia, with incidence variation between 2.6 and 300 per 100 000 population. An incidence above the country average was registered in eight of 33 administrative territories. In comparison to 1999, the incidence increased 48 times.
In 2001, there was a sharp increase in number of mumps cases in all territories in Latvia. The growth constituted 251% and 6834 cases (288 per 100 000 inhabitants) were registered. The incidence was between 4.7 and 655 per 100 000 inhabitants by territories. An above average incidence was registered in 11 of 33 administrative territories.
Starting in 2002, the number of mumps cases decreased: 231 cases or 9.9 per 100 000 population were registered, varying between territories from 2.3 to 53 per 100 000 population. An above republican average was registered in nine of 33 administrative territories.
In 2003, 107 cases (4.6 per 100 000 inhabitants) were registered. The incidence in the territories varied between 2.2 and 13 per 100 000 inhabitants. An above average incidence was registered in 10 of 33 administrative territories.
In total, 9021 persons had the disease from 2000 to 2003 (Fig. 1). In 2000-2003 the diagnosis was laboratory confirmed in on average 16% of the patients.
Fig. 2. Mumps incidence among males
Fig. 3. Mumps incidence among females
From 2000 to 2002, the highest incidence was registered among children at 3-5 years of age (Fig. 2 and 3). In the year of the epidemic peak (2001) a high incidence was also observed in the age group 10-17 years. In 2003, the highest incidence was observed in the age group 6-9 years (21 cases per 100 000 inhabitants) and in children at the age of one (26 cases per 100 000 inhabitants).
The incidence among males was 1.5 times higher than among females in 2000-2003 (on average 118 and 78 cases per 100 000 inhabitants respectively). Males constituted 56% of the patients.
Complications of mumps
An average 5.9% of the patients developed complications from the disease. Over the four-year period, orchitis (2.6%) and pancreatitis (2.1%) were the most common, 1% of the patients developed aseptic meningitis and less than 0.1% meningoencephalitis.
Table. 1. Complications of mumps (%)
Vaccination status, vaccination coverage and vaccine effectiveness
In accordance with the vaccination programme, children at the age of one had not been vaccinated. Vaccine coverage of children at the age of 2 had reached the WHO recommendation and constituted 98.6% by 2003.
The annual coverage of the second vaccine for 7-year old children increased, but was still not sufficient. In addition, the coverage of the second vaccine for teen-agers at 14-15 years was very low in 2000.
Among the mumps patients in 2000-2003, most were vaccinated (Table 3).
Table. 2. Vaccination coverage in given age-groups (%)
Table. 3. Number and share of vaccinated patients, 2000-2003
In 2001-2003, the interval between vaccination and disease was 3-4 years in 27.2% and nine years in 51.6%. An interval between vaccination and disease of more than nine years was observed in 74.5% of the vaccinated patients in 2000, in 53% in 2001, in 16% in 2002 and in 23.1% in 2003. In 2002, in 25.6% of the patients, the disease was established in less than a year after vaccination (Table 4).
Table. 4. Interval between vaccination and disease (only first dose)
Taking into account that large proportions of vaccinated persons were observed in different age groups, the effectiveness of the mumps vaccine was analyzed.
Table. 5. Effectiveness of mumps vaccine (%), 2000-2003
NA – No available (all patients were vaccinated)/расчет невозможен (все заболевшие вакцинированы)
The lowest effectiveness of the vaccine was observed at the age of 2 years, possibly in connection with the use of a low immunogenic vaccine. In the remaining groups, the vaccine efficacy was sufficiently high.
Population immunity against mumps
In 2000, 461 persons over the age of 5 were tested serologically. The highest prevalence of seronegative persons among males as well as females was observed in the age group 10-17 years, where the prevalence of all seronegative persons was between 38% and 55%. A rather high prevalence of all seronegative persons was also observed in the age group 18-29 years: males 43.2%, females – 26.7% (Fig. 4).
Fig. 4. Age distribution of seronegative persons and mumps incidence in 2000
Fig. 5. Age distribution of seronegative persons and mumps incidence in 2001
In 2001, 2812 persons from all age groups, beginning at age 2 years were tested serologically. The prevalence of seronegative children was especially high in the age group 4-5 years (boys 77%, girls 66%) and from 6 to 9 years (boys 51% and girls 55.4%).
Among adults, the highest prevalence of seronegative persons was in the age group from 18 to 29 years: 37.2% among males and 33.8% among females (Fig.5).
In 2002, only adults in one age group from 18 to 29 were tested serologically, and the prevalence of seronegative persons was 31% (Fig. 6).
Fig. 6. Age distribution of seronegative persons and mumps incidence in 2002
Fig. 7. Age distribution of seronegative persons and mumps incidence in 2003
In 2003, 3029 persons from 2 to 69 years were serologically tested. 13.9% of those examined were seronegative to mumps. The prevalence was high among children from 4 to 9 years (29.0%) and in the age group from 2 to 3 years - 23.3% of the children were seronegative. Among adults, a high prevalence of seronegative persons dominated in the age group from 18 to 29 years (15.2%). The highest prevalence of all seronegative persons among males as well as females was observed in children under 15 years (Fig. 7).
We believe there are three reasons for the epidemic of mumps in Latvia in 2000-2001:
а) Insufficient immunity in school children, for whom more than 10 years had passed after the first injection (at age 2) (revaccination of children at age 7 against mumps started only in 1998); therefore, many unvaccinated teen-agers were susceptible to infection.
b) Insufficient immunity in persons who were born before the introduction of vaccination (at the end of the 1970s) and who had not had the disease;
c) Insufficient coverage of revaccination among 8 year old children; a coverage of 95% was reached only in 2001-2003.
It is necessary to evaluate the sensitivity and specificity of the methods used in laboratory diagnostics of mumps and for seroepidemiological analysis.
Epidemiological investigation records, which had not in all cases been filled out with the use of data from medical documentation, were used as source for data on vaccination status, something which as a result may have led to incorrect classification of vaccine status in mumps patients.
During the epidemic it was decided to vaccinate also on epidemiological indicators: in total, more than 190 000 persons were vaccinated in 2000 - 2003.
We conclude that from 2000 to 2001 Latvia experienced an epidemic of mumps. In the peak year (2001) the incidence was 22 times higher than in 1997, the previous year of maximum rise in the incidence. The highest incidence during the epidemics was registered among children in the age groups 3 to 5 years and 10 to 17 years. The proportion of all vaccinated patients (at least one vaccine) was 71% during the epidemic, including 23% revaccinated. The average interval between vaccination and disease was 3-4 years, which may be related to the low efficacy of the given vaccine used for vaccination in the given years. The data from the serological tests show that among children older than 3 years, there was a high proportion of seronegative persons in the years 2000, 2001 and 2003.
We recommend that the organisation of the vaccination programme should be improved. This will include improving the account of the child population and planning of the vaccination, improved tracing of children who did not receive the vaccine at the planned time, improving the control and compliance with the cold-chain and performing an annual analysis of the coverage of immunization in children in given age groups (according to statistical forms prepared in advance). Furthermore, special attention should be given to public-relations work on the importance of vaccination.
We also recommend that the EU case definitions should be applied when diagnosing mumps.
The authors acknowledge the reviewers and especially I. Velicko for very helpful comments and suggestions.