This article describes the morbidity of vaccine preventable infections in the city of Severodvinsk. The study gives a detailed description of epidemiological trends over a long period of time, determines epidemiological characteristics of each of the investigated infections and shows timing charts as well as regularities in the increase and decrease of incidence rates. Cyclic trends for every disease within short-term and long-term periods have also been defined. The information helps to forecast the development of epidemiological process, to predict increases and decreases of morbidity and to plan organizational and financial measures. In addition, the epidemic process of each infection after introduction of immunization has been studied.
A decline in immunity during the 1900s resulted in an increase in the incidence of vaccine preventable infections in the city. For some infections, the incidence reached the pre-immunization rate. Lethal outcomes of some infections (e.g. diphtheria) were registered.
This study indicates that immunization with coverage rates over 95% is an effective means of decreasing the incidence of vaccine preventable infections to a non-significant level.
Communicable diseases play an important role in human pathology due to their direct and indirect influence on somatic, neuropsychic, physical and reproductive health of children, youth and adults. The goal of this study is to investigate specific trends in the incidence of vaccine preventable diseases over a 50-year period related to the introduction of immunization in the city of Severodvinsk.
The city of Severodvinsk has a population of 193,100 and is located on the south-eastern coast of the Dvina Gulf of the White Sea. Descriptive methods were used for analysis in order to evaluate morbidity of the diseases in different age groups (children, adults), for various nosological forms and periods of time.
Russian and foreign vaccines registered in the Russian Federation are used. The vaccination coverage is assessed by the documented number of vaccinated population. The vaccination coverage rate is calculated by the following formula:
C = (A/B) х 100, where
C is the coverage rate, in percent,
A is the number of children of a certain age (for the analyzed period) covered by vaccination,
B is the total number of children of this age registered at the health care facility.
The vaccination coverage includes also the children not fully immunized.
Results and Discussion
In Severodvinsk, cases of diphtheria started to be registered in 1944 (Fig.1). No data concerning disease incidence is available for the periods from 1946 to 1947 and from 1949 to 1954. In the 1950s, the reported incidence of diphtheria in the city was high, ranging from 392 per 100,000 population (in 1955) to 96 per 100,000 population (in 1960). In 1960 routine immunization was introduced, after which no new cases of diphtheria were registered in the city for 26 years (from 1963 to 1989). This favourable epidemic situation might have caused insufficient attention to preventive immunization. From 1984 vaccination coverage rates reduced to 88-92% leading to a decrease in the herd immunity. As a result, the infection returned in the early 1990s and the incidence increased and peaked at 16 per 100,000 population in 1994. From 1992 to 1995 a significant increase in the incidence of diphtheria was registered in Severodvinsk and in Russia on the whole . As a result of consistent routine vaccination a substantial decline in the incidence and severity of infection was reported in subsequent years. From 2004 no cases of diphtheria have been registered in the city.
Fig.1. Dynamics of diphtheria incidence (per 100,000 population) and diphtheria vaccination coverage in Severodvinsk (1944-2007)
Pertussis started to be registered in Severodvinsk in 1957 (Fig. 2). During the first 9 years the incidence was very high ranging from 1147 per 100,000 population in 1957 to 177 per 100,000 population in 1965. Following the introduction of vaccination the incidence decreased dramatically to 1-2 cases per 100,000 population. In the late 1970s and early 1980s, pertussis was almost eradicated. From 1969 to 1984 only sporadic cases were registered. The last fatal case of pertussis was reported in the city in 1958. As a result of the favourable epidemic situation, the importance of preventive measures was disregarded. In addition, this period was characterized by an unreasonable range of contraindications for preventive vaccination . From the 1970s, the number of vaccinated children decreased and the vaccination coverage rate varied from 37.6% to 77.7% until the mid 1990s. As a result herd immunity decreased and the incidence of disease increased. In 1993 the disease incidence in Severodvinsk reached 201 per 100,000 population and was thus two hundred times higher than in the early 1980s, comparable to the incidence reported in the city during the pre-immunization period. A whole decade, from 1989 (59 per 100,000 population) to 2000 (98 per 100,000 population), was characterized by a high incidence of pertussis. The incidence started to change in the beginning of the 21st century, indicating the reverse correlation between disease morbidity and immunization rate among children . The rapid decline of vaccination rates among children to 41.1% in 1990 caused the maximum increase in disease incidence in 1993. Starting from 1993 everything possible was done to raise the percentage of children vaccinated against pertussis. The amount of vaccination rejections due to medical contraindications was substantially reduced. Sanitary promotion measures were also taken. In 1993-1994, the vaccination coverage was restored. As a result, disease incidence stabilized and then rapidly declined to the level of the 1970s. At present morbidity due to pertussis is primarily among one year old children who have not been timely immunized for diphtheria-tetanus-pertussis (DTP) and among teenagers with a waning post-vaccination immunity .
Fig.2. Dynamics of pertussis incidence (per 100,000 population) and pertussis vaccination coverage (in %) in Severodvinsk (1957-2007)
The incidence of poliomyelitis has no correlation with population density and is usually observed as scattered cases. Since 1997 no case of acute poliomyelitis caused by wild-type virus has been registered in the territory of Russia . In Severodvinsk the poliomyelitis incidence was registered from 1955 to 1962 (Fig. 3) with two peaks reported in 1956-1957 (31 and 26 cases per 100,000 population, respectively) and in 1959 (18 cases per 100,000 population). There were also lethal cases of poliomyelitis during that period (1958-1962). Poliomyelitis vaccination started in 1960 for children at 2 months of age  and the coverage rate was high (more than 90% per year) during the whole monitoring period. The high coverage undoubtedly helped to support population immunity and since 1962 poliomyelitis has not been registered in Severodvinsk.
Fig.3. Dynamics of poliomyelitis incidence (per 100,000 population) and poliomyelitis vaccination coverage (in %) in Severodvinsk (1955-2007)
Natural susceptibility to measles is universal. In Severodvinsk registration of measles started in 1955 (Fig. 4). Since that time the incidence has been high and stable. During the pre-immunization period, increases in incidence were reported once every 4 or 5 years with peaks in 1966 (4217 per 100,000 population) and in 1970 (4904 per 100,000 population). In the first 5-6 years after the introduction of scheduled immunization, the incidence decreased by hundreds of times and just a few sporadic cases were reported. However, in 1978-1989 the incidence again increased to the level of the pre-immunization period. When vaccination started to be registered in 1976, the coverage rate among children ranged from 68% to 94%. The general decline in vaccination from the late 1980s to the early 1990s also influenced this infection. It was followed by a rapid increase in incidence and necessitated the re-evaluation of the significance of specific immunoprophylaxis. During the pre-immunization period children represented approximately 76% (ranging from 67 to 82%) of the patients. Following the introduction of mass immunization approximately 50% of the patients were more than 14 years of age. From the mid 1990s, disease morbidity among children and adults was sporadic and since 2000 no case has been registered. Sporadic lethal cases of measles among children registered in the city in 1964, 1965, 1970 and 1978.
Fig.4. Dynamics of measles incidence (per 100,000 population) and measles vaccination coverage (in %) in Severodvinsk (1944-2007)
Susceptibility to mumps is high (Fig.5) with a contagiousness index of 70-80% . In the first decade after cases of mumps were first registered, from 1955 to 1965, increases and decreases in incidence were quite regular and occurred once every 5 years: in 1955 (1059 per 100,000 population), in 1960 (1925 per 100,000 population) and in 1965 (1943 per 100,000 population). However, from 1967 the incidence grew radically and formed a morbidity plateau that exceeded the previously registered rates by 2-3 times. Against the background of this general growth tendency, there were also peaks reported every 2-3 years with incidence ranging from 1974 per 100,000 population (in 1974) to 3459 per 100,000 population (in 1982). In the city vaccination for mumps started in 1983 and 54% of children from 3 to 5 years of age were vaccinated. In the 15 years that followed the immunization coverage was not adequate and varied from 28% to 74%. The epidemic process continued. Since 2001 immunization coverage has been over 90% resulting in a rapid decline in incidence. From 1984, the incidence has decreased by 10 times: to 189 per 100,000 population (in 1985) and 200 per 100,000 population (in 1988). The incidence has continued to decrease with an epidemic growth reported once every 4-5 years. No case of mumps has been registered in the last several years, although in 2007 sporadic cases were reported.
Fig.5. Dynamics of mumps incidence (per 100,000 population) and mumps vaccination coverage (in %) in Severodvinsk (1948-2007)
The morbidity of viral hepatitis B is mainly associated with poor social and economic conditions. The epidemic process is actively fuelled by young able-bodied citizens. Patients of this age group prevail because the disease is mainly spread through parenteral and sexual transmission. Young drug users under the age of 30 constitute 80% of patients with a lethal outcome of viral hepatitis B . At present in our country parenteral hepatitis is changing in status from medical to social. The disease is often reported among family members as a result of active sexual and contact transmission .
In Severodvinsk, viral hepatitis B registration started in 1966 (Fig. 6). Prior to 1984 the infection was known as parenteral hepatitis and since 1985 has been registered as viral hepatitis B. Before 1983 the incidence was low and stable with minor increases in 1968 (10 cases per 100,000 population) and 1974 (14 cases per 100,000 population). There were 9 lethal outcomes of the disease among children registered between 1958 and 1975. From 1984 a significant increase in the morbidity and incidence of viral hepatitis B started in the city. Starting from the mid 1980s and continuing through the following two decades the incidence was high and stable with peaks in 1992 (42 cases per 100,000 population) and in 2000-2001 (58 and 85 per 100,000 population, respectively). As the incidence of viral hepatitis B is mainly associated with poor social and economic conditions, the increase in morbidity during the 1990s is quite explainable. Vaccination against viral hepatitis B was added to the immunization schedule in 2001. At first the immunization coverage was low; however, it grew steadily and reached 89.8% in 2007 that resulted in a significant decline in the morbidity of this severe infection. In 2006 and 2007 the incidence of the disease decreased to 3.6 per 100,000 population.
Fig.6. Dynamics of viral hepatitis B incidence (per 100 000 population) and viral hepatitis B vaccination coverage (in %) in Severodvinsk (1966-2007)
Regular increases in the incidence of rubella are very typical. Moderate growth is observed every 3-5 years and more intensive growth every 10-12 years . In Severodvinsk rubella started to be registered in 1979 (Fig. 7). During the monitoring period that lasted 30 years the incidence has been high and stable. During the first decade epidemic increases occurred every 2-3 years, while in the second decade (from 1989 to 1999) there were significant epidemic peaks in 1993 (1571 per 100,000 population) and in 1999 (4912 per 100,000 population), the last being the highest in city history. Rubella vaccination in Severodvinsk started in 2000. The vaccination coverage rate among children gradually increased from 11.9% (in 2000) to 72.8% (in 2007). Due to the introduction of vaccination the incidence of rubella has decreased and the decline has been particularly evident during the last 5 years.
Fig.7. Dynamics of rubella incidence (per 100,000 population) and rubella vaccination coverage (in %) in Severodvinsk (1979-2007)
The characteristics of epidemiological trends of vaccine preventable diseases that have been monitored in Severodvinsk during the past 50 years help to determine the specifics of epidemic for each of the investigated infections before and after the introduction of vaccination. In addition, data obtained through monitoring also defines the specifics of epidemic processes after the introduction of vaccination.
Analysis of vaccination coverage over a long-term period showed periodic decreases (typical of the 1990s) that resulted in a rapid increase in the incidence of vaccine preventable diseases in the city. The incidence of some infections even reached the rate observed during pre-immunization period. The study has shown that immunization at coverage rates over 95% is an effective means of decreasing the incidence of the diseases down to a non-significant rate.
This information helps to predict increases and decreases in the incidence of vaccine preventable infections and to plan organizational and financial measures. In addition, it enables an estimation of funds necessary for purchasing required vaccines as well as antibacterial and antiviral medicines for treating the infections.
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