During the 2009-2010 influenza epidemic season, Pskov oblast witnessed the emergence of a new virus, А(H1N1) pdm, to which most of the population had no immunity. This virus caused irregular and extensive outbreaks and demonstrated extremely high morbidity during the autumn. This pandemic virus exhibited nearly total dominance over other seasonal influenza viruses and an uncommon clinical manifestation, i.e. the most severe cases were usually registered among young people.
Influenza and other acute respiratory viral infections (ARVI) pose a threat to the health of the population of all age groups. During the 2009-2010 epidemic season, the epidemiological situation was aggravated by the emergence of the highly pathogenic variant of the influenza virus that necessitated the intensification of epidemiological surveillance.
The article reports a general analysis addressing the spread of influenza in Pskov oblast during the 2009-2010 epidemic season. The basic tendencies in the development of the epidemiological situation are described in relation to various cohorts of patients; the common characteristic of the epidemic is provided alongside with the results of laboratory diagnostics for influenza and other types of ARVIs.
Data from the Main Base of the Federal Centre for Hygiene and Epidemiology in Pskov oblast have been used. For many years the Main Base has been compiled using routine epidemiological analysis. The method is based on the determination of epidemiological thresholds and estimation of control levels of weekly non-epidemic incidence for influenza and ARVI given in absolute values for the city of Pskov and Pskov oblast and presented in the guidelines on efficient analysis and forecasting of influenza and ARVI (1). Basic criteria for early recognition of the beginning of an epidemic are established when epidemic thresholds are steadily exceeded, and the weekly disease incidence and hospitalization rates start growing more and more rapidly.
This article includes information about the number of registered cases of influenza and ARVI, lethal outcomes and hospitalization cases per calendar week for the whole population, including children in the age from 0 to 2 years, from 3 to 6 years and from 7 to 14 years, adults from 15 years of age and older, and from 65 years of age and older. A total of over 230 clinical samples were examined by a molecular genetic method and over 2500 immunofluorescence assays were performed.
The hospitalization rates were analyzed with regard to the patients who were diagnosed with influenza or ARVI during the pandemic (H1N1) 2009. The case fatality dynamics was analyzed based on the information about the laboratory-confirmed cases of the pandemic influenza (H1N1) 2009.
Pskov oblast has a population of 688,600 and is located in the northwest of the European part of Russia. It shares borders with Estonia, Latvia and Belarus and neighbours Leningrad, Novgorod, Tver and Smolensk oblasts.
Analysis of the epidemiological situation shows that the influenza epidemic in Pskov oblast started in week 44 (26.10 – 01.11.2009). During this week, all age groups were affected by the epidemic except for children of 3-6 years (Figure 1). In the weeks that followed, the epidemic began to spread in other cities and districts of the oblast.
Fig. 1. Weekly incidence of influenza and acute respiratory viral infections in the city of Pskov in 2009
The incidence peak was reached in week 47. The average duration of epidemiological threat was 8 weeks. During the epidemic rise, 55,642 people were infected in the oblast, or 8.0% of the population. In Pskov and Velikiye Luki 11.5% and 9.2% of the population was infected, respectively. The most affected by the infection group were children of 7-14 years of age. Teaching at educational institutions was suspended on November 3, 2009 and resumed 14 days later. The closing of schools helped to reduce the incidence of the disease. The situation in the region stabilized in the middle of December 2009.
Fig. 2. Number of tested for pandemic infuenza (H1N1) 2009 and number of positive cases by week in 2009 in Pskov oblast
The results of monitoring over the circulation of causative agents of influenza and ARVI show that the viruses of non-influenza etiology prevailed in the etiological structure of infections diagnosed in September. Before the epidemic growth of influenza and ARVI started (in week 44), examined patients received the following diagnoses: 0.5% pandemic influenza, 2.1% seasonal influenza (H1N1) and (H3N2), 14% parainfluenza, 7.8% respiratory syncytial virus (RSV) infection and 5.2% adenoviral infection.
After October 20, the proportion of (H1N1) 2009 pandemic influenza started to grow. By the end of November (week 47), the pandemic influenza began to dominate and was detected in 22.4% of examined patients. In the beginning of December the proportion increased to 27.3%. Meanwhile, the circulation of other respiratory viruses during this period continued (Figure 2). By November20, parainfluenza was detected in 12% of the examined patients, in addition to 4% adenoviral infection, 8.7% RSV infection, 1.8% seasonal influenza A(H1N1), 1.5% influenza А (H3N2) and 0.4% influenza В.
Circulation of the A(H1N1)pdm pandemic influenza virus was registered sporadically as confirmed by laboratory diagnostics of the А/California/07/2009 influenza virus.
The incidence of seasonal influenza was controlled by immunization. A special immunization campaign in the oblast organized for this purpose. Funded by the federal budget, 110 thousand people were vaccinated against seasonal influenza in the frames of the national priority project. The following vaccines were used:
- “Grippol Plus” - 23,500 children in kindergarten and young school children in classes 1-4 were vaccinated (100% of the plan),
- “Grippol” - 86,500 older children and healthcare workers, teaching staff, people over 60 years of age were vaccinated (100% of the plan).
In addition, vaccination of about 8 thousand people was financed from other sources at the expense of companies, institutions and individual citizens. In total 17% of the population of Pskov oblast was vaccinated. Besides the seasonal vaccination, immunization of risk groups against pandemic influenza was also conducted. Persons at risk included life support workers, health care workers and social workers, teachers, patients with chronic diseases, students, schoolchildren, children and some other categories of the population. More than 62 thousand people were immunized against the pandemic influenza.
Most of the patients were adults (60.5% of all cases) between 18 to 39 years (44.2%). Among the children, the highest share of cases was registered among schoolchildren of 7-14 years (17.2%) and teenagers of 15-17 years (8.7%). The average incidence among the population at large was 9.9 per 100,000 population, 32.1 per 100,000 population among children under 2 years of age and 33.7 per 100,000 population among children from 3 to 6. Among schoolchildren from 7 to 14 years the incidence was 27.6 per 100,000 population. Among adults the average incidence was 6.3 per 100,000 population. It is worth noting that the disease prevalence among the elderly was quite low. The average incidence among people over the age of 65 was 1.2 per 100,000 population, which was 6 times less than among people of 15-64 years of age.
The hospitalization of patients suffering from influenza and ARVI during the influenza pandemic was analyzed. The average share of hospitalized patients from the total number of people infected with influenza and ARVI in all groups of the population was 3.4%, ranging from 6.4% (for children of 0-2 years of age) to 2.1% (for schoolchildren of 7-14 years). The share of hospitalized patients among adult population was 3.5%, for people of 15-64 years of age this value was 3.6%. The highest hospitalization rates were registered among children aged 0-2 years followed by adults aged 15-64 years. The lowest rate was reported for the elderly (3). The high share of hospitalization among patients aged of 15-64 years who suffered from influenza and ARVI indicates that the clinical form of the disease in this age group was especially severe. The share of hospitalized people with influenza and ARVI related to the total population for all age groups was 0.3% ranging from 2.1% for children under the age of 2 years to 0.2% among people over the age of 15 years. This value was higher for people of 15-64 years of age (0.3%) than for people of over 65 years.
The case fatality dynamics among the laboratory-confirmed influenza cases was analyzed. Starting from the beginning of the epidemic, 17 fatal cases caused by the (H1N1) 2009 influenza were registered in the oblast (mortality was 2.4 per 100,000 population). The first lethal outcome was registered in week 46. The highest number of lethal outcomes was registered at the incidence peak and during the two following weeks. In the age structure of fatal cases in the 2009 epidemic 94.1% were registered among people aged 15 to 64 years. People aged 18 to 53 years amounted to 70.5% of fatal cases. The proportion of fatal cases among three age categories (18-31 years, 32-41 years and 42-53 years) differed slightly (11.8% to 29.4%). The share of fatal cases among people aged 54 to 64 years was 4 times lower, and the share of cases among people of over 65 years of age was only 5.8%. Lethal outcomes were not registered among children under 14 years of age.
The pandemic influenza (H1N1) 2009, showed more intense epidemic process indicated by the greater number of people infected in different cities and districts of the oblast.
In Pskov oblast, the 2009-2010 epidemic season started uncommonly early, earlier than any other epidemics of the last few decades, on 26.10.2009, together with the seasonal growth of ARVI. The epidemic differed from other epidemics by a greater proportion of the population in all cities and districts of the oblast that was infected, that it lasted for 8 weeks, and the incidence was high among all groups of the population (4). The epidemic was characterized by evident monoetiology (5).
The highest epidemic activity was registered among schoolchildren aged 7-14 years and people over the age of 15. Persons in these age groups were the first to be infected. The lowest hospitalization rates were reported among the elderly (65 years and older) as compared to the total number of infected people and the total population of the region during the epidemic. The epidemic of influenza (H1N1) 2009 was also characterized by severe forms of the disease and the high fatality among adults aged 18 to 64 years with laboratory confirmed diagnoses (2).
During the epidemic, the elderly over 65 years were infrequently infected and hospitalized. The share of hospitalized patients in this age group was 6 times lower than among people of 15-64 years of age. This may be due to the fact that the elderly had a less severe clinical form of the disease. The share of lethal outcomes among the elderly was low. This age group was previously exposed to a virus of the А(Н1N1) subtype in the period from 1946 to 1956, a virus that was similar to the currently circulating influenza, and to have acquired anamnestic immunity against the disease.
To prevent the emergence of and to localize the foci of influenza and ARVI, a set of preventive and counter-epidemic measures were taken. Teaching in educational institutions was suspended and public cultural and athletic events were cancelled. In addition, out-of-schedule sanitary inspections in public places were held. The population was informed about personal and collective prevention measure for influenza and ARVI infection. All these measures helped to prevent the wider spread of the pandemic influenza virus in the oblast.
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