Epidemiological Investigation of an Influenza Сluster in a Pre-School Institution in Tashkent

  Published: 21.06.11 Updated: 21.06.2011 15:37:18

R.A. Rakhimov, R.R. Rakhimov, N. Ibadullaeva, D. Suyarkulova

Research Institute of Virology, Tashkent, Uzbekistan

Citation: Rakhimov RA, Rakhimov RR, Ibadullaeva N, Suyarkulova D. Epidemiological Investigation of an Influenza Cluster in a Pre-School Institution in Tashkent. EpiNorth 2011; 12: 18-23.

An epidemiological investigation of a cluster of an influenza-like disease at an orphanage in Tashkent during October – December 2009 that occurred simultaneously with a seasonal increase of influenza was conducted. In total, 77 children aged 9 – 16 years were infected within 12 days. A PCR-based laboratory test confirmed influenza A virus in 34 children and influenza A(H1N1) 2009 in 5 children. The clinical and epidemiological data as well as the laboratory test results indicated that the clustered case was a rather severe local influenza (H1N1) 2009 outbreak. The infectiousness rate was as high as 0.83 and the secondary attack rate was 1.1-29.4%.

The data enabled an accurate evaluation of the epidemic situation and an improvement in preventive and counter-epidemic measures that were implemented at the beginning of the influenza pandemic.

A rapid spread of the new influenza virus A (H1N1) 2009 that caused a worldwide pandemic was first observed in 2009. Central Asia was also affected by the virus [1,2]. In Uzbekistan, an early seasonal increase in the incidence of acute respiratory infections (ARI), mainly among children, was observed amid the influenza pandemic at the end of October and beginning of November 2009 in Tashkent, the largest city of the country. In November 2009, the reported incidence among the population of Uzbekistan was 2,063.7 per 100 000 population.
During the previous 5 years, the seasonal activity of influenza and acute respiratory disease epidemic process started later and was not as intense. The average incidence rate in November usually ranged from 508.2 to 691.4 per 100 000 population.

The influenza outbreak in Tashkent (October 24, 2009 – week 43) occurred simultaneously with the seasonal increase of influenza and ARI incidence in the city. The number of cases grew by 22.7% on average every week. The incidence was above the epidemic threshold for 7 weeks (from week 43 to week 49). The maximum influenza and ARI incidence was observed in week 47 [3]. Compared to the incidence registered in many countries participating in the EuroFlu surveillance system [2], in general the epidemic activity of influenza and ARI in Uzbekistan was not high at that time.

The influenza and ARI incidence varied for different age categories (0 – 2 years, 3 – 6 years, 7 – 14 years, 15 years and older) during the epidemic period of 2009. Figure 1 shows the incidence in different age groups during weeks 43-49 of 2008 and 2009.

Fig.1 Influenza and ARI incidence by age groups during weeks 43-49 of 2008 and 2009 in Tashkent

Fig.1 Influenza and ARI incidence by age groups during weeks 43-49 of 2008 and 2009 in Tashkent















The highest incidence of influenza and ARI is normally reported among children aged 2 years or younger. Among older children, the rate is 1.5 – 7.7 times lower. However, during the 2009 epidemic period the number of cases grew rapidly among children aged 2 years and younger as well as children aged 3 – 14 years. The peak incidence among children of all ages was reported in weeks 46 and 47. In adults, an increase in the incidence was also observed, but it was much less intense than in children.

The goal of the study was to conduct an epidemiological and laboratory investigation of the major cluster of an influenza-like disease that was registered in a pre-school institution in Tashkent.

Materials and Methods
An epidemiological investigation of an ARI cluster in an orphanage sheltering 93 children of both sexes aged 9 – 16 years was conducted in Tashkent (population of 2 239.4 thousand) during the early seasonal increase of influenza and ARI incidence in the beginning of November 2009.

The outbreak investigation and examination of children started on the 10th day after the first case had been reported (October 24, 2009). A provisional diagnosis and disease severity analysis was made in accordance with the standard case definition approved by the Ministry of Healthcare. The case definition for influenza includes a rapid rise in body temperature to 38ºC or higher, dry cough, pain in the nasopharynx and behind the chest sometimes combined with muscle pain and/or headache. The definition for ARI of the upper respiratory tract includes a rise in body temperature to 37.5ºС or higher, inflammation of the mucous membranes in the upper respiratory tract sometimes combined with mucous and/or purulent discharge from the upper respiratory passages. ARI of the lower respiratory tract is characterized by a rise in body temperature to 38.0ºC or higher, dry cough with pain behind the chest or productive cough with serous sputum sometimes combined with short breath of more than 24 breaths per minute and/or auscultatory dry or moist and/or crackling rales in the lungs.

Nasopharynx swabs were taken from 50 children who were infected within 5 days prior to the beginning of the investigation. The swabs were tested by PCR for influenza [4]. The test consisted of 2 stages. First, the swabs were tested for influenza A. Positive swabs were then tested for influenza A(H1N1) 2009. Due to the limited availability of primers, only 5 swabs randomly chosen from 39 influenza A-positive samples were analyzed at the second stage of the study.

During the epidemiological investigation, 7 children suspected of having influenza were examined in the department of a children’s hospital. The girl who had been the first to get infected in the orphanage was also treated at this hospital. Sixteen cases of acute respiratory disease were reported among 34 employees of the orphanage, but they were not examined because they were absent from work on the first days of sickness. None of the children had been vaccinated against seasonal flu and influenza (H1N1) 2009. The infected children had not been given any antiviral drugs before the examination. None of the children had ever left the country.

The infectiousness rate in the focus of the infection (i) was determined by calculating the ratio between the number of children infected within 12 days (x) and the total number of contacting children (n) using the following formula: i = x:n. The secondary attack rate (p) was calculated by dividing the number of disease cases revealed among the contacting children (y) during the influenza (H1N1) 2009 incubation time (from 2 to 7 days) [1] since their contact with the infection source by the total number of contacting children (n) using the following formula: p = y∙100:n.

By the day of examination, 69 children had been infected with acute respiratory diseases. In the next 2 days additional 8 cases were detected. Based on the standard case definition and the clinical manifestations of the disease, all patients were preliminarily diagnosed with “suspected influenza”.

A 14 year old girl was the first to be infected. She fell ill after returning from a children’s hospital after inpatient treatment. A laboratory test for influenza performed on the 10th day of her illness was negative. Among 50 infected children who were examined, there were 39 (78.0%) laboratory-confirmed influenza A cases. Five randomly selected influenza А-positive samples were confirmed to be the pandemic influenza A (H1N1) 2009. Laboratory analysis of samples obtained from 77 children confirmed 34 cases of influenza A of which 5 (14.7%) were influenza (H1N1) 2009.

Laboratory results confirmed 2 cases of influenza (H1N1) 2009 among 7 children in the children’s hospital. This indicated intrahospital influenza (H1N1) 2009 infection and the possibility that the girl from the orphanage was infected at the hospital through contact with other patients. Among the children who had contacted this girl, there were some laboratory-confirmed influenza (H1N1) 2009 cases detected. Thus she was considered to be the source of influenza virus A (H1N1) 2009 in the orphanage.

Among 77 infected orphanage children, 5 had laboratory-confirmed influenza (H1N1) 2009. The outbreak was short-term and local; the source of the infection was contact with people who had influenza (H1N1) 2009; the clinical manifestations of the diseases were similar in the infection focus; laboratory tests confirmed that the majority of the infected children had influenza A and some of them had new pandemic influenza (H1N1) 2009. As for the rest 72 children from this orphanage, their probable diagnosis was also influenza (H1N1) 2009. Therefore the cluster case of influenza among the children in the orphanage was classified as a local influenza (H1N1) 2009 outbreak.

The initial 6 cases of the disease among contacting children took place within 2 days. One case occurred on the third day after appearance of the infection source in the orphanage and five cases were reported on the fifth day. In total, 76 of 92 children who had been in contact with the infection source became infected (Fig.1.) The infectiousness rate was 0.83. On the 7th day (at the end of the maximum incubation time), 27 children who had been in contact with the source got infected. The secondary attack rate was 1.1 – 29.4%. The peak morbidity was reached on the 8th day. The cumulative prevalence among contacting children was 54.4% on the 8th day.

The clinical condition of 46 infected children (59.7%) was of moderate severity, while the remaining 31 individuals (40.3%) experienced only mild disease. No severe clinical cases were identified. In 9 cases, complications were observed as the disease progressed. The clinical condition was complicated by acute pneumonia (2.6%) in 2 children, by acute bronchitis (6.5%) in 5 children and by acute otitis (2.6%) in 2 children.

Fig.2 Changes in the Number of Disease Cases over Time and the Results of Examination of the Children in the Orphanage in Tashkent (n=77)

Fig.2 Changes in the Number of Disease Cases over Time and the Results of Examination of the Children in the Orphanage in Tashkent (n=77)














An epidemiological investigation of a cluster of acute respiratory disease among children residing in a pre-school institution indicated that the source was probably at the children’s hospital where several cases of influenza (H1N1) 2009 were confirmed. Laboratory analyses conducted on samples from 50 children from the orphanage confirmed influenza A among 34 children, and 5 children were all confirmed influenza A(H1N1) 2009-positive. Due to limited resources, random sampling was performed to determine dominance of influenza A and circulation of the new (H1N1) 2009 influenza. Uniform clinical manifestations of the disease, the short period between contact and infection, and positive laboratory results thus indicate that the clustered case was a local outbreak of influenza (H1N1) 2009.
In addition to the infection source, 76 children in the orphanage had clinically manifested influenza. The consecutive interval, i.e. the average time between onset of consecutive cases in the chain of transmission, for the new (H1N1) 2009 influenza for the next 6 children was 4 days, similar to that reported in other investigations [5]. The infectiousness rate in relation to the first case of the disease (the assumed source of the infection) was 0.83. The secondary attack rate in the focus during the first 2-7 days was 1.1 – 29.4%. In previous seasons, the epidemic process of acute respiratory diseases in the facility was less intense, with prevalence ranging from 11.7±3.2% to 26.7±4.2%.

The short duration of the outbreak and high prevalence rate among children show that the local epidemic process of influenza in the institution was rather intensive [6]. This influenza outbreak took place simultaneously with an increase of disease incidence in the city that was probably caused by the emergence of the new influenza virus A(H1N1) 2009. The incidence in the city occured earlier than usual among a population, especially school children and young people, with a higher susceptibility to this virus [1,7].

In general, the epidemiological situation regarding influenza and ARI in Uzbekistan and other countries of Central Asia was characterized as moderate [2]. However, even under moderate conditions intensive local influenza outbreaks can take place. The intensity of the influenza epidemic process in the pre-school institution can be explained by exposure to the new pandemic virus to which the population was very susceptible and by the close living conditions at the institutions [1,7]. Under other conditions the transmission of influenza (H1N1) 2009 was not as intense [8, 9].

The disease incidence in the facility reached a peak on the 8th day when the general prevalence was 54.4%. After the 8th day the intensity of the epidemic process started to decline.

Despite the relative low activity of influenza in the region, an intensive local outbreak of influenza (H1N1) 2009 was revealed in the pre-school institution. The severity of the epidemic process can be explained by the high infectiousness of the new virus and general susceptibility of children in the orphanage to this virus as well as the close living conditions at the orphanage. The results of the epidemiological investigation proved that the new virus A(H1N1) 2009 circulated in the city. This explained the intensity of the seasonal increase of the disease incidence and changes in patients’ age categories. The data collected enabled an accurate evaluation of the epidemic situation and improved preventive and counter-epidemic measures for the influenza pandemic.


  • World Helath Organization.
  • The WHO Regional Office for Europe. /cgi-files/bulletin
  • Rakhimov R.A., Pereyaslov D., Ibadullaeva N.S., Rakhimov R.R. Experience in modernizing the system of epidemiological surveillance for influenza in Uzbekistan. EpiNorth 2010; 11: 87-93.
  • CDC Realtime RT PCR Protocol for Detection and Characterization of Swine Influenza (version 2009). Available from:
  • Morgan O.W., Parks S., Shim T. Household transmission of pandemic (H1N1) 2009, San Antonio, Texas, USA, April-May 2009. Emerging Infectious Diseases 2010;16:631-7.
  • Marchbanks T.L., Bhattarai A., Fagan R.P. et al. An Outbreak of 2009 Pandemic Influenza A (H1N1) Virus Infection in an Elementary School in Pennsylvania. Clin Infect Dis 2011; 52: 154-60.
  • Cutter JL, Ang LW, Lai FYL, et al. Outbreak of pandemic influenza A (H1N1-2009) in Singapore, May to September 2009. Annals of the Academy of Medicine Singapore 2010;39:273-82.
  • Donnelly CA, Finelli L, Cauchemez S, et al. Serial Intervals and the Temporal Distribution of Secondary Infections within Households of 2009 Pandemic Influenza A (H1N1): Implications for Influenza Control Recommendations. Clin Infect Dis 2011;52:123-30.
  • France AM, Jackson M, Schrag S, et al. Household transmission of 2009 influenza A (H1N1) virus after a school-based outbreak in New York City, April-May 2009. J Infect Dis 2010; 201: 984-92.