Results of influenza surveillance during the pandemic and post-pandemic influenza season in AP Vojvodina, Serbia

Abstract

Introduction. Although influenza is longest and most studied infectious disease, to date, not much has been achieved in preventing and combating this disease. Many infectious diseases are now eliminated or significantly reduced, but only influenza remained epidemic and pandemic character. Influenza epidemics occur every year. Increase in number of patients occurs in early winter. Pandemics occur over long intervals of time and are caused by new subtypes of the influenza virus. Objective. To analyze the epidemiological characteristics of influenza through surveillance of influenza during 3 seasons of monitoring. Method. Data for the analysis were gathered from epidemiological surveillance of influenza through sentinel surveillance, virological surveillance and control of severe acute respiratory distress syndrome. Sentinel physician network consisted of doctors from the public sector, general practitioners, pediatricians, and specialists of emergency medicine and pulmonologists from intensive care units. Supervisors were experts from the Institute of Public Health of Vojvodina, Novi Sad. Samples from the patients (nasopharyngeal swabs) were tested by PCR in National Reference Laboratories (Institute of Virology, Vaccines and Sera 'Torlak' in Belgrade) and in Institute of Public Health of Vojvodina). Results. During the pandemic season, based on the recorded incidence or intensity of clinical activity, incidence rate of the influenza virus infections was above the intermediate threshold (246.3) for the territory of Vojvodina in the period from 45th to 52nd week of surveillance with the peak incidence at 50th week (1090.3 / 100,000). In the next two seasons of sentinel surveillance intensity of virus activity threshold was above the average over the period of 4 to 10 weeks (season 2010/11) and of 11 to 13 weeks of surveillance (season 2011/12). The largest number of patients was registered in the age group of 5-14 years, and the lowest number of patients was in age group over 65. Summarized results of virological surveillance of influenza show that during the pandemic season the most frequent isolate was the virus influenza A (H1N1). The largest number (24) of confirmed cases was registered in 51st week of surveillance, which coincided with the highest activity of the virus influenza during sentinel surveillance of ILI. Within virological surveillance in the season 2010/11 out of total of 213 tested samples of patient material, infection was confirmed in 112 samples: influenza A (H1N1) was confirmed in 90% (101/112), type B in 6.3% (7/112) and influenza A (H3N2) in 3.6% (4/112) samples. During the last season dominant infection was influenza A (H3N2) with a share of 96.5% (55/57) of all confirmed cases of sentinel and non-sentinel specimens tested. The average age of the 38 patients with fatal outcome during the seasons 2009/10 and 2010/11 was 47.1 (range: 12 to 76 years), and all were confirmed to infection with influenza virus A (H1N1). Obesity had the largest share as a factor of comorbidity, and was registered in 36.8% (14/38) patients with fatal outcome. Conclusion. The influenza virus is constantly present in a population with different intensity from year to year. Disease is the most often registered in the preschool and school children, but fatalities due to influenza are usually registered among the working population.

Keywords

• influenza
• sentinel surveillance
• pandemic influenza
• virological surveillance