Influenza surveillance in five consecutive seasons during post pandemic period: results from National Influenza Center, Turkey

Abstract

Influenza surveillance provides data about the characteristics of influenza activity, types, sub-types and antigenic properties of the influenza viruses in circulation in a region. Surveillance also provides for the preparation against potential influenza pandemics with the identification of the genetic properties of viruses and the mutant strains that could pose a threat. In this study, data in the scope of national influenza surveillance carried out by National Influenza Center, Turkey for five consecutive influenza seasons between 2010-2015, following the A(H1N1)pdm09 virus pandemic, have been presented and evaluated. A total of 15.149 respiratory samples, including 8.894 sentinel and 6.255 non-sentinel specimens, during 2010-2015 influenza seasons, within the periods between September and May, were evaluated in our center. All samples were tested using real-time reverse transcriptase PCR (rRT-PCR) for the presence of influenza virus types and subtypes. Within the sentinel influenza surveillance, the samples that were detected negative for influenza viruses, have also been tested for the other respiratory viruses (respiratory syncytial virus, rhinoviruses, paramyxoviruses, coronaviruses) using the same technique. Further analysis, including virus isolation by cell culture inoculation and antigenic characterization by hemagglutination inhibiton test were performed for the samples found positive for influenza A and B viruses. Selected representative virus isolates have been sent to WHO reference laboratory for the sequence analysis. In the study, influenza virus positivity rates detected for all of the samples (sentinel+non-sentinel) were as follows; 34% (779/2316) in 2010-11 season; 25% (388/1554) in 2011-12; 20% (696/3541) in 2012-13; 23% (615/2678) in 2013-14; and 26% (1332/5060) in 2014-15. When all the samples were considered for influenza A and B viruses, the positivity rates for the seasons of 2010-11; 2011-12; 2012-13; 2013-14; 2014-15 were determined as follows; 49.9% and 50.1%, 71.6% and 28.4%; 98.3% and 1.7%; 73.6% and 26.4%; 48.1% and 51.9%, respectively. The frequency of respiratory viruses detected only in sentinel samples other than influenza, were found as follows; 10% (148/1435) in 2010-11; 18% (175/963) in 2011-12; 23% (415/1768) in 2012-13; 22% (468/2108) in 2013-14; and 21% (546/2620) in 2014-15 seasons. When the distribution of influenza virus subtypes were considered, the detection rates of A(H1N1)pdm09 and A(H3N2) viruses in all of the samples were 55% and 45%; 0% and 100%; 95% and 5%; 2% and 98%; and 79% and 21% in the seasons of 2010-11; 2011-12; 2012-13; 2013-14; and 2014-15, respectively. For B/Victoria and B/Yamagata lineages, those rates in the same order of seasons were found as 15% and 85%; 98% and 2%; 0% and 100%; 16% and 84%; and 2% and 98%, respectively. When the data were evaluated, for 2010-2011 and 2014-2015 seasons, cocirculation of influenza A and B were observed within the same periods and similar proportions, but the peak activity occurred 7-8 weeks later for 2014-2015 season. For both seasons, A(H1N1)pdm09 viruses were predominant for non-sentinel, while A(H3N2) viruses were predominant for sentinel detections. During 2011-2012 and 2013-2014 seasons, influenza activity presented similar profile; predominance of A(H3N2) and B viruses observed while A(H1N1)pdm09 virus detections remained low. In contrast, for the 2012-2013 season, A(H1N1)pdm09 viruses were predominant and the detection of A(H3N2) and B viruses remained low. For the seasons in which A(H3N2) was predominant, the peak activity seen earlier than the other seasons. For all seasons, A(H1N1)pdm09 viruses in circulation were antigenically compatible with the vaccine virus, while A(H3N2) viruses were compatible for three seasons (2010-11, 2012-13, 2013-14) and incompatible in two seasons (2011-12, 2014-15). Influenza B viruses were determined as antigenically compatible with the vaccine viruses in all except 2010-2011 season. Predominance of B/Victoria lineage were observed during 2011-2012 season while the rest of the majority were B/Yamagata. The positivity rates of other respiratory viruses which were also analyzed in the scope of sentinel influenza surveillance were similar to influenza positivity for all seasons except 2010-2011. This fact has emphasized that, those viruses were also responsible for influenza-like illness especially during the early and late phases of the season. In conclusion, monitoring of the antigenic and genetic characteristics of influenza viruses by surveillance studies is essential for the early detection of potential pandemic variants as well as to ensure similarities among the circulating strains and the corresponding vaccine strains.