Influenza Viruses in Nigeria, 2009-2010: Results From the First 17 Months of a National Influenza Sentinel Surveillance System

Background. Influenza surveillance data from tropical, sub-Saharan African countries are limited. To better understand the epidemiology of influenza, Nigeria initiated influenza surveillance in 2008. Methods. Outpatients with influenza-like illness (ILI) and inpatients with severe acute respiratory illness (SARI) were enrolled at 4 sentinel facilities. Epidemiologie data were obtained, and respiratory specimens were tested for influenza viruses, using real-time reverse-transcription polymerase chain reaction assays. Results. During April 2009-August 2010, 2841 patients were enrolled. Of 2803 specimens tested, 217 (7.7%) were positive for influenza viruses (167 [8%] were from subjects with ILI, 17 [5%] were from subjects with SARI, and 33 were from subjects with an unclassified condition). During the prepandemic period, subtype H3N2 (A[H3N2]) was the dominant circulating influenza A virus subtype; 2009 pandemic influenza A virus subtype H1N1 (A[H1N1] pdm09) replaced A(H3N2) as the dominant circulating virus during November 2009. Among persons with ILI, A(H1N1) pdm09 was most frequently found in children aged 5-17 years, whereas among subjects with SARI, it was most frequently found in persons aged > 65 years. The percentage of specimens that tested positive for influenza viruses peaked at 18.9% in February 2010, and the majority were A(H1N1) pdm09. Conclusions. Influenza viruses cause ILI and SARI in Nigeria. Data from additional years are needed to better understand the epidemiology and seasonality of influenza viruses in Nigeria.