Laboratory confirmed circulating Influenza virus: Analysis of trend and seasonality in Nigeria from 2008–May 2019

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Background: National Reference Laboratory detected different subtypes of influenza virus circulating among human population through national influenza sentinel surveillance for Ifluenza Like Ilnesses (ILI) and Severe Acute Respiratory Infection (SARI) across four geopolitical zones of Nigeria to help explain the epidemiology of influenza in Nigeria. Methods and materials: Nasopharyngeal/Oropharyngeal samples were collected from patients who met the criteria for ILI/ SARI in four sentinel sites, were tested by Real Time rtPCR technique. Data generated from April 2008 to May 2019 were analyzed by Epi info data analysis software and Microsoft Excel. Results: A total of 13792 samples were documented from April 2008–May 2019 with a total of 1351 positive and 12441 negative. Of which 869 (6.3%) tested positive to Flu A, 480 (3.5%) to Flu B, 2 (0.01%) to Flu A/B co-infection. The circulating influenza subtypes were H1N1pdm09 404 (3.2%), AH1N1 15 (0.1%), AH3N1 345 (2.8%), Flu B-Victoria 24 (0.2%), Flu B-Yamagata 3 (0.02%), Flu not subtype 107 (0.9%), Flu A un-subtype 453 (3.6%). Influenza positivity in females was 625 (46%) and 711 (53%) in males, missing variables for gender was 15 (1.%). Patient with Influenza Like Illnesses (ILI) with highest proportion of Flu A was among age-group 5-9 and 55–59 with 8%. Cases with Severe Acute Respiratory Infection (SARI) having highest proportion of FluA were age-group 50–54 and 55–59. Influenza B virus among ILI cases are higher among age-group 5–9 with 15%. There was no observed commonality in seasonal peaks across the years. Subtype A/H1N1pdm09 circulate the most after the pandemic of 2009 alongside A/HI & A/H3. While recently A/H3 and influenza B-Victoria are commonly isolated. The least influenza subtype circulating is influenza B-Yamagata. Seasonal trend of all the circulating subtypes was around September each year. Conclusion: Laboratory data showed year round influenza virus circulation with the younger and older age-groups affected the most in both ILI and SARI cases. Influenza AH1N1pdm(09) and Influenza A/H3 are the most isolated influenza virus strains in Nigeria with a recent increase in the incidence of FluA and FluB coinfection.