Epidemiology of influenza A and B viruses in Mauritius over an 8-year period, 2009-2016.

Influenza viruses show clear seasonality in temperate regions, while circulation in tropical countries is less well defined. This study characterised the seasonality, genetic diversity, antigenicity, vaccine match and antiviral susceptibility of influenza A and B viruses in Mauritius from 2009 -2016. A total of 10612 patients were enrolled through the National Influenza Surveillance Programme. Influenza virus detection was performed by rRT-PCR and virus isolation. Antigenicity was assessed using haemagglutination inhibition (HI) assays against viruses and reference antisera. HA and NA sequences were analysed for phylogenetic relationships, amino acid substitutions and neuraminidase inhibitor resistance markers. Influenza was detected in 18.1% (1917/10612) of cases, with similar proportions among ILI and SARI cases. A/H1N1pdm09 predominated in 2009, 2010 and 2014 whereas A/H3N2 dominated in 2012, 2013, 2015 and 2016. Influenza B prevailed in 2011 with co-circulation of both B/Yamagata and B/Victoria lineages in 2016. A/H1N1pdm09 remained antigenically similar to A/California/7/2009 vaccine strain. A/H3N2 isolates demonstrated reduced reactivity to egg-propagated vaccine strains. Influenza B viruses showed gradual antigenic drift. All viruses were phenotypically susceptible to oseltamivir and zanamivir. Mauritius exhibits year-round circulation of influenza viruses with ongoing antigenic drift, supporting the need for continued influenza surveillance. • Year-round influenza circulation observed in Mauritius with shifting lineages. • A/H1N1pdm09 remained antigenically matched to A/California/7/2009 • A/H3N2 showed reduced HI reactivity to egg-propagated vaccine strains • All Mauritius isolates were susceptible to neuraminidase inhibitor except one A/H3N2 I222V isolate.