Influenza surveillance in Zambia, 2008-2009.

Background: Limited information exists about influenza viruses in Africa. We used data from a new sentinel surveillance system to investigate the seasonality and characteristics of influenza, including pandemic (pdm) influenza A H1N1, in Zambia.

Methods: In June 2008, we established sentinel surveillance for influenza-like illness (ILI) and severe acute respiratory illness (SARI) at 4 healthcare facilities in Zambia. Nasopharyngeal and oropharyngeal swabs and structured questionnaires were collected from eligible patients and samples were tested by real-time reverse-transcription polymerase chain reaction for influenza virus types and subtypes.

Results: From June 2008 to December 2009, we collected 1234 specimens, of which 334 (27%) were ILI, and 900 (63%) were SARI. Overall, 4% (57) of specimens were positive for influenza. The influenza detection rate in ILI and SARI cases was 5% (17/334) and 4% (40/900), respectively. Among all influenza cases, 54 (95%) were influenza A and 3 (5%) were influenza B. Of the influenza A viruses, 16 (30%) were A(H1N1)pdm09, 29 (54%) were seasonal A(H1N1), 6 (11%) were A(H3N2), and 4 (7%) were unsubtyped. The detection rate for A(H1N1)pdm09 cases was highest in persons aged 5-24 years (5/98; 5%), 25-44 years (4/78; 5%), and 45-64 years (1/17; 6%). Conversely, for seasonal influenza the detection rate was highest in children aged 1-4 years (18/294; 6%). Influenza virus circulation peaked during June-August in both years and A(H1N1)pdm09 occurred at the end of the influenza season in 2009.

Conclusions: Seasonal influenza virus infection was found to be associated with both mild and severe respiratory illness in Zambia. Future years of surveillance are necessary to better define the seasonality and epidemiology of influenza in the country.