Delayed 2009 Pandemic Influenza A Virus Subtype H1N1 Circulation in West Africa, May 2009-April 2010

To understand 2009 pandemic influenza A virus subtype H1N1 (A[H1N1] pdm09) circulation in West Africa, we collected influenza surveillance data from ministries of health and influenza laboratories in 10 countries, including Cameroon, from 4 May 2009 through 3 April 2010. A total of 10 203 respiratory specimens were tested, of which 25% were positive for influenza virus. Until the end of December 2009, only 14% of all detected strains were A(H1N1) pdm09, but the frequency increased to 89% from January through 3 April 2010. Five West African countries did not report their first A(H1N1) pdm09 case until 6 months after the emergence of the pandemic in North America, in April 2009. The time from first detection of A(H1N1) pdm09 in a country to the time of A(H1N1) pdm09 predominance varied from 0 to 37 weeks. Seven countries did not report A(H1N1) pdm09 predominance until 2010. Introduction and transmission of A(H1N1) pdm09 were delayed in this region.