Worldwide transmission and seasonal variation of pandemic influenza A(H1N1)2009 virus activity during the 2009-2010 pandemic

BACKGROUND: Seasonal influenza activity varies with geography and time of year.

OBJECTIVE: To describe how pandemic influenza A(H1N1)2009 [A(H1N1)pdm09] activity varied during the 2009-2010 pandemic.

METHODS: We analyzed influenza virological data compiled by the World Health Organization from June 2009-August 2010. We calculated weekly proportions of A(H1N1)pdm09-positive specimens out of all A(H1N1)pdm09-positive specimens detected during the study period for each country. We compared parameters of pandemic activity (e.g., peak A[H1N1]pdm09 weekly proportion [peak activity], number of weeks between the 5th and 95th percentiles of A(H1N1)pdm09 cumulative weekly proportion [duration of activity]) between countries in temperate and tropical-subtropical regions. We quantified the proportion of A(H1N1)pdm09 out of all influenza A specimens by country and correlated it with countries' central latitudes.

RESULTS: We analyzed data from 80 countries (47 temperate, 33 tropical-subtropical). The median proportion of cases identified during the peak week was higher in temperate (0·12) than in tropical-subtropical (0·09) regions (P<0·01). The median duration of activity was longer in tropical-subtropical (27 weeks) than in temperate countries (20 weeks) (P < 0·01). In most temperate countries (98%), peak pandemic activity occurred during the fall-winter period. There was a positive correlation between country central latitude and proportion of A(H1N1)pdm09 out of all influenza A specimens (r: 0·76; P<0·01).

CONCLUSIONS: The transmission of A(H1N1)pdm09 exhibited similarities with seasonal influenza transmission in that activity varied between temperate and tropical-subtropical countries and by time of year. Our findings suggest the potential utility of accounting for these factors during future pandemic planning.