A total of 22,994 ILI cases were reported to the Peruvian Ministry of Health from May 1 to December 31, 2009 . Of all reported ILI cases, 18,139 were laboratory tested for A/H1N1 influenza infection (78.9%), and 8,994 cases were confirmed as having A/H1N1 influenza (39.1%). The average RT-PCR positivity throughout the pandemic was 49.6% (see also Figure  A in Text S1 for time trends in RT-PCR positivity rate). Daily timeseries of ILI and A/H1N1 influenza cases in the greater Lima metropolitan area and the rest of Peru were highly synchronized (Spearman rho = 0.8-0.9, P,0.0001).