We relied on the epidemiological and virological surveillance network for influenza and other respiratory viruses conducted since 1998 by the Peruvian Ministry of Health (MoH) [18] . In 2006, the MoH of Peru expanded the surveillance network by joining efforts with the US Naval Medical Research Unit -6 (NAMRU-6) and scaled up its coverage to a total of 50 inpatients and outpatients Sentinel health care centers, uniformly distributed across geographic regions of Peru [13, 19] . Moreover, as a result of the global pandemic alert issued by the World Health Organization (WHO) on May 9, 2009, the MoH of Peru intensified surveillance efforts through a public health directive [20] and expanded the identification of influenza-like-illnesses (ILI) to include all public and private health care centers in the country. ILI was defined as a sudden onset of fever ($38uC) and cough or sore throat fewer than five days in duration, with or without general symptoms such as myalgias, prostration, headache, or malaise [21] . Nasal and/or oropharyngeal swabs were taken from a random sample of ILI patients and sent to the Instituto Nacional de Salud and NAMRU-6 for testing by reverse transcriptase polymerase chain reaction (RT-PCR) [18] . Case definitions and laboratory diagnostics did not change throughout the pandemic period covered in our study. However on July 7, 2009, public health authorities ordered prioritization of hospitalized ILI cases for A/H1N1 influenza testing in all health care centers, except for the 50 official sentinel units where testing of mild and severe ILI cases was maintained [22] . A chronology of events relating to the 2009 A/H1N1 influenza pandemic in Peru is given in Table 1 .