Human Infection with Avian Influenza Virus, Pakistan, 2007 Mukhtiar Zaman, Saadia Ashraf, Nancy A. Dreyer, and Stephen Toovey Human infection with avian influenza (H5N1) virus raises concern for the possibility of a pandemic. We report 20 cases, which ranged from asymptomatic to fatal, in Pakistan in 2007. These cases indicate human-to-human- to-human transmission of this virus, and the number of cases may be higher than realized. E vidence of human-to-human transmission of influenza A (H5N1) virus raises concern over a possible pandemic (1). Previous epidemiologic investigation of the outbreak of influenza (H5N1) among persons in the Northwest Frontier Province of Pakistan (Figure 1) in 2007 found 5 cases—3 confirmed, 1 asymptomatic, and 1 probable—as defined by the World Health Organization (WHO) (2). We report a larger set of 20 cases during this outbreak in Pakistan, supporting human-to-human-to-human transmission. The Cases Records were examined from all hospitals that treated patients with influenza (H5N1) virus in Northwest Frontier Province during 2007. Data were rendered anonymous and entered into a secure database with predetermined clinical and epidemiologic fields. Cases matching predefined criteria (Table 1) were classified as laboratory confirmed, likely, or possible. Cases not meeting classification criteria were excluded. We slightly modified WHO criteria to resemble criteria that clinicians might adopt during an actual outbreak, especially in a resource-poor setting (3). We identified 20 cases—4 laboratory confirmed, 7 likely, and 9 possible—resulting in a ratio of 4 likely/ possible cases for each laboratory-confirmed case. Median age was 29 years (range 7–60 years) for all patients and 30 years (range 23–35 years) for confirmed case-patients; 16 (80%) patients were male. The infecting exposure could not be established for all patients because multiple exposures, human and avian, were recorded for some. Of the 4 patients with laboratory-confirmed cases, 3 were treated with oseltamivir (2 [67%] of whom survived), and 1 had asymptomatic disease and received no antiviral treatment. DISPATCHES 1056 Emerging Infectious Diseases  www.cdc.gov/eid  Vol. 17, No. 6, June 2011 Author affiliations: Khyber Teaching Hospital, Peshawar, Pakistan (M. Zaman, S. Ashraf); Outcome Sciences, Cambridge, Massachusetts, USA (N.A. Dreyer); and University College and Royal Free Medical School, London, UK (S. Toovey) DOI: 10.3201/eid1706.091652 Figure 1. Areas of influenza (H5N1) cases in humans, Pakistan, 2007. Red shading indicates districts that reported suspected human cases of influenza (H5N1). Light brown shading indicates Northwest Frontier Province. Source: World Health Organization (WHO). Districts of avian influenza suspected cases in Northwest Frontier Province, Pakistan. WHO map no. WHO-PAK-002 (www. whopak.org/disaster). Table 1. Case classification definitions used to diagnose influenza (H5N1) infection in humans, Pakistan, 2007* Classification Definition Laboratory confirmed Laboratory confirmation of influenza (H5N1) virus at local/regional or World Health Organization confirmatory laboratory Likely Definition 1 Epidemiologically linked by time, place, and exposure to a likely or confirmed human or avian influenza (H5N1) case AND Equivocal test OR positive laboratory confirmation of an influenza A virus infection but insufficient laboratory evidence for influenza (H5N1) virus infection AND Clinical signs or symptoms consistent with disease (regardless of severity): fever or flu- like Definition 2 Epidemiologically linked by time, place, and exposure to a likely or confirmed influenza (H5N1) case-patient AND Death due to unexplained acute respiratory illness AND Negative test/test not performed Possible Epidemiologically linked by time, place and exposure to a likely or confirmed human or avian influenza (H5N1) case AND Test not performed/negative test AND Clinical signs and symptoms consistent with disease (regardless of severity): fever or flu- like Noncase Confirmed positive for non-H5N1 influenza A virus subtype OR Patient condition determined to have etiology other than avian influenza