In conclusion, we have established SAFV virus as a cause of invasive infection and a highly probable cause of severe disease in children. More studies are needed to further illuminate the role of SAFV as a human pathogen.  Pandemic (H1N1) 2009 among Quarantined Close Contacts, Beijing, People’s Republic of China We estimated the attack rate of pandemic (H1N1) 2009 and assessed risk factors for infection among close contacts quarantined in Beijing, People’s Republic of China. The first 613 confirmed cases detected between May 16 and September 15, 2009, were investigated; 7,099 close contacts were located and quarantined. The attack rate of confirmed infection in close contacts was 2.4% overall, ranging from 0.9% among aircraft passengers to >5% among household members. Risk factors for infection among close contacts were younger age, being a household member of an index case-patient, exposure during the index case-patient’s symptomatic phase, and longer exposure. Among close contacts with positive test results at the start of quarantine, 17.2% had subclinical infection. Having contact with a household member and younger age were the major risk factors for acquiring pandemic (H1N1) 2009 influenza virus infection. One person in 6 with confirmed pandemic (H1N1) 2009 was asymptomatic. I n early April 2009, human cases of infection with a novel infl uenza virus of swine origin, pandemic (H1N1) 2009 virus, were identifi ed in the United States and Mexico, and this virus spread rapidly across the world (1-3). On June 11, 2009 , the World Health Organization raised the pandemic level to 6, the highest level for pandemic alert (4) .