The demographic and epidemiologic characteristics of the 3 patients with confirmed pandemic (H1N1) 2009, including the estimated disease incubation period and their travel histories, are listed in Table 1. Their clinical characteristics are shown in Table 2. All 3 patients were Chinese students who had been studying abroad (2 in United States and 1 in Canada).

Patient 1 left St. Louis, Missouri, USA, on May 7, 2009 (US date), and flew to St. Paul, Minnesota, USA, where he transferred to a flight to Tokyo, Japan, then to Beijing, China, arriving on May 8 (Beijing date). The patient did not experience illness, and fever did not develop during the flight from St. Louis to Beijing. At the Beijing airport, his temperature was normal. The patient stayed in Beijing for 1 night and boarded a flight to Chengdu on May 9. He did not report feeling ill before the flight. On the flight from Beijing to Chengdu, fever developed. Upon landing, the patient went to a hospital for medical attention for his illness. The case was reported to the Chinese CDC, and specimens were collected on May 9. A test result for pandemic (H1N1) 2009 virus was positive on May 10 by real-time RT-PCR. All 144 close contacts (including his father, girlfriend, taxi driver, and all passengers on the flight from Chendu to Beijing) were quarantined in hotels for medical observation for 7 days.

Patient 2 left Winnipeg, Manitoba, Canada, on May 7 (Canada date) and arrived in Beijing on May 8 (Beijing date). He did not feel ill while traveling from Winnipeg to Beijing. He stayed for 2 days in Beijing, where a fever developed, but he did not seek medical attention. The patient subsequently took a train from Beijing to Jinan on May 11 and, on arrival, went to the local infectious diseases hospital. There was no thermal scanner for fever at the train station in Beijing. The case was reported to the Chinese CDC, and specimens were collected on May 11. The pandemic (H1N1) 2009 virus test result was positive on May 12 by real-time RT-PCR. All 40 close contacts sitting in the same car of the train were quarantined for medical observation for 7 days.

Patient 3 left New York, New York, USA, on May 10 (US date) and arrived at home in Beijing on May 11 (Beijing date). She was well when traveling from New York to Beijing. Her temperature was normal at border health/temperature monitoring. She stayed home with her mother and did not meet other persons and visit other places. A fever developed on May 13 (Beijing date), and she sought medical attention on the evening of May 14. The case was reported to Chinese CDC, and specimens were collected on May 15. The test result for pandemic (H1N1) 2009 virus was positive on May 15 by real-time RT-PCR. Her mother and a taxi driver were the only close contacts.

All 3 patients were positive for pandemic (H1N1) 2009 virus by real-time RT-PCR, and further confirmed PCR products sequencing the partial hemagglutinin sequence from all 3 patients were 99.5–100% homologous with A/California/4/2009 but only 79.4%–79.6% homologous with seasonal influenza virus (H1N1) (A/Brisbane/59/2007) (Table 3). The dynamic virologic monitoring of pharyngeal swab samples from the 3 case-patients showed that real-time RT-PCR results for influenza A pandemic (H1N1) 2009 virus were negative on day 5, day 7, and day 6, respectively (Table 3).

Only 1 sample from a total of 186 close contacts tested positive for pandemic (H1N1) 2009 virus by real-time RT-PCR. This sample came from the 48-year-old mother of patient 3, who had lived with patient 3 for 2 days before her illness and had taken care of her for 2 days after her illness began. On the fifth day (May 16) after exposure to patient 3, the woman’s sample became positive (Table 3). As with all the other close contacts, this woman had no fever or ILI symptoms.

The 3 cases were mild, and the patients were young, which is consistent with the profile of other pandemic (H1N1) 2009 infections reported around the world (6). The most frequent signs and symptoms in the patients were fever and other manifestations that were nonspecific for pandemic (H1N1) 2009 and indistinguishable from those of seasonal influenza. None of them had evidence of severe lower respiratory tract illness or unusual symptoms of influenza, such as diarrhea. All patients recovered quickly, with a median duration of fever of 3 days. All 3 patients had complete blood counts performed during the course of their disease, but none had leukopenia (leukocyte count <4,000/mm³), or lymphopenia (total lymphocyte count <800/mm³), or thrombocytopenia (total platelet count <100,000/mm³).

The transmissibility of influenza A pandemic (H1N1) 2009 virus is uncertain. One study assumed that its transmissibility (R₀) is substantially higher than that of seasonal influenza and comparable with the viruses in previous influenza pandemics (12). Our study demonstrates, on the basis of PCR testing, that the average time of pandemic (H1N1) 2009 virus shedding is 4–6 days. Positive RT-PCR results do not necessarily confirm virus shedding, but the PCR-based method is more sensitive than culture methods for detecting virus shedding (13). Our study also shows that the risk for person-to-person transmission is greatest in households. Among 186 close contacts, only the mother of patient 3, who lived with her, had a positive result for pandemic (H1N1) 2009 virus by real-time RT-PCR, although she had no symptoms of influenza. All other close contacts, including taxi drivers, passengers in an airplane or the same car of a train, were negative for pandemic (H1N1) 2009 virus at screening, and illness did not develop subsequently while they were being observed. Asymptomatic infections appear to be possible among household members, as demonstrated by case-patient 3 and her mother.

Our study has several limitations. First, numbers in this series were low. Second, the exact date of exposure to a known infectious source was difficult to trace. Third, pharyngeal swab samples may have a lower sensitivity (than nasopharyngeal), and thus some false-negative results might have occurred, which could lead to underestimation of viral shedding.

A national network for the surveillance and control of pandemic (H1N1) 2009 was quickly organized in China, and the response was quick and thorough. The Chinese government is moving swiftly to contain the new influenza, drawing on lessons from the severe acute respiratory syndrome and bird influenza outbreaks in recent years. In all airports, thermal scanners have been installed, and all asymptomatic contacts of case-patients were quarantined for 7 days as part of the early response. As the number of imported cases has increased, the quarantine policy is changing. During the past 4 weeks, only symptomatic travelers at ports of entry and passengers sitting within a short distance (<2 m) to a person with a suspected or confirmed case were quarantined. As local transmission has been documented, more efforts have been paid to communities. Since June 22, hand temperature monitors were used in all schools in Beijing. All students with body temperatures >37.2°C were asked to stay at home until their temperature returned to normal. Further evaluation is needed to continue to clarify the nature of pandemic (H1N1) 2009, including its clinical features, severity, incubation period, and transmission patterns (14).