During August 31–September 12, 2009, an outbreak of pandemic (H1N1) 2009 occurred among students of a university in northern China. On August 31, pandemic (H1N1) 2009 was laboratory confirmed in 6 students who had fever and acute upper respiratory symptoms. A subsequent investigation found that all of the confirmed case-patients, along with 27 other students, had traveled by train from Shanghai to the university on the afternoon of August 28. One of the students (the index case-patient) had a cough during the trip on August 27. Another 5 students also had fever and influenza-like symptoms and visited the school medical services for treatment during August 28–30. When the outbreak was identified, a total of 202 contacts (average age 21 years, range 19–23 years) were traced and immediately quarantined in a separate dormitory on September 1. Eighty-nine rooms (each with 1 toilet) and 9 apartments (each with 2 bedrooms and 1 toilet) were occupied. One or 2 contacts were assigned to each bedroom. Other control measures, such as ventilating and disinfecting each room, wearing masks, and washing hands, were strictly implemented in accordance with guidance provided by the Chinese Ministry of Health (8).

Oropharyngeal swabs from all contacts were collected and tested on the first day of quarantine (September 1). Virologic laboratory testing subsequently indicated that 39 contacts were positive for pandemic (H1N1) 2009. Among the 163 virus-negative contacts, 11 had fever (>38°C) or influenza-like symptoms at the beginning of the quarantine; the other 152 had no fever or influenza-like symptoms on September 1.

Among the 152 virus-negative contacts who had fever or influenza-like symptoms, 20 were exposed to a virus-positive contact during quarantine; 19 shared a bedroom and a toilet with a virus-positive contact; and 1 shared a toilet but not a bedroom with a virus-positive contact. The other 132 were not exposed to any virus-positive contacts during quarantine, including 6 persons housed in single rooms and 126 sharing double rooms.

The temperature and symptoms of contacts were recorded 3× per day (8:00 am, 2:00 pm, and 6:00 pm). We defined a suspected case as a virus-negative contact with sudden onset of fever >38°C and at least 1 of the following symptoms: cough, sore throat, runny nose, shortness of breath, headache, body aches, fatigue, vomiting or diarrhea, and absence of other diagnoses. Students with suspected pandemic (H1N1) 2009 infection who had high fever (>38.5°C) or severe respiratory symptoms (bad cough or dyspnea) were transferred to hospital for treatment; others who had fever <38.5°C or mild influenza-like symptoms were treated in the quarantine building. Because of a shortage of resources, laboratory testing of suspected case-patients in quarantine was not performed after the first day.

Data were analyzed by using SPSS 15.0 (SPSS Inc., Chicago, IL, USA). χ² test and Fisher exact test were used to compare the attack rates of suspected cases between the groups under different quarantine arrangements. A p value <0.05 was considered significant.

Figure

Figure. Number of new suspected cases of pandemic (H1N1) 2009 infection per day among 152 quarantined persons who were virus-negative at the start of quarantine during an outbreak in northern People’s Republic...

During the quarantine, 14 suspected cases were identified among 152 virus-negative contacts, but none had high fever (>38.5°C) or severe respiratory symptoms. Of 19 initially virus-negative contacts sharing a room with a virus-positive contact, 5 suspected cases were identified; onset of symptoms occurred during September 2–8. Also, of 126 initially virus-negative contacts who shared a room with a virus-negative contact, 9 suspected cases were identified; onset of symptoms occurred during September 2–7 (Table, Figure). The attack rate of suspected cases differed significantly between the 2 groups (p = 0.02, 2-tailed Fisher exact test).

Among 132 initially virus-negative contacts, who were not exposed to a virus-positive contact during the quarantine, no suspected cases were identified among the 6 students in single rooms, but 9 students with suspected infection were identified among 126 students in double rooms; onset of symptoms occurred during September 2–7 (Table; Figure). Attack rates for suspected cases between the virus-negative contacts in single rooms and those in double rooms did not differ significantly (p = 1.00, 2-tailed Fisher exact test). All students with suspected infection had normal temperature with no influenza-like symptoms after September 9, and all quarantined persons were released by September 12.

In our study, the attack rate of suspected cases among pandemic (H1N1) 2009 virus–negative contacts increased significantly when persons were quarantined in the same room or used the same bathroom as a virus-positive contact (p = 0.02, 2-tailed Fisher exact test). Nevertheless, quarantining each virus-negative contact in 1 room did not significantly decrease the attack rate of suspected cases among virus-negative contacts in comparison with quarantining 2 persons in 1 room. Our results support the effectiveness of quarantine in preventing a secondary outbreak of pandemic (H1N1) 2009 among contacts of confirmed cases. Our results also support quarantining 2 virus-negative contacts in 1 room in situations where a large number of contacts have been traced but space is limited. Whether our findings support quarantining >2 contacts in 1 room deserves further study.

During the quarantine, every room was routinely disinfected by university staff 1 × per day. Compliance of all contacts with regulations governing personal protection and hygiene was good. Also, staff were assigned to supervise the behavior of contacts in quarantine. These control measures did not contribute to the differences of the attack rate of suspected cases between the different cohorts in this study.

Our study is limited in that virologic laboratory confirmation of suspected cases was not available. However, no other respiratory infections were known to be circulating at that time in the student population, and pandemic (H1N1) 2009 is probably the most likely explanation for influenza-like illness in recent contacts of laboratory-confirmed cases. Lack of laboratory testing also means that we may have underestimated the attack rate during quarantine; some secondary infections may have been associated with asymptomatic or subclinical disease.