[Non-airborne transmission during outbreak of pandemic (H1N1) 2009 among tour group members, China, June 2009].

OBJECTIVE

During June 2 - 8, 2009, 11 cases of the novel influenza A (H1N1) occurred in Sichuan Province, China. We investigated this outbreak to identify the source of infection, mode of transmission and risk factors for infection.

METHODS

The primary case, a U.S. citizen, developed disease on June 2. From June 3 to 5, she joined Tour Group A for a trip to Jiuzhaigou. We telephoned passengers of the three flights on which the primary case had traveled in China, and members of Tour Group A. We asked whether they had any influenza-like symptoms during May 27 to June 12. Health authorities placed passengers whose seats were within three rows of the primary case on flights and members of Tour Group A on medical observation, and isolated individuals if they developed symptoms. We used real-time RT-PCR to test the throat swabs from symptomatic persons for the novel influenza virus and defined a confirmed case as one with influenza-like symptoms and laboratory confirmation. A retrospective cohort investigation to identify the risk factors for infection was conducted. We interviewed all members of Tour Group A about their detailed contact history with the primary case.

RESULTS

During June 5 to 6, 9 (30%) of the primary case's 30 fellow tour group members developed disease, compared with none of her 87 fellow passengers to Jiuzhaigou and 1 of her 87 fellow passengers on the returning trip (when several of the members of Tour Group A were symptomatic). 56% of the tourists who had talked with the primary case in close range (< 2 m) for ≥ 2 minutes developed disease, whereas none of the 14 other tour group members developed disease (RR = ∞;exact 95%CI: 2.0 - ∞). Having conversed with the primary case for ≥ 10 minutes (vs. 2 - 9 minutes) increased the risk by almost five fold (RR = 4.8, exact 95%CI: 1.3 - 180). Conversely, other kinds of contact, such as dining at the same table, receiving chewing gum from the primary case and sharing bus rides or planes with the primary case played no roles during this outbreak.

CONCLUSION

This novel influenza A (H1N1) outbreak was caused by an imported case, and transmitted mainly via droplet transmission when the primary case was talking with her fellow tourists during a tour. These findings highlight the importance of preventing droplet transmission during a pandemic.