Protection by Face Masks against Influenza A(H1N1)pdm09 Virus on Trans-Pacific Passenger Aircraft, 2009

Lijie Zhang¹, Zhibin Peng¹, Jianming Ou¹, Guang Zeng¹, Robert E. Fontaine, Mingbin Liu, Fuqiang Cui, Rongtao Hong, Hang Zhou, Yang Huai, Shuk-Kwan Chuang, Yiu-Hong Leung, Yunxia Feng, Yuan Luo, Tao Shen, Bao-Ping Zhu, Marc-Alain Widdowson, and Hongjie Yu

Author affiliations: Chinese Center for Disease Control and Prevention, Beijing, China (L. Zhang, Z. Peng, G. Zeng, M. Liu, F. Cui, H. Zhou, Y. Feng, Y. Luo, T. Shen, B.-P. Zhu, H. Yu); Fujian Center for Disease Control and Prevention, Fuzhou, China (J. Ou, R. Hong); Nanchang Center for Disease Control and Prevention, Nanchang, China (M. Liu); Centers for Disease Control and Prevention, Atlanta, Georgia, USA (R. E. Fontaine, M.-A. Widdowson); China–US Collaborative Program on Emerging and Re-emerging Infectious Diseases, Beijing (Y. Huai); Hong Kong Department of Health, Hong Kong, China (S.-K. Chuang, Y.-H. Leung)

Abstract

In response to several influenza A(H1N1)pdm09 infections that developed in passengers after they traveled on the same 2 flights from New York, New York, USA, to Hong Kong, China, to Fuzhou, China, we assessed transmission of influenza A(H1N1)pdm09 virus on these flights. We defined a case of infection as onset of fever and respiratory symptoms and detection of virus by PCR in a passenger or crew member of either flight. Illness developed only in passengers who traveled on the New York to Hong Kong flight. We compared exposures of 9 case-passengers with those of 32 asymptomatic control-passengers. None of the 9 case-passengers, compared with 47% (15/32) of control-passengers, wore a face mask for the entire flight (odds ratio 0, 95% CI 0–0.71). The source case-passenger was not identified. Wearing a face mask was a protective factor against influenza infection. We recommend a more comprehensive intervention study to accurately estimate this effect.

After influenza A(H1N1)pdm09 virus was identified in April 2009 (1), it spread rapidly, largely through air travel by infected passengers (2). On May 2, 2009, China implemented intensive screening of arriving air passengers by using thermal cameras to detect fever and a short questionnaire about existing respiratory symptoms and fever; passengers were advised to seek medical consultation if fever or respiratory symptoms developed ≤ 7 days of arrival (3,4). Nasopharyngeal swab specimens collected from all arriving febrile passengers were tested for virus at the nearest provincial, city, or county Centers for Disease Control (CDC) laboratory by using real-time reverse transcription PCR (RT-PCR) (5). If any of these results were positive results, all passengers on the same flight were quarantined.
On May 11, 2009, this system detected the first confirmed influenza A(H1N1)pdm09 infection in mainland China in a US traveler (6). As of May 29, the system detected 21 other imported infections in passengers arriving on international flights. On May 29, the first locally acquired influenza A(H1N1)pdm09 infection was detected.
On May 30, acute onset of fever (38.3°C), cough, sore throat, and headache developed in a 22-year-old man. He sought treatment at a clinic in Fuzhou, China, where medical staff learned that he recently arrived from New York, New York, USA (hereafter referred to as New York) and reported a suspected case of influenza A(H1N1)pdm09 infection to the county CDC. On May 31, duplicate nasopharyngeal swabs specimens from the patient were positive for influenza A(H1N1)pdm09 virus at Fuzhou CDC and Fujian Provincial CDC.
On May 27 at 10:40 am (all times Beijing local time), the patient had departed New York on a flight to Hong Kong, China. After flying for 5 hours and 50 min, the plane made a scheduled stopover in Vancouver, British Columbia, Canada. All passengers remained on board during the stopover, which lasted 1 hour and 15 min (4:30 pm–5:45 pm). Air-handling systems were fully operational. The aircraft left Vancouver and flew for 13 hours and 15 min and arrived in Hong Kong at 7:00 am on May 28. In Hong Kong, 63 passengers transferred to a Hong Kong to Fuzhou flight, which departed Hong Kong at 8:50 am and arrived at in Fuzhou City Airport at 10:30 am (flight time 1 hour and 40 min.).
The aforementioned patient had no fever or respiratory symptoms when screened on arrival in Fuzhou. The Fujian Provincial CDC, concerned that other passengers on the Hong Kong to Fuzhou flight might be infected, traced and quarantined (involuntary social distancing) the arriving passengers and crew members in their own homes, designated hotels, or hospitals. According to Chinese Ministry of Health guidelines (7), social contacts of this confirmed case-patient were traced and quarantined. These passengers, crew members, and contacts were monitored for 7 days for fever and respiratory illness; nasopharyngeal swab specimens were obtained from symptomatic persons. This effort identified 7 additional case-passengers on the Hong Kong to Fuzhou flight in whom symptoms developed during May 30–June 1 and had influenza A(H1N1)pdm09 infection confirmed by RT-PCR. All 8 case-passengers had arrived in Hong Kong on the same New York to Hong Kong flight. The China CDC and Fujian Provincial CDC initiated an outbreak investigation to assess possible transmission of influenza A(H1N1)pdm09 virus on those flights and better understand risks for influenza spread in confined settings.