Evidence of Airborne Transmission of SARS
http://www.100md.com
《新英格兰医药杂志》
To the Editor: Yu et al. (April 22 issue)1 report evidence of airborne transmission of the severe acute respiratory syndrome (SARS) virus. In Figure 1 of their article, the Amoy Gardens housing complex appears to be isolated, when in fact, just 20 m east of building F there is a high school and 50 m southwest of buildings A and B there is an even more densely populated housing estate (Lower Ngau Tau Kok Estate). Were there any cases of SARS in those two areas?
To laypeople, the computational fluid-dynamics analysis described in the article is not readily comprehensible. Questions that will inevitably be asked are whether other experimental results concur with those of the computer analysis and how robust the analysis is. The first question can be addressed with the use of other methods of computer analysis, with the use of the same method of computer analysis by other researchers (to measure reproducibility), and with "wet" experiments involving the use of tracers. Robustness in this case refers to the ability of the computer analysis to predict outcomes given different input parameters, such as the origin of the aerosol, the direction of the wind, and the environmental temperature. Assessing the robustness of this computational approach might require multiple wet experiments but might demonstrate the usefulness of such computer analysis in other circumstances, such as smart landscaping and the design of hospital environments.
Tommy R. Tong, M.D.
Princess Margaret Hospital
Hong Kong, China
tommy.tong@members.nyas.org
Chun Liang, Ph.D.
Hong Kong University of Science and Technology
Hong Kong, China
References
Yu ITS, Li Y, Wong TW, et al. Evidence of airborne transmission of the severe acute respiratory syndrome virus. N Engl J Med 2004;350:1731-1739.
To the Editor: Yu et al., in their investigation of the distribution of cases of SARS in the Amoy Gardens outbreak, support the hypothesis that SARS coronavirus–laden aerosols may spread from a common source to buildings far from that of the index patient. However, this finding does not exclude the possibility that after the physical decay of the virus-laden aerosol plume, the prevalent mode of exposure could still be contact with inanimate materials or objects contaminated by infection-competent virus, rather than airborne exposure. Moreover, diarrhea was a symptom more frequently observed among patients from Amoy Gardens (prevalence, 73 percent)1,2 than in the other SARS clusters (prevalence, 10 percent to 27 percent).3,4 Fecal excretion has already been recognized as an important mode of viral shedding in the Amoy Gardens outbreak,2 and fecal–oral exposures could have been the prevalent route of transmission in this setting. No data are available on the prevalence of diarrhea among patients in different buildings at the onset of their illness or while they were ill. Such data could be useful for gaining further insight into the route of transmission of the SARS coronavirus.
Emanuele Nicastri, M.D., Ph.D.
Nicola Petrosillo, M.D.
Vincenzo Puro, M.D.
National Institute for Infectious Diseases IRCCS Lazzaro Spallanzani
00149 Rome, Italy
nicastri@inmi.it
References
Cheng VCC, Hung IFN, Tang BSF, et al. Viral replication in the nasopharynx is associated with diarrhea in patients with severe acute respiratory syndrome. Clin Infect Dis 2004;38:467-475.
Outbreak of severe acute respiratory syndrome (SARS) at Amoy Gardens, Kowloon Bay, Hong Kong. Government of Hong Kong Special Administrative Region: Department of Health, April 2003.
Booth CM, Matukas LM, Tomlinson GA, et al. Clinical features and short-term outcomes of 144 patients with SARS in the greater Toronto area. JAMA 2003;289:2801-2809.
Donnelly CA, Ghani AC, Leung GM, et al. Epidemiological determinants of spread of causal agent of severe acute respiratory syndrome in Hong Kong. Lancet 2003;361:1761-1766.
To the Editor: In Toronto, we have also witnessed transmission of SARS that could not easily be explained by droplet spread.1 SARS developed in some hospitalized patients when the only known source of exposure was a patient who had SARS and respiratory distress but who was a considerable distance away. Somogyi et al. therefore examined the spatial flow of aerosols arising from persons receiving high-flow oxygen by noninvasive masks.2 They observed that exhaled aerosol plumes with potentially virus-laden particles frequently extend many meters beyond the patient (Figure 1). 2,3 This observation provides one plausible mechanism to explain distant spread of the SARS virus during acute care or during transport within hospitals.
Figure 1. Exhaled Aerosol Dispersal Pattern during High-Flow Oxygen Administration with a Conventional, Noninvasive Face Mask.
In addition to the use of appropriate personal protective equipment by health care workers when they are treating patients with SARS, we advocate careful attention to the source control of aerosols from infected patients, including the use of oxygen-delivery systems with exhalation ports and submicron filters. This may minimize the spread of virus-laden aerosols and reduce the risk of transmission.2,4
Robert A. Fowler, M.D.
Damon C. Scales, M.D.
Roy Ilan, M.D.
University of Toronto
Toronto, ON M4N 3M5, Canada
rob.fowler@sw.ca
References
Scales DC, Green K, Chan AK, et al. Illness in intensive care staff after brief exposure to severe acute respiratory syndrome. Emerg Infect Dis 2003;9:1205-1210.
Somogyi R, Vesely AE, Azami T, et al. Dispersal of respiratory droplets with open vs closed oxygen delivery masks: implications for the transmission of severe acute respiratory syndrome. Chest 2004;125:1155-1157.
Respiratory droplet dispersal. Toronto: Isocapnia Research Laboratory, 2004. (Accessed July 15, 2004, at http://www.isocapnia.com/SARS.htm.)
Centers for Disease Control and Prevention. Infection control for prehospital emergency medical services (EMS), 2004. (Accessed July 15, 2004,at http://www.cdc.gov/ncidod/sars/guidance/I/prehospital.htm.)
The authors reply: Figure 1 of our article shows only 7 of the 19 buildings of the Amoy Gardens complex. There was no report of SARS in the high school. In the Lower Ngau Tau Kok Estate, which comprises 14 buildings, 47 cases of SARS were reported; 45 of them occurred in the 7 buildings situated directly downwind (southwest) from buildings A through G of the Amoy Gardens. The directional spatial distribution of SARS cases in nearby housing estates supports the possibility that there was airborne spread beyond the Amoy Gardens.
Our computer-simulation studies included sensitivity studies of various input parameters, including the source variation, the wind direction, and occupants' behavior in their apartments. Both field measurements and small-scale model studies in wind tunnels are possible, and we have used these approaches in our investigation of the Amoy Gardens outbreak. Computational air-flow modeling has been used in investigating the spread of other infectious diseases.1,2
It is possible that a small number of infected persons might have contracted the infection through routes other than inhalation. The frequent occurrence of diarrhea does not mean that fecal–oral transmission was responsible for the mass outbreak. Unlike bacteria that cause food poisoning, there is no evidence that any virus, including the SARS coronavirus, can replicate on surfaces or in food or water to amplify the source. The volume of material (a reflection of the viral dose) moving in and out of the respiratory tract is thousands of times that entering and leaving the gastrointestinal tract. A higher pH favors the survival of the SARS virus,3 and it is doubtful that the virus can survive the highly acidic environment of the stomach before entering the intestines. There is also no evidence that the SARS virus enters the body more efficiently through the gastrointestinal tract than it does through the respiratory tract.
In Hong Kong, we also saw that the transmission of SARS was not easily explained by droplet spread within the hospital environment. Our epidemiologic analysis of the largest nosocomial outbreak of SARS in Hong Kong suggested that virus-laden aerosols generated by the index patient could have been responsible for the spread of the infection to persons at a considerable distance.4
We totally agree that in treating patients with SARS, careful attention to the source control of aerosols from infected patients should be given top priority. In fact, a group of occupational health professionals in Hong Kong has designed equipment for source control in the health care setting.5
Ignatius T.S. Yu, M.B., B.S., M.P.H.
Chinese University of Hong Kong
Hong Kong, China
iyu@cuhk.edu.hk
Yuguo Li, Ph.D.
University of Hong Kong
Hong Kong, China
References
Meselson M, Guillemin J, Hugh-Jones M, et al. The Sverdlovsk anthrax outbreak of 1979. Science 1994;266:1202-1208.
S?rensen JH, Jensen C?, Mikkelsen T, Mackay DK, Donaldson AI. Modelling the atmospheric spread of foot-and-mouth disease virus for emergency preparedness. Phys Chem Earth 2001;26: 93-7.
World Health Organization. Consensus document on the epidemiology of severe acute respiratory syndrome (SARS). (Accessed July 15, 2004, at http://www.who.int/csr/sars/en/WHOconsensus.pdf.)
Wong TW, Lee CK, Tam W, et al. Cluster of SARS among medical students exposed to a single patient, Hong Kong. Emerg Infect Dis 2004;10:269-276.
The Hong Kong University of Science & Technology. Novel devices to control infection. (Accessed July 15, 2004, at http://www.ust.hk/en/pa/e_pa030731-742.html.)
To laypeople, the computational fluid-dynamics analysis described in the article is not readily comprehensible. Questions that will inevitably be asked are whether other experimental results concur with those of the computer analysis and how robust the analysis is. The first question can be addressed with the use of other methods of computer analysis, with the use of the same method of computer analysis by other researchers (to measure reproducibility), and with "wet" experiments involving the use of tracers. Robustness in this case refers to the ability of the computer analysis to predict outcomes given different input parameters, such as the origin of the aerosol, the direction of the wind, and the environmental temperature. Assessing the robustness of this computational approach might require multiple wet experiments but might demonstrate the usefulness of such computer analysis in other circumstances, such as smart landscaping and the design of hospital environments.
Tommy R. Tong, M.D.
Princess Margaret Hospital
Hong Kong, China
tommy.tong@members.nyas.org
Chun Liang, Ph.D.
Hong Kong University of Science and Technology
Hong Kong, China
References
Yu ITS, Li Y, Wong TW, et al. Evidence of airborne transmission of the severe acute respiratory syndrome virus. N Engl J Med 2004;350:1731-1739.
To the Editor: Yu et al., in their investigation of the distribution of cases of SARS in the Amoy Gardens outbreak, support the hypothesis that SARS coronavirus–laden aerosols may spread from a common source to buildings far from that of the index patient. However, this finding does not exclude the possibility that after the physical decay of the virus-laden aerosol plume, the prevalent mode of exposure could still be contact with inanimate materials or objects contaminated by infection-competent virus, rather than airborne exposure. Moreover, diarrhea was a symptom more frequently observed among patients from Amoy Gardens (prevalence, 73 percent)1,2 than in the other SARS clusters (prevalence, 10 percent to 27 percent).3,4 Fecal excretion has already been recognized as an important mode of viral shedding in the Amoy Gardens outbreak,2 and fecal–oral exposures could have been the prevalent route of transmission in this setting. No data are available on the prevalence of diarrhea among patients in different buildings at the onset of their illness or while they were ill. Such data could be useful for gaining further insight into the route of transmission of the SARS coronavirus.
Emanuele Nicastri, M.D., Ph.D.
Nicola Petrosillo, M.D.
Vincenzo Puro, M.D.
National Institute for Infectious Diseases IRCCS Lazzaro Spallanzani
00149 Rome, Italy
nicastri@inmi.it
References
Cheng VCC, Hung IFN, Tang BSF, et al. Viral replication in the nasopharynx is associated with diarrhea in patients with severe acute respiratory syndrome. Clin Infect Dis 2004;38:467-475.
Outbreak of severe acute respiratory syndrome (SARS) at Amoy Gardens, Kowloon Bay, Hong Kong. Government of Hong Kong Special Administrative Region: Department of Health, April 2003.
Booth CM, Matukas LM, Tomlinson GA, et al. Clinical features and short-term outcomes of 144 patients with SARS in the greater Toronto area. JAMA 2003;289:2801-2809.
Donnelly CA, Ghani AC, Leung GM, et al. Epidemiological determinants of spread of causal agent of severe acute respiratory syndrome in Hong Kong. Lancet 2003;361:1761-1766.
To the Editor: In Toronto, we have also witnessed transmission of SARS that could not easily be explained by droplet spread.1 SARS developed in some hospitalized patients when the only known source of exposure was a patient who had SARS and respiratory distress but who was a considerable distance away. Somogyi et al. therefore examined the spatial flow of aerosols arising from persons receiving high-flow oxygen by noninvasive masks.2 They observed that exhaled aerosol plumes with potentially virus-laden particles frequently extend many meters beyond the patient (Figure 1). 2,3 This observation provides one plausible mechanism to explain distant spread of the SARS virus during acute care or during transport within hospitals.
Figure 1. Exhaled Aerosol Dispersal Pattern during High-Flow Oxygen Administration with a Conventional, Noninvasive Face Mask.
In addition to the use of appropriate personal protective equipment by health care workers when they are treating patients with SARS, we advocate careful attention to the source control of aerosols from infected patients, including the use of oxygen-delivery systems with exhalation ports and submicron filters. This may minimize the spread of virus-laden aerosols and reduce the risk of transmission.2,4
Robert A. Fowler, M.D.
Damon C. Scales, M.D.
Roy Ilan, M.D.
University of Toronto
Toronto, ON M4N 3M5, Canada
rob.fowler@sw.ca
References
Scales DC, Green K, Chan AK, et al. Illness in intensive care staff after brief exposure to severe acute respiratory syndrome. Emerg Infect Dis 2003;9:1205-1210.
Somogyi R, Vesely AE, Azami T, et al. Dispersal of respiratory droplets with open vs closed oxygen delivery masks: implications for the transmission of severe acute respiratory syndrome. Chest 2004;125:1155-1157.
Respiratory droplet dispersal. Toronto: Isocapnia Research Laboratory, 2004. (Accessed July 15, 2004, at http://www.isocapnia.com/SARS.htm.)
Centers for Disease Control and Prevention. Infection control for prehospital emergency medical services (EMS), 2004. (Accessed July 15, 2004,at http://www.cdc.gov/ncidod/sars/guidance/I/prehospital.htm.)
The authors reply: Figure 1 of our article shows only 7 of the 19 buildings of the Amoy Gardens complex. There was no report of SARS in the high school. In the Lower Ngau Tau Kok Estate, which comprises 14 buildings, 47 cases of SARS were reported; 45 of them occurred in the 7 buildings situated directly downwind (southwest) from buildings A through G of the Amoy Gardens. The directional spatial distribution of SARS cases in nearby housing estates supports the possibility that there was airborne spread beyond the Amoy Gardens.
Our computer-simulation studies included sensitivity studies of various input parameters, including the source variation, the wind direction, and occupants' behavior in their apartments. Both field measurements and small-scale model studies in wind tunnels are possible, and we have used these approaches in our investigation of the Amoy Gardens outbreak. Computational air-flow modeling has been used in investigating the spread of other infectious diseases.1,2
It is possible that a small number of infected persons might have contracted the infection through routes other than inhalation. The frequent occurrence of diarrhea does not mean that fecal–oral transmission was responsible for the mass outbreak. Unlike bacteria that cause food poisoning, there is no evidence that any virus, including the SARS coronavirus, can replicate on surfaces or in food or water to amplify the source. The volume of material (a reflection of the viral dose) moving in and out of the respiratory tract is thousands of times that entering and leaving the gastrointestinal tract. A higher pH favors the survival of the SARS virus,3 and it is doubtful that the virus can survive the highly acidic environment of the stomach before entering the intestines. There is also no evidence that the SARS virus enters the body more efficiently through the gastrointestinal tract than it does through the respiratory tract.
In Hong Kong, we also saw that the transmission of SARS was not easily explained by droplet spread within the hospital environment. Our epidemiologic analysis of the largest nosocomial outbreak of SARS in Hong Kong suggested that virus-laden aerosols generated by the index patient could have been responsible for the spread of the infection to persons at a considerable distance.4
We totally agree that in treating patients with SARS, careful attention to the source control of aerosols from infected patients should be given top priority. In fact, a group of occupational health professionals in Hong Kong has designed equipment for source control in the health care setting.5
Ignatius T.S. Yu, M.B., B.S., M.P.H.
Chinese University of Hong Kong
Hong Kong, China
iyu@cuhk.edu.hk
Yuguo Li, Ph.D.
University of Hong Kong
Hong Kong, China
References
Meselson M, Guillemin J, Hugh-Jones M, et al. The Sverdlovsk anthrax outbreak of 1979. Science 1994;266:1202-1208.
S?rensen JH, Jensen C?, Mikkelsen T, Mackay DK, Donaldson AI. Modelling the atmospheric spread of foot-and-mouth disease virus for emergency preparedness. Phys Chem Earth 2001;26: 93-7.
World Health Organization. Consensus document on the epidemiology of severe acute respiratory syndrome (SARS). (Accessed July 15, 2004, at http://www.who.int/csr/sars/en/WHOconsensus.pdf.)
Wong TW, Lee CK, Tam W, et al. Cluster of SARS among medical students exposed to a single patient, Hong Kong. Emerg Infect Dis 2004;10:269-276.
The Hong Kong University of Science & Technology. Novel devices to control infection. (Accessed July 15, 2004, at http://www.ust.hk/en/pa/e_pa030731-742.html.)