Evidence SARS virus may spread through the air

Mar 23 2005

Two new studies present evidence that the virus causing severe acute respiratory syndrome (SARS) may spread through the air, not just through direct contact with contaminated water droplets as previous research had shown.

SARS coronavirus was detected in the air in a patient's room during the 2003 outbreak in Toronto, according to a new study published in The Journal of Infectious Diseases. Another study, from Hong Kong, shows patients in hospital bays near a SARS patient had a much higher infection rate than patients in distant bays, consistent with the possibility of airborne SARS transmission, according to an article in Clinical Infectious Diseases. Both articles are published in the journals' May 1 issues, and are now available online.

The Toronto research was conducted by Timothy F. Booth, PhD and colleagues during the SARS outbreak there in March 2003. Their results mark the first experimental confirmation of the presence of the SARS virus in the air of an infected patient's hospital room.

The authors cautioned that their results do not document any cases of airborne transmission of the SARS virus from one person to another, only the dissemination of the virus from an infected patient to the air, via breathing or coughing.

During the outbreak in Toronto hospitals, health care workers became infected with the virus despite observance of strict infection control precautions. The investigators wondered whether environmental contamination of hospital air or surfaces could explain the ongoing risk of SARS coronavirus transmission to health care workers. To answer this question, they collected patient information and environmental samples from the SARS units of four Toronto hospitals.

SARS coronavirus was detected in the air in one of the four rooms tested. The researchers also detected virus in four of 85 surface samples taken from frequently touched surfaces, highlighting the importance of strict adherence to infection control precautions to prevent SARS coronavirus transmission in the health care setting.

In the Hong Kong study, which focused on the 2003 SARS outbreak at the Prince of Wales Hospital, 41 percent of patients admitted to the ward in which the first SARS patient was staying became infected. Proximity to the bed of the first case seemed to be strongly linked with incidence of infection-two-thirds of patients in the same bay and half of patients in an adjacent bay were infected with SARS, while only 18 percent of patients in distant bays were infected.

The Hong Kong researchers, led by Ignatius T.S. Yu, MBBS, MPH, of the Chinese University of Hong Kong, speculate that the increased risk of infection with closer proximity to the index SARS case suggests airborne transmission. Although they do not have "direct proof" of airborne transmission, according to Dr. Yu, "no other known routes of infectious diseases transmission could adequately explain the spread of the disease in the outbreak, and hence we feel that the evidence is quite strong."

An editorial accompanying the Toronto study, by Tommy Tong, MBBS, of Princess Margaret Hospital in Hong Kong, emphasized the scientific significance of discovering SARS coronavirus in the air in a patient's room. "Although the possibility of airborne dissemination of SARS coronavirus has been controversial," said Dr. Tong, "this important work shows beyond doubt that SARS coronavirus aerosol generation can occur from a patient with SARS." The Hong Kong study provides additional, complementary evidence that the virus may be capable of spreading through the air.