Report: SARS Can Spread Through Air
Virus-Laden Plume Spread SARS in 2003 Hong Kong Outbreak
April 21, 2004 -- Contaminated air can spread deadly cases of SARS, a new report suggests.
Public health measures -- some of them drastic -- ended the 2003 SARS outbreak. The world came close to a public health disaster. If airborne virus particles can indeed spread SARS, we may have been even closer to disaster than anyone knew at the time.
A SARS Mystery
The findings come from an investigation into one of the great mysteries of the Hong Kong SARS epidemic. That mystery is known as the Amoy Gardens outbreak, where 187 SARS cases appeared in a multi-building housing complex in just a few days.
A man with SARS infection carried the virus into Amoy Gardens when he stopped by for two visits five days apart. The man was suffering from diarrhea, one of the symptoms of SARS infection. While the most serious symptoms of SARS come from lung infections, huge amounts of the virus grow in the gut.
Everyone who's tried to solve the mystery agrees that the problem started when the infected man used the toilet. What happened next has been a mystery. An official WHO inquiry concluded that the infection probably spread from exhaust fans in the sewage system. But that doesn't explain why people living in different buildings got infected at almost the same time as those in the affected building -- too soon for efficient person-to-person spread.
Another educated guess suggested that rats carried the virus from apartment to apartment. But that doesn't explain why people on the buildings' upper floors -- and not those on lower floors -- had a higher risk of becoming infected. And since roof rats are highly territorial, it still doesn't explain why the infections appeared almost simultaneously in different buildings populated by different packs of roof rats.
Detective Work Pays Off
It seems few experts believed that SARS might have been airborne. Except for tuberculosis bacillus and measles virus -- two widely acknowledged airborne bugs -- modern science believes most respiratory germs are spread by large droplets and should be easily detected. These droplets fly over short distances or contaminate surfaces where people are exposed to them.
Ignatius T.S. Yu, MD, PhD, of the Chinese University of Hong Kong, and colleagues thought otherwise. They obtained detailed floor plans, site plans, and weather reports for Amoy Gardens on the dates in question.
Using sophisticated airflow-dynamics studies, Yu's team showed that a SARS-laden air plume rose up from the drainage pipes through floor drains with faulty seals. The air plume was carried outside by exhaust fans and blown by prevailing wind patterns into some apartments but not others.
Yu and colleagues describe the spread of the killer air plume in chillingly scientific language. Here's an excerpt from their report in the April 22 issue of The New England Journal of Medicine: