Jul 7, 2005 (CIDRAP News) Some healthcare workers who were exposed to SARS (severe acute respiratory syndrome) early in the epidemic became infected without ever falling ill, according to a recent report in Emerging Infectious Diseases.
Six of 80 (7.5%) healthcare workers exposed to SARS patients in a Singapore hospital had asymptomatic infections, according to the report by Annelies Wilder-Smith of Tan Tock Seng Hospital in Singapore and her colleagues with National Healthcare Group and Singapore General Hospital. They studied healthcare workers exposed to patients with SARS in the first 22 days of the outbreak, before the hospital began infection control measures.
The initial cases at the hospital involved three patients admitted to three wards in early March 2003, before the SARS coronavirus (Co-V) was identified. The first patient brought SARS from Hong Kong, and the second patient was the first patient's nurse. The third patient was admitted for other health problems, but shared a room with patient 2 and became infected.
Patient 1 arrived Mar 1. By Mar 6, healthcare workers were using N95 masks, gowns, and gloves when nursing patient 1 and any other suspected SARS patients. But since SARS was not suspected in patients 2 and 3, workers caring for them initially did not use protection.
The researchers identified 105 staff members who had worked in the three affected hospital wards between Mar 1 and 22 and had contact with any of the three patients. Eighty of these responded to a questionnaire and consented to a serologic test.
Of the 80 workers, 45 (56%) had positive serum samples for antibodies to SARS Co-V. Thirty-seven of the 45 (82%) were classified as having pneumonic SARS, 2 (4%) as having subclinical SARS, and 6 (13%) as having asymptomatic infection, the report says.
The investigators found no difference between the pneumonic SARS patients and the asymptomatic workers in age, glove use, handwashing, and closeness or time of contact with SARS patients. However, three of the six asymptomatic workers had worn N95 masks, whereas only 8% of the pneumonic SARS patients had used masks, a significant difference.
In addition, levels of antibodies to SARS Co-V were significantly lower in the workers who remained asymptomatic than in those who fell ill, the article says. "These observations suggest that the extent of exposure to SARS in persons who remained asymptomatic may have been lower, possibly resulting in a lower viral load of SARS-CoV, associated with less severe symptoms," the report states. This possible association needs to be tested in animal studies, the authors add.
Three earlier studies turned up no evidence of asymptomatic SARS infections, while two studies in Hong Kong did find some cases, though the rates were lower than in this study, the researchers say.
Because the cases occurred before SARS diagnostic tests were available, the researchers could not determine whether the asymptomatic workers had shed the SARS virus during their infection.
Wilder-Smith A, Teleman MD, Heng BH, et al. Asymptomatic SARS coronavirus infection among healthcare workers, Singapore. Emerg Infect Dis 2005;11(7):1142-5 [Full text]