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Letter
Asymptomatic Severe Acute
Respiratory Syndrome–associated Coronavirus Infection
Harold K.K. Lee,* Eugene Y.K. Tso,* T. N. Chau,* Owen T.Y. Tsang,*
K.W. Choi,* and Thomas S.T. Lai*
*Princess Margaret Hospital, Hong Kong
Suggested citation
for this article:
Lee HKK, Tso EYK, Tsang OTW Choi KW, Lai TST. Asymptomatic severe acute
respiratory syndrome–associated coronavirus infection. Emerg Infect
Dis [serial online] 2003 Nov [date cited]. Available from: URL:
http://www.cdc.gov/ncidod/EID/vol9no11/03-0401.htm
To the Editor: An outbreak of severe acute respiratory syndrome
(SARS) began in Hong Kong in March 2003. As of May 29, 2003, a total of
1,732 cases were confirmed; 381 case-patients were healthcare workers
and medical students. Clinical features, treatment protocols, and outcomes
have been previously reported by various local experts (1–3).
The etiologic agent is a SARS-associated coronavirus (SARS-CoV) (1).
However, no asymptomatic case of SARS-CoV infection has been previously
reported (4). In addition, in Hong Kong, blood donors
have not shown any detectable antibody to SARS-CoV (1).
We report a case of possible asymptomatic SARS-CoV infection in Hong Kong.
The case-patient is a registered nurse working in Princess Margaret Hospital,
the major infectious diseases hospital that treated >600 SARS patients
in Hong Kong. Within this hospital, >800 frontline staff members have
participated in direct care of SARS patients, and SARS developed in 62
of these staff members. All healthcare workers working in SARS wards followed
the same infection control measures, wearing a N-95 respirator, eye shield,
disposable cap, water-resistant gown, and gloves. Gowns and equipment
were removed before the staff left the SARS wards.
We performed serologic testing of the first 101 healthcare workers (doctors,
nurses, healthcare assistants) who worked in the SARS wards but in whom
SARS did not develop. The serologic testing was performed 7–8 weeks after
the healthcare workers were first exposed to SARS patients.
We identified a nurse who was asymptomatic for SARS-CoV infection, worked
in the SARS ward since the disease outbreak, and used full infection control
procedures as recommended by the World Health Organization (WHO). The
nurse performed procedures, including nasopharyngeal aspiration, handling
of fecal matter, and oral feeding of SARS patients. SARS developed in
six colleagues who worked in the same ward. She had unprotected exposure
to a colleague who contracted SARS and required hospitalization. Serologic
testing for SARS-CoV antibody was performed in the microbiology laboratory
of Princess Margaret Hospital on week 8 of the nurse’s SARS ward duty.
The result of the test was positive by enzyme-linked immunosorbent assay.
The test was repeated by the Government Virus Unit of the Department of
Health, one of the reference laboratories in Hong Kong. The second test
also showed a positive result with an antibody titer of 400 by immunofluorescence
assay (normal: <25). We performed another serologic test on week 10
of her SARS ward duty; the result was again positive. The nurse was interviewed
by two physicians and questioned about her health condition since February
2003. She did not report any symptoms typical of SARS, such as fever,
chills, rigors, malaise, myalgia, cough, dyspnea, and diarrhea (1,3)
during and after her SARS ward duty. She did have a mild, short-term headache,
which she has had periodically for many years. She did not take sick leave
since February 2003. She did not record any rise in body temperature >37°C
and had a leukocyte count of 5.9 x 109/L and a lymphocyte count
of 1.6 x 109/L. Results of liver and renal function tests were
all normal. Reverse transcription-polymerase chain reaction results for
SARS-CoV in stool, urine, throat, and nasal swabs collected during weeks
10 and 14 of her SARS ward duty were all negative. No abnormal radiologic
change was identified in the lungs. She lived with four family members
and had close contact with them. None of her family members contracted
SARS, and all showed a negative result in the serologic testing for SARS-CoV.
We think that asymptomatic and subclinical infection of SARS-CoV exists
and can result in seroconversion; however, this kind of asymptomatic seroconversion
is probably uncommon. Why a person infected with SARS-CoV did not have
typical symptoms, and the infectivity of an asymptomatic person is unknown.
A person’s genetic makeup may determine susceptibility to SARS-CoV and
the final clinical outcome. We agree with Seto et al. (5)
that recall bias is a concern. However, recall bias probably had little
effect since the events took place recently. Moreover, the hospitalization
of the nurse’s infected colleague would have made her more alert and aware
of symptoms of the illness.
References
- Peiris JSM, Lai ST, Poon LLM, Guan Y, Yam LYC, Lim
W, et al. Coronavirus
as a possible cause of severe acute respiratory syndrome. Lancet
2003;361:1319–25.
- So LKY, Lau ACW, Yam LYC, Cheung TMT, Poon E, Yung RWH, et al. Development
of a standard treatment protocol for severe acute respiratory syndrome.
Lancet 2003;361:1615–7.
- Lee N, Hui D, Wu A, Chan P, Cameron P, Joynt GM, et al. A
major outbreak of severe acute respiratory syndrome in Hong Kong.
N Engl J Med 2003;348:1986–94.
- Vu TH, Cabau JF, Nguyen NT, Lenoi M. SARS
in northern Vietnam. N Engl J Med 2003;348:2035.
- Seto WH, Tsang D, Yung RWH, Ching TY, Ng TK, Ho M, et al. Effectiveness
of precautions against droplets and contact in prevention of nosocomial
transmission of severe acute respiratory syndrome (SARS). Lancet
2003;361:1519–20.
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