Natural Course of Severe Acute Respiratory Syndrome–Associated Coronavirus Immunoglobulin after Infection

Correspondence Figure 1. Severe acute respiratory syndrome– associated coronavirus immunoglobulin (SARS CoV Ig) titers, measured over the course of 1 year in 62 survivors of SARS (shown are mean titers with 95% confidence intervals) (A) and in 1 asymptom-atic infected health-care worker (B). To the Editor—In Chen et al.'s recent study, severe acute respiratory syndrome (SARS)–associated coronavirus (SARS CoV) IgG was shown to be persistent for up to 60 days [1]. Their results suggested that production of this antibody is dependent on CD4 + cells and might play a role in protective immunity against SARS CoV. On the other hand, IgM has been found to decrease and become undetectable 11 weeks into the recovery phase [2]. Whether survivors of SARS can have persistent an-tibody and lifelong immunity against SARS CoV is unknown. We conducted a 1-year prospective study to investigate the natural course of SARS CoV immunoglobulin titer in 62 survivors of SARS and in 1 asymptomatic infected health-care worker, as described in our previous reports [3, 4]. All of the serologic tests were performed at the Government Virus Unit, Hong Kong, China , following a standard protocol. Serum samples were diluted 1:25, and 15 mL of the diluted samples were incubated by use of microscopic slides coated with SARS CoV–infected fetal rhesus kidney cells in a moist chamber for 30 min at 37ЊC. The slides were then washed with 2 changes of Tween 20 (Sigma). Fifteen microliters of polyvalent anti–human immunoglobulin labeled with fluorochrome was added, and the slides were incubated again for 30 min at 37ЊC. This was followed by another wash with 2 changes of Tween 20. The slides were then examined by use of a fluo-rescence microscope, under a low-power field (20ϫ). Any positive signals showing cytoplasmic fluorescence were confirmed by examination under a high-power field (40ϫ), and all tests with indeterminate results were repeated with uninfected cells, to exclude nonspecific reactions. Results were quantified by use of serial titrations of serum samples from patients and were For all of the survivors of SARS in our study, serum samples were collected on the day of admission to the hospital and 15 days, 1 month, 3 months, 6 months, 9 months, and 12 months after the onset of SARS symptoms. The mean age of the survivors of SARS was 37.07 years (SD, 12.96 years), and the male:female ratio was 0.82. On admission to the hospital, all of …