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Previous Volume 349:508-509 July 31, 2003 Number 5 Next

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To the Editor: A novel coronavirus called the severe acute respiratory syndrome (SARS)–associated coronavirus (CoV) has been identified as the causal agent of SARS.^1,2,3 To understand the humoral immunity to this virus, we studied the profile of IgM and IgG antibody responses to SARS-CoV. IgM and IgG antibodies were analyzed by an indirect enzyme-linked immunosorbent assay in 20 patients with SARS from week 1 of their illness to week 12 and in 103 healthy contacts.

All 20 patients tested negative for IgM and IgG at week 1 after the onset of symptoms. Of these patients, 16 tested positive for IgM and 17 tested positive for IgG at week 2 (Figure 1). All 20 patients were IgG-positive after week 3 and continued to have high levels of IgG up to three months after the onset of symptoms. The IgG titers were low at the beginning of week 2 (mean, 1:40, with the cutoff for a positive result being 1:10), increased to an average of 1:256 at week 3, and peaked at 1:640 at week 12. The IgM titers peaked during the acute or early convalescent phase and then declined with IgM disappearing by the end of week 12. All 103 healthy contacts tested negative for IgM and IgG.

View larger version (5K): [in this window] [in a new window]   Figure 1. Changing Titers of IgM and IgG Antibodies to the SARS-Associated Coronavirus from the Onset of Illness through the Convalescent Phase. IgM and IgG were measured at weeks 1, 2, 3, 4, 8, and 12; the mean IgG titer was 1:40 at week 2, 1:256 at week 3, 1:368 at week 4, 1:640 at week 8, and 1:640 at week 12. The mean IgM titer was 1:120 at week 2, 1:320 at week 3, 1:160 at week 4, and 1:40 at week 8. The cutoff value for a positive result was 1:10, and patients with negative results were considered to have a titer of 0 for the calculation of the mean titer.

 
Our results suggest that 100 percent of patients had antibody responses to SARS-CoV during the convalescent phase. The SARS-specific IgG antibody persisted for a long time, but the SARS-specific IgM remained measurable for a much shorter period, suggesting that IgG antibody to SARS-CoV may represent the primary humoral immune response protecting patients against SARS. The profile of antibodies against SARS-CoV was consistent with common findings with regard to acute viral infectious diseases such as hepatitis A.⁴ The profile of anti-SARS antibodies may be helpful in the diagnosis and in epidemiologic surveys. The presence of high titers of IgG antibody to SARS-CoV in the patients at the convalescent stage also suggests that a live attenuated or inactivated vaccine for active immunization and a concentrated human SARS-specific IgG antibody for passive immunization could be developed for the treatment of SARS.

Gang Li, M.D., Ph.D.
Xuejuan Chen
Anlong Xu, Ph.D.
Sun Yat Sen University
Guangzhou 510275, China
ligangzh{at}pub.guangzhou.gd.cn

References

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2. Drosten C, Günther S, Preiser W, et al. Identification of a novel coronavirus in patients with severe acute respiratory syndrome. N Engl J Med 2003;348:1967-1976. [Free Full Text]
3. Poutanen SM, Low DE, Henry B, et al. Identification of severe acute respiratory syndrome in Canada. N Engl J Med 2003;348:1995-2005. [Free Full Text]
4. Kawai H, Feinstone SM. Acute viral hepatitis. In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennett's principles and practice of infectious diseases. 5th ed. Vol. 1. Philadelphia: Churchill Livingstone, 2000:1279-97.

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