Antibody avidity in the diagnosis of SARS coronavirus infection

Department of Microbiology, The University of Hong Kong and Queen Mary Hospital, Hong Kong, SAR, People's Republic of China; EUROIMMUN AG, Luebeck, Germany; Robert Koch Institute, Berlin, Germany

^* To whom correspondence should be addressed. Email: malik{at}hkucc.hku.hk.

	Abstract

An indirect immunofluorescent (IIF) assay (EUROIMMUN AG, Luebeck, Germany) was used to investigate the avidity of the IgG, IgM, IgA and total Ig (IgGAM) antibody responses to SARS coronavirus (SARS CoV) infections. Serial sera from 8 patients collected during the first, third and ninth month post onset of infection were evaluated. It was found that low avidity IgG antibodies were detected in 15/15 (100%), 1/5 (20%) and 0/8 (0%) of serum samples collected from the first, third and ninth month after onset of symptoms, respectively. Low avidity antibodies of IgA and IgM subclasses were detected in 14/14 (100%) and 3/14 (21%) of sera collected in the first month post onset of infection. However, IgA antibodies remained low in avidity in a proportion of patients even during late convalescence. As a consequence, IgG antibody avidity assays gave better discrimination between acute from late convalescent sera than IgM, IgA or total IgGAM assays. In two of these patients, sequential sera were also tested IgG avidity against human coronavirus OC43 and 229E in parallel. While SARS CoV infections induced an anamnestic IgG antibody response to 229E and OC43 viruses, these cross-reactive antibodies remained of high avidity from early (first month) post infection. The results showed that assays for low avidity antibody may be useful for discriminating early from late antibody responses and also in distinguishing anamnestic cross-reactive antibody responses from primary specific responses. This may be useful in some clinical situations.