While
IgM and
IgG response to SARS-CoV-2 has been extensively studied, relatively little is known about
secretory IgA (
sIgA) response in respiratory mucosa. Here we report
IgA response to the SARS-CoV-2 in sputum, throat swabs, and serum with
nucleocapsid protein (NP)
enzyme-linked
immunosorbent assays (ELISA) in a cohort of 28
COVID-19 patients and 55
vaccine recipients. The assays showed
sIgA in respiratory mucosa could be detected on the first day after illness onset (AIO), and the median conversion time for
sIgA in sputum, throat swabs, and serum was 3, 4, and 10 days, respectively. The positive rates of
sIgA first week AIO were 100% (24/28) and 85.7% (24/28) in sputum and throat swabs, respectively, and were both 100% during the mid-onset (2-3 weeks AIO). During the recovery period,
sIgA positive rates in sputum and throat swabs gradually decreased from 60.7% (17/28) and 57.1% (16/28) 1 month AIO and the
sIgA antibodies were all undetectable 6 months AIO. However, serum
IgA positive rate was still 100% at 4 months and 53.6% (15/28) at 6 months. Throat swabs obtained from volunteers who received inactivated
SARS-CoV-2 vaccines by intramuscular delivery all showed negative results in
IgA ELISA. These findings will likely improve our understanding of respiratory mucosal immunity of this emerging disease and help in containing the pandemic and developing
vaccines.