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The time point of positive or negative results of IgA antibody against SARS-CoV-2 in different samples of 28 COVID-19 patients during the observation period. The abscissa represented the number of 28 COVID-19 patients during the observation period. The ordinate represented days after symptom onset. Red icons meant that IgA antibody test results are positive. Blue icons meant that IgA antibody turns negative.
