Characteristic of IgA and IgG antibody response to SARS-CoV-2 infection in an Italian referral COVID-19 Hospital

INTRODUCTION: Antibody response plays a fundamental role in the natural history of infectious disease. A better understanding of the immune response in patients with SARS-CoV-2 infection could be important for identifying patients at greater risk of developing a more severe form of disease and with a worse prognosis. METHODS: We performed a cross-sectional analysis to determine the presence and the levels of both anti-SARS-CoV-2 IgG and IgA in a cohort of hospitalized patients with confirmed infection at different times in the natural history of the disease. Patients enrolled when admitted at the emergency department were prospectively followed up during hospital stay. RESULTS: Overall, 131 patients were considered with a total of 237 samples processed. Cross-sectional analysis showed that seroconversion for IgA seems to occur between days 6 and 15, while IgG response seems to occur slightly later, peaking at day 20 after symptoms onset. Both IgA and IgG were maintained beyond 2 months. Severe patients showed a higher IgA response compared with mild patients when analyzing optical density (8.3 versus 5.6, p < 0.001). Prospective analysis conducted on 55 patients confirmed that IgA appear slightly earlier than IgG. After stratifying for the severity of disease, both the IgA and IgG responses were more vigorous in severe cases. Moreover, while IgG tended to stabilize, there was a relevant decline after the first month of IgA levels in mild cases. CONCLUSION: IgA and IgG antibody response is closely related, although seroconversion for IgA occurs earlier. Both IgA and IgG are maintained beyond 2 months. Severe patients showed a more vigorous IgA and IgG response. IgA levels seem to decline after 1 month since the onset of symptoms in mild cases. Our results should be interpreted with cautions due to several limitations in our study, mainly the small number of cases, lack of data on viral load and clinical setting.