Abstract
Background and Objectives: SARS-CoV-2 infection initiates at mucosal surfaces, and mucosal immunity may influence the nature and severity of infection. Little is known about the induction of mucosal immunity by vaccination in COVID-19 convalescents. Methods: Sera from 205 healthcare workers were collected one month after the first Covishield vaccination and 1/3/6 months after the second vaccination, while paired sera and stimulated whole-mouth fluid (SWMF) was collected 1/3/6 months after the third vaccination (N = 10) and at 0/30/90 days after a COVID-19 episode (N = 8). Anti-SARS-CoV-2 spike antibody detection by ECLIA/ELISA and cytokine detection by ELISA/CBA were performed. Results: One month post-second vaccination, serum antibodies had increased significantly (6-fold) in the COVID-19-naïve group (CNG) but declined (1.5-fold) in the previously COVID-19-exposed group (CEG), who already had high antibody titres. The serum regulatory cytokine IL-10 levels were higher after three antigen exposures (p = 0.0002). New infections (breakthrough infections-BTIs) or reinfections (RIs) with asymptomatic/mild disease occurred in 44% of the CNG and 27% of the CEG (p < 0.01). The mucosal cytokine IL-17 levels were significantly higher in the CEG. Salivary IgG/IgA and secretory IgA antibodies were detectable both after vaccination and COVID-19. Innate cytokines (MIG, MCP-1, IL-8, IL-1β) were higher and sustained in SWMF in contrast to serum. Conclusions: Two vaccinations in the CNG resulted in an antibody boost, but the second vaccination in the CEG induced antibody anergy. Serum/mucosal antibodies declined by six months after vaccination, but the rapid increase at subsequent exposures were indicative of a good T cell/B cell memory response to SARS-CoV-2. A higher percentage of BTI among the CNG than RI among the CEG may indicate better protection due to higher antibody responses in the latter group.