Detection of SARS-CoV-2-Specific Secretory IgA and Neutralizing Antibodies in the Nasal Secretions of Exposed Seronegative Individuals.

Mucosal immunity may contribute to clearing SARS-CoV-2 infection prior to systemic infection, thereby allowing hosts to remain seronegative. We describe the meaningful detection of SARS-CoV-2-specific nasal mucosal antibodies in a group of exposed-household individuals that evaded systemic infection. Between June 2020 and February 2023, nasopharyngeal swab (NPS) and acute and convalescent blood were collected from individuals exposed to a SARS-CoV-2-confirmed household member. Nasal secretory IgA (SIgA) antibodies targeting the SARS-CoV-2 spike protein were measured using a modified ELISA. Of the 36 exposed individuals without SARS-CoV-2 detected by the RT-PCR of NPS specimens and seronegative for SARS-CoV-2-specific IgG at enrollment and convalescence, 13 (36.1%) had positive SARS-CoV-2-specific SIgA levels detected in the nasal mucosa at enrollment. These individuals had significantly higher nasal SIgA (median 0.52 AU/mL) compared with never-exposed, never-infected controls (0.001 AU/mL) and infected-family participants (0.0002 AU/mL) during the acute visit, respectively (both p < 0.001). The nasal SARS-CoV-2-specific SIgA decreased rapidly over two weeks in the exposed seronegative individuals compared to a rise in SIgA in infected-family members. The nasal SARS-CoV-2-specific SIgA may have a protective role in preventing systemic infection.