Relationship between nasal viral load and SARS-CoV-2 antibody levels
SARS-CoV-2 viral load was measured in nasal wash collected at the first visit. SARS-CoV-2 was detected in about 48% (25/52) of anosmic and 45% (24/53) of normosmic patients. Among SARS-CoV-2 positive patients, viral load was not significantly different in anosmic and normosmic patients (data not shown). Anosmic patients tended to have higher levels of SARS-CoV-2 specific nasal and serum IgG and this difference reached significance for RBD IgG in nasal wash (Figure 2a) . No significant difference was observed for nasal or serum IgA. These results contrast with our previous report indicating that patients with prolonged anosmia had lower levels of nasal, but not serum IgG, 2 months after onset of symptoms7. This difference suggests that anosmia is associated with dynamic changes in antibody levels that may be related to the changes in SARS-CoV-2 antigen load16. The association with serum and nasal IgG, and not IgA, levels suggests that antigen challenge occurs at the systemic level. To further explore the relationship between viral replication and antibody levels, nasal SARS-CoV-2 PCR-positive and negative patients were compared. Among anosmic patients, SARS-CoV-2 PCR-positive patients had lower levels of nasal and serum IgG as compared to SARS-CoV-2 PCR-negative patients (Figure 2b) . A similar trend was observed for nasal IgA levels. In serum, SARS-CoV-2 negative patients had higher levels of SARS-CoV-2 specific IgA, contrasting with IgG levels. Similar opposite trends for serum IgG and IgA levels have been observed in previous studies of COVID-19 convalescent patients, although the mechanism involved has not been determined17.
Among SARS-CoV-2 PCR-positive patients, a negative correlation was observed between SARS-CoV-2 viral load and nasal and serum IgG and IgA(Figure 2c) . Together these data suggest that, among patients with more severe OD, SARS-CoV-2 replication in the nasal cavity may be controlled by the presence of high levels of IgG and IgA. The most significant negative correlations were observed with antibodies to RBD and S1 subunit, suggesting that neutralizing antibodies may play an important role (Figure 2c) . As the relationship between viral load and levels of antibodies was observed in both nasal wash and in serum, both mucosal immunity and systemic immunity transferred to the mucosa may participate in the control of nasal SARS-CoV-2 replication18. Surprisingly, no significant association between nasal SARS-CoV-2 viral load and antibody levels was observed in normosmic patients (Figures 2b and 2c) . This intriguing observation suggests an interaction between more severe OD and antibody-mediated control of SARS-CoV-2 replication in the nasal cavity. Such interaction may involve other immune effectors, including innate immune cells, participating in the anti-viral activity of antibodies19.