Mucosal immunity has a pivotal role in protection from respiratory viral infections. 1 The current authors have showed substantial protection from omicron infection by high concentrations of nasal mucosal SARS-CoV-2 WT spike immunoglobulin-A (M-IgA) over a 4-week screening period. 2 A sharp increase in M-IgA concentrations following BA. 1 or BA. 2 breakthrough infection in triple vaccinated healthcare workers was also observed. 2 Here, we present follow-up data with prospectively collected omicron infection rates and systemic and mucosal antibody concentrations from the same cohort (appendix pp 7–9, 12–14).

The association between M-IgA concentrations at the 75th percentile or higher at enrolment and a reduced risk of symptomatic BA. 1, BA. 2, or BA. 5 breakthrough infection remained over an 8-month followup period, with a hazard ratio (HR) of 0· 55 (95% CI 0· 35–0· 87), much due to the initial risk difference (figure A). Serum WT spike-specific IgG (S-IgG) concentrations waned over 8 months