Nasal mucosal secretory immunoglobulin A but not serum antibodies is resistant to Omicron evasion

Intramuscular vaccines against severe acute respiratory syndrome coronavirus 2(SARS-CoV-2)can effectively alleviate the severity of SARS-CoV-2-caused disease and reduce mortality.However,they cannot effectively prevent SARS-CoV-2 infection and transmission due to the failure to elicit protective viral-specific mucosal secretory immunoglobulin A(sIgA)[1,2].Unlike intramuscular immunization,natural infection,irrespective of prior vaccination,can induce neutralizing mucosal sIgA in the upper respiratory tract,the first site where SARS-CoV-2 infection establishes,replicates,and persists until the final viral clearance[3].The local presence of viral antigens in the respiratory tract is essential for generating mucosal immunity,which guards against subsequent infection.Several studies showed that the Omicron breakthrough infection rate is lower in people with a higher spike-specific mucosal sIgA level[4–6].

Intranasal booster using an Omicron vaccine confers broad mucosal and systemic immunity against SARS-CoV-2 variants

The highly contagious SARS-CoV-2 Omicron subvariants severely attenuated the effectiveness of currently licensed SARS-CoV-2 vaccines based on ancestral strains administered via intramuscular injection.In this study,we generated a recombinant,replication-incompetent human adenovirus type 5,Ad5-S-Omicron,that expresses Omicron BA.1 spike.Intranasal,but not intramuscular vaccination,elicited spike-specific respiratory mucosal IgA and residential T cell immune responses,in addition to systemic neutralizing antibodies and T cell immune responses against most Omicron subvariants.We tested intranasal Ad5-S-Omicron as a heterologous booster in mice that previously received intramuscular injection of inactivated ancestral vaccine.In addition to inducing serum broadly neutralizing antibodies,there was a significant induction of respiratory mucosal IgA and neutralizing activities against Omicron subvariants BA.1,BA.2,BA.5,BA.2.75,BF.7 as well as pre-Omicron strains Wildtype,Beta,and Delta.Serum and mucosal neutralizing activities against recently emerged XBB,BQ.1,and BQ.1.1 could also be detected but were much lower.Nasal lavage fluids from intranasal vaccination contained multimeric IgA that can bind to at least 10 spike proteins,including Omicron subvariants and pre-Omicron strains,and possessed broadly neutralizing activities.Intranasal vaccination using Ad5-S-Omicron or instillation of intranasal vaccinee’s nasal lavage fluids in mouse nostrils protected mice against Omicron challenge.Taken together,intranasal Ad5-S-Omicron booster on the basis of ancestral vaccines can establish effective mucosal and systemic immunity against Omicron subvariants and multiple SARS-CoV-2 variants.This candidate vaccine warrants further development as a safe,effective,and user-friendly infection and transmission-blocking vaccine.