It’s always a race between viral replication and immune response. Asymptomatic spread of coronavirus requires some degree of viral replication and amplification. The presence of neutralizing antibodies can “trap” virus before it spreads, but how much neutralizing antibody is really present in the upper respiratory mucosa? Intramuscular vaccines induce systemic IgG, rather than much mucosal IgA. The real heroes with the mRNA and viral vectored vaccines are the cytotoxic T cells, which attack virus infected cells and prevent the propagation of more virus. This response can often be active within days of vaccination. I would not be surprised, therefore, that these vaccines can reduce spread by asymptomatic, infected persons.