ABSTRACTRising breakthrough infections with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron BA.4/5 led to the performance of various studies investigating systemic immunity and neutralizing antibodies in sera, but mucosal immunity remains understudied. In this cohort study, the humoral immune responses, including immunoglobulin levels and the presence of virus-neutralizing antibodies, of 92 vaccinated and/or BA.1/BA.2 convalescent individuals were investigated. Cohorts received two doses of ChAdOx1, BNT162b2, or mRNA-1273 and subsequent booster vaccination with either BNT162b2 or mRNA-1273, following BA.1/BA.2 infection. In addition, vaccinated and nonconvalescent or unvaccinated and BA.1 convalescent individuals were studied. Serum and saliva samples were used to determine SARS-CoV-2 spike-specific IgG and IgA titers and neutralizing activity against replication-competent SARS-CoV-2 wild-type virus and the Omicron BA.4/5 variant. Vaccinated/convalescent cohorts demonstrated strongest neutralization against BA.4/5, with 50% neutralization titer (NT50) values reaching 174.2; however, neutralization was reduced up to 11-fold, compared to wild-type virus. Both BA.1 convalescent and vaccinated nonconvalescent cohorts displayed the weakest neutralization against BA.4/5, with NT50values being reduced to 4.6, accompanied by lower numbers of positive neutralizers. Additionally, salivary neutralization against wild-type virus was strongest in vaccinated and BA.2 convalescent subjects, but this elevated neutralization efficiency was lost when challenged with BA.4/5. Our data support the contention that current coronavirus disease 2019 (COVID-19) vaccines efficiently induce humoral immunity. However, antiviral effectiveness in serum and saliva is greatly reduced against novel variants of concern. These results suggest an adjustment of current vaccine strategies to an adapted or alternative vaccine delivery, such as mucosal booster vaccinations, which might establish enhanced or even sterilizing immunity against novel SARS-CoV-2 variants.IMPORTANCERising incidences of breakthrough infections caused by SARS-CoV-2 Omicron BA.4/5 have been observed. Although various studies were conducted investigating neutralizing antibodies in sera, mucosal immunity was barely evaluated. Here, we investigated mucosal immunity, since the presence of neutralizing antibodies at mucosal entry sites plays a fundamental role in disease limitation. We found strong induction of serum IgG/IgA, salivary IgA, and neutralization against SARS-CoV-2 wild-type virus in vaccinated/convalescent subjects but detected 10-fold reduced (albeit positive) serum neutralization against BA.4/5. Interestingly, vaccinated and BA.2 convalescent patients demonstrated the greatest serum neutralization against BA.4/5, but this advantageous neutralizing effect was not observed in the saliva. Our data support the contention that current COVID-19 vaccines are very efficient against severe/critical disease progression. Moreover, these results suggest an adjustment of the current vaccine strategy to adapted and alternative vaccine delivery, such as mucosal booster vaccinations, to establish robust sterilizing immunity against novel SARS-CoV-2 variants.