SARS-CoV-2 mRNA vaccines are administered as effective prophylactic measures for reducing virus transmission rates and disease severity. To enhance the durability of post-vaccination immunity and combat SARS-CoV-2 variants, boosters have been administered to two-dose vaccinees. However, long-term humoral responses following booster vaccination are not well characterized. A 16-member cohort of healthy SARS-CoV-2 naïve participants were enrolled in this study during a three-dose BNT162b2 vaccine series. Serum samples were collected from vaccinees over 420 days and screened for antigen (Ag)-specific antibody titers, IgG subclass distribution, and neutralizing antibody (nAb) responses. Vaccine boosting restored peak Ag-specific titers with sustained α-RBD IgG and IgA antibody responses when measured at six months post-boost. RBD- and spike-specific IgG4 antibody levels were markedly elevated in three-dose but not two-dose immune sera. Although strong neutralization responses were detected in two- and three-dose vaccine sera, these rapidly decayed to pre-immune levels by four and six months, respectively. While boosters enhanced serum IgG Ab reactivity and nAb responses against variant strains, all variants tested showed resistance to two- and three-dose immune sera. Our data reflect the poor durability of vaccine-induced nAb responses which are a strong predictor of protection from symptomatic SARS-CoV-2 infection. The induction of IgG4-switched humoral responses may permit extended viral persistence via the downregulation of Fc-mediated effector functions.

Background/Objectives: Effective prophylaxis for Mycobacterium tuberculosis (Mtb) requires greater understanding of immune correlates of protection. With renewed interest in BCG as an Mtb vaccine, particularly via the intravenous (IV) route, our objective was to characterize both innate and adaptive immune correlates of vaccine-induced pulmonary immunity as potential biomarkers for protective efficacy in a murine model of Mtb infection. Methods: Mice were given BCG via different routes and some boosted with recombinant virus constructs encoding Mtb Ag85B. Responding innate lymphoid cell (ILC) populations, T cells and B cells were analyzed by fluorescence activated cell sorting (FACS) for surface markers and by intracellular cytokine staining or antibody ELISPOT. Some immunized mice were challenged with aerosolized Mtb and monitored for bacterial growth in the lungs and spleen. Results: BCG given IV, but not intranasally or subcutaneously, resulted in marked increases in IFNγ expression at 72 h by pulmonary CD49⁺ NK cells, CD69⁺ ILC1, and two ILC3 populations, NCR-ILC3 and LTi cells, the latter also producing IL-22. Pulmonary ILC2 populations in these mice had significantly increased IL-13 expression at 24 h compared to the other routes. Interestingly, high levels of NK cells and ILC1 expressing IFNγ and/or TNFα were sustained at 8 wk, with sustained expression of IL-17A by pulmonary NCR-ILC3 and pronounced tissue-resident and effector memory CD4⁺ and CD8⁺ T cell responses. Intranasal boosting with Ad-Ag85B enhanced these T cell responses and generated Mtb-specific pulmonary IgA and IgG B cells, correlating with significantly reduced bacterial loads following Mtb challenge. Conclusions: BCG given IV primed for both early and persistent pulmonary ILC1/ILC3 responses of a predominantly Th1/Th17-type profile along with local Mtb-specific memory T cell and B cell populations, correlating with enhanced protective efficacy. These are worthy of further study as compartmentalized biomarkers for effective vaccine-induced local immunity against Mtb.