Publication date: May 20, 2025

The binding and neutralising activity of SARS-CoV-2 antibodies are important correlates of protection of current COVID-19 vaccines. SARS-CoV-2 exposure status and COVID-19 vaccine types can influence these responses and the breadth of cross-reactivity to variants. In this longitudinal cohort study, we used SARS-CoV-2-specific multiplex Luminex antibody assays and live virus neutralisation of ancestral (VIC01/2020), Delta and Omicron (BA1, BA2 and BA5) SARS-CoV-2 variants to compare antigen-specific binding and neutralising antibody (nAb) responses to primary vaccination (two doses) of adenovirus vectored (AdVV) or mRNA vaccines followed by a booster dose of mRNA vaccine in convalescent (n=51) and infection-naive individuals (n=47). In a subset of individuals, we performed additional analysis of antibody responses following breakthrough infection. We found that titres of anti-SARS-CoV-2 nAb following primary vaccination (2 doses) with AdVV vaccine were significantly lower than those following mRNA vaccine, irrespective of prior SARS-CoV-2 infection status. However, an mRNA vaccine booster dose resulted in equivalent binding and nAb titres to the ancestral virus in all individuals, irrespective of primary vaccine type. Notably, vaccinated infection-naive, but not convalescent individuals required the third dose of vaccine (mRNA) to induce nAbs to Omicron subvariants BA1, BA2 and BA5, though titres against the variants were lower than those against the ancestral strain. Importantly, breakthrough infection with Omicron strains induced higher nAb titre rises against the ancestral strain than against Omicron variants consistent with imprinting of the immunologic response and recall of pre-existing immunity to the ancestral strain.

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