Antibody levels induced by a single dose of ChAdOx1 nCoV-19 decreased gradually but remained above baseline levels after 1 year. We have previously shown that administration of a second dose of vaccine induces higher antibody responses by 1 month after the second dose than before the second dose, with higher responses with a dose interval up to 3 months between the first two doses.
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Here, we found that a long extension of the dose interval (up to 45 weeks) between the first and second dose further enhances the immune response to the second dose when compared with shorter dose intervals. Furthermore, for the first time, we showed that a third dose of ChAdOx1 nCoV-19 can induce a strong boost to immune responses to the transgene product, SARS-CoV-2 spike protein, and that these responses result in increased neutralising antibody titres and enhanced antibody activity against variants.