The trajectory of immune responses following the primary dose series determines the decline in vaccine effectiveness over time. Here we report on maintenance of immune responses during the year following a two-dose schedule of ChAdOx1 nCoV-19/AZD1222, in the absence of infection, and also explore the decay of antibody after infection. Total spike-specific IgG antibody titres were lower with two low-doses of ChAdOx1 nCoV-19 vaccines (LDLD) (p=0.0006) than with SDSD (the approved dose) or LDSD regimens. Longer intervals between first and second doses resulted in higher antibody titres (p<0.0001), however, there was no evidence that the trajectory of antibody decay differed by interval or by vaccine dose, and the decay of IgG antibody titres followed a similar trajectory after a third dose of ChAdOx1 nCoV-19. Trends in post-infection samples were similar with an initial rapid decay in responses but good persistence of measurable responses thereafter. Extrapolation of antibody data, following two doses of ChAdOx1 nCov-19, demonstrates a slow rate of antibody decay with modelling suggesting that antibody titres are well-maintained for at least two years. These data suggest a persistent immune response after two doses of ChAdOx1 nCov-19 which will likely have a positive impact against serious disease and hospitalisation.
Clin Exp Immunol
Anti-Viral Immunity, Antibodies, Vaccination, Vaccine