Persistence of the immune response after two doses of ChAdOx1 nCov-19 (AZD1222): 1 year of follow up of two randomised controlled trials.
Voysey M., Flaxman A., Aboagye J., Aley PK., Belij-Rammerstorfer S., Bibi S., Bittaye M., Cappuccini F., Charlton S., Clutterbuck EA., Davies S., Dold C., Edwards NJ., Ewer KJ., Faust SN., Folegatti PM., Fowler J., Gilbride C., Gilbert SC., Godfrey L., Hallis B., Humphries HE., Jenkin D., Kerridge S., Mujadidi YF., Plested E., Ramasamy MN., Robinson H., Sanders H., Snape MD., Song R., Thomas KM., Ulaszewska M., Woods D., Wright D., Pollard AJ., Lambe T.
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.