BACKGROUND: Heterologous COVID vaccine priming schedules are immunogenic and effective. This report aims to understand the persistence of immune response to the viral vectored, mRNA and protein-based COVID-19 vaccine platforms used in homologous and heterologous priming combinations, which will inform the choice of vaccine platform in future vaccine development. METHODS: Com-COV2 was a single-blinded trial in which adults ≥50 years, previously immunised with single dose 'ChAd' (ChAdOx1 nCoV-19, AZD1222, Vaxzevria, Astrazeneca) or 'BNT' (BNT162b2, tozinameran, Comirnaty, Pfizer/BioNTech), were randomised 1:1:1 to receive a second dose 8-12 weeks later with either the homologous vaccine, or 'Mod' (mRNA-1273, Spikevax, Moderna) or 'NVX' (NVX-CoV2373, Nuvaxovid, Novavax). Immunological follow-up and the secondary objective of safety monitoring were performed over nine months. Analyses of antibody and cellular assays were performed on an intention-to-treat population without evidence of COVID-19 infection at baseline or for the trial duration. FINDINGS: In April/May 2021, 1072 participants were enrolled at a median of 9.4 weeks after receipt of a single dose of ChAd (N= 540, 45% female) or BNT (N=532, 39% female) as part of the national vaccination programme. In ChAd-primed participants, ChAd/Mod had the highest anti-spike IgG from day 28 through to 6 months, although the heterologous vs homologous geometric mean ratio (GMR) dropped from 9.7 (95%CI: 8.2,11.5) at D28 to 6.2 (95%CI: 5.0, 7.7) at D196. The heterologous/homologous GMR for ChAd/NVX similarly dropped from 3.0 (95%CI:2.5-3.5) to 2.4 (95%CI:1.9-3.0). In BNT-primed participants, decay was similar between heterologous and homologous schedules with BNT/Mod inducing the highest anti-spike IgG for the duration of follow-up. The aGMR for BNT/Mod compared with BNT/BNT increased from 1.36 (95%CI: 1.17, 1.58) at D28 to 1.52 (95%CI: 1.21, 1.90) at D196, whilst for BNT/NVX this aGMR was 0.55 (95%CI: 0.47, 0.64) at day 28 and 0.62 (95%CI: 0.49, 0.78) at day 196. Heterologous ChAd-primed schedules produced and maintained the largest T-cell responses until D196. Immunisation with BNT/NVX generated a qualitatively different antibody response to BNT/BNT, with the total IgG significantly lower than BNT/BNT during all follow-up time points, but similar levels of neutralising antibodies. INTERPRETATION: Heterologous ChAd-primed schedules remain more immunogenic over time in comparison to ChAd/ChAd. BNT-primed schedules with a second dose of either mRNA vaccine also remain more immunogenic over time in comparison to BNT/NVX. The emerging data on mixed schedules using the novel vaccine platforms deployed in the COVID-19 pandemic, suggest that heterologous priming schedules might be considered as a viable option sooner in future pandemics. ISRCTN: 27841311 EudraCT:2021-001275-16 FUNDING: UK Vaccine Task Force (VTF), Coalition for Epidemic Preparedness Innovations (CEPI) and National Institute for Health and Carte Research (NIHR). NVX was supplied for trial use by Novavax, Inc.
Covid, Protein, SARS-CoV2, T cell, T-cell, Tcell, adenovirus vector, adjuvant, antibody, cellular, heterologous, homologous, humoral, immunology, mRNA lipid nanoparticle, persistence, subgroup, vaccination, vaccine