Impact of priming interval on reactogenicity, peak immunological response and waning after homologous and heterologous COVID-19 vaccine schedules: Exploratory analyses of Com-COV, a randomised control trial

Background: Priming COVID-19 vaccine schedules have been deployed at variable intervals globally, which may influence immune persistence and the relative importance of third-dose ‘booster’ programmes. Here, we report on the impact of 4- versus 12-week priming intervals on reactogenicity and the persistence of immune response up to 6 months following homologous and heterologous priming schedules using BNT162b2 (BNT, tozinameran, Comirnaty, Pfizer/BioNTech) and ChAdOx1 nCoV-19 (ChAd, Vaxzevria, AstraZeneca). Methods: Com-COV is a participant-blinded, randomised immunogenicity trial. Results are reported here for the ‘General’ cohort, in which adults aged over 50 years were randomised to four homologous and heterologous schedules using BNT and ChAd with 4- or 12-week priming intervals. Immunogenicity analyses were on the intention-to-treat population (ITT), without evidence of COVID-19 infection at baseline or for the trial duration, with the purpose of describing the effect of priming interval on humoral and cellular immune response at peak and later timepoints, in addition to the effects on reactogenicity and safety Findings: Between 11th–26th Feb 2021, 730 participants were randomised in the general cohort, with 77-89 per arm in the ITT analysis. At 28-days and 6-months post-second dose, the geometric mean concentration (GMC) of anti-SARS-CoV-2 spike IgG was significantly higher in 12- versus 4-week interval arms for homologous schedules. In heterologous arms, there was only a significant difference between intervals for the BNT/ChAd arm at 28-days. Pseudotyped virus neutralisation titres were significantly higher in all 12-week versus 4-week schedules, 28-days post-second dose, with geometric mean ratios 1.4 (95%CI: 1.1-1.8, BNT/BNT), 1.5 (95%CI: 1.2-1.9, ChAd/BNT), 1.6 (95%CI 1.3-2.1, BNT/ChAd) and2.4 (95%CI: 1.7-3.2, ChAd/ChAd). At 6 months post-second dose, anti-spike IgG GMCs fell to 0.17-0.24 of the 28-day post-second dose value across all eight study arms, with only BNT/BNT displaying a slightly slower decay for the 12-week versus 4-week schedule in the adjusted analysis. The rank order of schedules by humoral response was unaffected by interval with BNT/BNT remaining the most immunogenic by antibody response. T-cell responses were reduced in all 12-week priming intervals versus their 4-week counterparts. 12-week schedules for BNT/BNT and ChAd/BNT schedules were up to 80% less reactogenic than 4-week schedules. Interpretation: These data support flexibility in priming interval in all studied COVID-19 vaccine schedules. Longer priming intervals may result in lower reactogenicity in schedules with BNT as a second-dose and higher humoral immunogenicity in homologous schedules, but overall lower T-cell responses across all schedules. Future vaccines employing these novel platforms may benefit from prolonged-interval schedules. ISRCTN:69254139, EudraCT:2020-005085-33.