Flaxman A, Marchevsky NG, Jenkin D, et al. - Increased antibody titres are recorded in correlation with an extended interval before the second dose of ChAdOx1 nCoV-19. A third dose of ChAdOx1 nCoV-19 after the second dose resulted in induction of antibodies to a level that links with high efficacy along with a boost in T-cell responses.

• In this sub study, invitation for vaccination was given to volunteers who were enrolled in the phase 1/2 (COV001) controlled trial and received either a single dose or two doses of 5 × 10¹⁰ viral particles.

• For comparison purposes, researchers used data from volunteers enrolled in either the phase 1/2 (COV001) or phase 2/3 (COV002), single-blinded, randomized controlled trials of ChAdOx1 nCoV-19 and had previously received a single dose or two doses of 5 × 10¹⁰ viral particles.

• Those with a longer interval between first and second dose had higher antibody titers at 28 days after vaccination relative to those with a short interval.

• Antibody titers were significantly higher 28 days after a third dose than 28 days after a second dose.

• A third dose also resulted in a boost of T-cell responses.

• Relative to reactogenicity after a first dose, a lower reactogenicity was recorded after a late second dose or a third dose.