Fig. 4. Antibody responses and neutralising function after boosting.

The fractional dose of the viral vector vaccine was delivered intradermally, 4–8 weeks (Group2) and 8–12 weeks (Group3) after two doses of inactivated SARS-CoV-2 vaccines. Intramuscular viral vector boosting was also given at 4–8 weeks after the last vaccination (Group 1). The blood samples were collected before (D0) and after the booster dose for 14, 28 days (D14, D28). a Serum samples were analyzed using CMIA to measure anti-RBD IgG. b Neutralising function of antibodies against the delta variant were tested using PRNT. Each symbol represents one participant, and the number is the geometric mean with 95% CI (n = 30–34 volunteers). Statistical significance was determined using Kruskal–Wallis test, with Dunn’s multiple comparisons test between vaccinated groups (a) and between time points (b). *p ≤ 0.05, **p ≤ 0.01, ****p ≤ 0.0001, ns non significance.