The anti-SARS-CoV-2 immunoglobulin G levels and neutralising capacities against alpha and delta virus variants of concern achieved after initial immunisation with vector vaccine followed by mRNA vaccine boost are comparable to those after double immunisation with mRNA vaccines

BackgroundThe humoral immune response after primary immunisation with a SARS-CoV-2 vector vaccine (AstraZeneca AZD1222, ChAdOx1 nCoV-19, Vaxzevria) followed by an mRNA vaccine boost (Pfizer/BioNTech, BNT162b2; Moderna, m-1273) was examined and compared with the antibody response after homologous vaccination schemes (AZD1222/AZD1222 or BNT162b2/BNT162b2). MethodsSera from 59 vaccinees were tested for anti-SARS-CoV-2 immunoglobulin G (IgG) and virus-neutralising antibodies (VNA) with four IgG assays, a surrogate neutralisation test (sVNT), and a Vero cell-based neutralisation test (cVNT) using the B.1.1.7 variant of concern (VOC; alpha) as antigen. Investigation was done before and after heterologous (n=31 and 42) or homologous booster vaccination (AZD1222/AZD1222, n=8/9; BNT162b2/BNT162b2, n=8/8). After the second immunisation, 26 age and gender matched sera (AZD1222/mRNA, n=9; AZD1222/AZD1222, n=9; BNT162b2/BNT162b2, n=8) were also tested for VNA against VOC B.1.617.2 (delta) in the cVNT. The strength of IgG binding to separate SARS-CoV-2 antigens was measured by avidity. ResultsAfter the first vaccination, prevalence of IgG directed against (trimeric) SARS-CoV-2 spike (S)-protein and its receptor-binding domain (RBD) varied from 55-95 % (AZD1222) to 100% (BNT162b2), depending on the vaccine used and the SARS-CoV-2 antigen used. The booster vaccination resulted in 100 percent seroconversion and appearance of highly avid IgG as well as VNA against VOC B.1.1.7. The results of the anti-SARS-CoV-2 IgG tests showed an excellent correlation to the VNA titres against this VOC. The agreement of cVNT and sVNT results was good. However, the sVNT seems to overestimate non and weak B.1.1.7-neutralising titres. The mean anti-SARS-CoV-2 IgG and B.1.1.7-neutralising titres were significantly higher after heterologous vaccination compared to the homologous AZD1222 scheme. If VOC B.1.617.2 was used as antigen, significantly lower mean VNAs were measured in the cVNT, and three (33.3%) vector vaccine recipients had a VNA titre <1:10. ConclusionsThe heterologous SARS-CoV-2 vaccination leads to a strong antibody response with anti-SARS-CoV-2 IgG and VNA titres at a level comparable to that of a homologous BNT162b2 vaccination scheme. Irrespectively of the chosen immunisation regime, highly avid IgG antibodies can be detected just two weeks after the second vaccine dose indicating the development of a robust humoral immunity. The observed reduction in the VNA titre against VOC B.1.617.2 is remarkable and may be attributed to a partial immune escape of the delta variant.