Fig. 2: Haemagglutination test antibodies to the ancestral Wuhan SARS-CoV-2 virus and variants of concern in older and younger adult vaccinees and after natural infection.

Haemagglutination test (HAT) antibodies to the SARS-CoV-2 virus receptor binding domains of homologous founder virus (a–d: Wuhan-like) and variants of concern (e–h B.1.1.7 alpha; i–l: B.1.351 beta; m–p: P.1 gamma and q–t: B.1.617.2 delta). Endpoint HAT titres are presented in (b–d, f–h, j–l, n–p, r–t). The percentage of responders with haemagglutination test titre ≥ 40 (a, e, i, m, q) and endpoint HAT titres (b, f, j, n, r) in seronegative older (n = 89) and seronegative adults (n = 309) post 1^st dose (3 weeks) and post 2^nd dose (6–8 weeks after 1^st dose) mRNA BNT162b2 COVID-19 vaccination. In infected individuals, convalescent serum was collected 3–10 weeks after SARS-CoV-2 confirmed infection (infected, n = 307) with D614G virus during the first pandemic wave (a–b, e–f, i–j, m–n, q–r). HAT endpoint titres to Wuhan-like and VOC in previously infected older individuals (n = 7) and adults (n = 7) who were vaccinated are shown in different colours, with the grey dashed line showing comparison of the geometric mean HAT titres for the corresponding seronegative (not previously infected) old (n = 89) and adult (n = 309) vaccinees (c–d, g–h, k–l, o–p, s–t). For endpoint HAT titres (b–d, f–h, j–l, n–p, r–t), negative values were assigned a value of 5. The geometric mean titres (GMT) and error bars with 95% confidence intervals are shown in black and each symbol represents one subject (b, f, j, n, r).