Fig. 1: Low titre antibodies and neutralisation develop following primary Omicron infection.

SARS-CoV-2-specific antibody responses were determined in children with recent Omicron BA.1/2 infection. A Wuhan-Hu-1 (Wu-Hu-1) spike antibody binding in samples from children who were SARS-CoV-2 seronegative (n = 4) or seropositive (n = 11) prior to Omicron infection (yellow). Dotted lines indicate seropositive cut-off for Wuhan-Hu-1-specific antibody response; dashed lines indicate below the limit of detection. Data were presented as arbitrary units (AU)/ml. One-way Friedman Test with Dunn’s multiple comparisons test. B Wuhan-Hu-1 RBD-specific antibody binding in the total cohort of 43 children with recent Omicron BA.1/2 infection shows a bimodal distribution of primary (green; n = 20) or secondary (orange; n = 23) SARS-CoV-2 infection status. Data were presented as arbitrary units (AU)/ml. Bar indicates geometric mean; representative of primary or secondary Omicron infection. C Wuhan-Hu-1 and Omicron BA.1 spike-specific antibody binding in children with primary (green; n = 20) or secondary (orange; n = 23) Omicron BA.1/2 infection. The dotted line indicates seropositive cut-offs for Wuhan-Hu-1. Data were presented as arbitrary units (AU)/ml. One-way Kruskal–Wallis test with Dunn’s multiple comparisons test, bars indicate geometric mean ±95% CI. D Neutralisation of pseudovirus-bearing B.1 (Wuhan-Hu-1 and D614G) ancestral or Omicron BA.1 or BA.2 sequence spike protein in children following primary (green; n = 20) or secondary (orange; n = 21) Omicron infection. Dashed lines indicate the limit of detection. Data were presented as NT50, the titre at which infectivity had been reduced by 50% relative to controls. One-way Kruskal–Wallis test with Dunn’s multiple comparisons test, bars indicate geometric mean ±95% CI. E Bar graph indicating the percentage of children with detectable neutralising titres to B.1 or Omicron BA.1 or BA.2 following primary (green, n = 20) or secondary (orange, n = 21) Omicron infection. Source data are provided as a Source Data file.