LAUSR

Data on cross‐neutralization of the SARS‐CoV‐2 omicron variant more than 1 year after SARS‐CoV‐2 infection are urgently needed, especially in children, to predict the likelihood of reinfection and to guide vaccination strategies. In a prospective observational cohort study, we evaluated live‐virus neutralization of the SARS‐CoV‐2 omicron (BA.1) variant in children compared with adults 14 months after mild or asymptomatic wild‐type SARS‐CoV‐2 infection. We also evaluated immunity to reinfection conferred by previous infection plus COVID‐19 mRNA vaccination. We studied 36 adults and 34 children 14 months after acute SARS‐CoV‐2 infection. While 94% of unvaccinated adults (16/17) and children (32/34) neutralized the delta (B.1.617.2) variant, only 1/17 (5.9%) unvaccinated adults, 0/16 (0%) adolescents and 5/18 (27.8%) children <12 years of age had neutralizing activity against omicron (BA.1). In convalescent adults, one or two doses of mRNA vaccine increased delta and omicron neutralization 32‐fold, similar to a third mRNA vaccination in uninfected adults. Neutralization of omicron was 8‐fold lower than that of delta in both groups. In conclusion, our data indicate that humoral immunity induced by previous SARS‐CoV‐2 wild‐type infection more than 1 year ago is insufficient to neutralize the current immune escape omicron variant.