Having antibodies against COVID-19 provides children with little or no protection against infection, a study published in JAMA has found. Instead, the main determinant of protection is cellular immunity (i.e., T cells). Worse, against Omicron having antibodies actually increased the risk of infection, which may help to explain the negative vaccine effectiveness seen in a number of studies, where infection rates are higher in the vaccinated than the unvaccinated.
The study assessed protection from SARS-CoV-2 infection in 79 children aged four to 11 years in England. The results are depicted below. Notice that the two cellular-positive bars (pink and beige) are much lower than the two cellular-negative bars. This shows that being cellular positive entailed much lower infection risk than being cellular negative, regardless of whether the person had antibodies (i.e., was seropositive). Having antibodies here did result in a slightly reduced risk of infection, but not much.
The chart above shows the infections over time. The most striking aspect is the huge green column in January for Omicron, which is for the cellular-negative. Note also that the pink bar in January is slightly higher than the beige bar. The pink bar is antibody-positive whereas the beige bar is not (both are cellular positive), suggesting antibodies here were a disadvantage against Omicron.