Humoral and cellular immunity of two-dose inactivated COVID-19 vaccination in Chinese children: a prospective cohort study.

Children are the high-risk group for COVID-19, and in need of vaccination. However, humoral and cellular immune responses of COVID-19 vaccine remain unclear in vaccinated children. To establish the rational immunization strategy of inactivated COVID-19 vaccine for children, the immunogenicity of either one dose or two doses of the vaccine in children was evaluated. A prospective cohort study of 322 children receiving inactivated COVID-19 vaccine was established in China. The baseline was conducted after 28 days of the first dose, and the follow-up was conducted after 28 days of the second dose. The median titers of RBD-IgG, and neutralizing antibody (NAb) against prototype strain and Omicron variant after the second dose increased significantly compared to those after the first dose (first dose: 70.0, [IQR, 30.0-151.0] vs second dose: 1261.0 [636.0-2060.0] for RBD-IgG; 2.5 [2.5-18.6] vs 252.0 [138.6-462.1] for NAb against prototype strain; 2.5 [2.5-2.5] vs 15.0 [7.8-26.5] for NAb against Omicron variant, all P <0.05). The flow cytometry results showed that the first dose elicited SARS-CoV-2 specific cellular immunity, while the second dose strengthened SARS-CoV-2 specific IL-2 + or TNF-α + monofunctional, IFN-γ + TNF-α + bifunctional, and IFN-γ - IL-2 + TNF-α + multifunctional CD4 + T cell responses (P <0.05). Moreover, SARS-CoV-2 specific memory T cells were generated after the first vaccination, including the central memory T cells and effector memory T cells. The present findings provide scientific evidence for the vaccination strategy of inactive vaccine among children against COVID-19 pandemic. This article is protected by copyright. All rights reserved.