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 receptor binding domain (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, [interquartile range, 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-alpha(+) monofunctional, IFN-gamma+TNF-alpha(+) bifunctional, and IFN-gamma-IL-2(+)TNF-alpha(+) 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 the inactive vaccines among children against COVID-19 pandemic.

Automated summary

Children are at high risk for COVID-19 and require vaccination. This study evaluates the humoral and cellular immune responses of COVID-19 vaccines in vaccinated children to establish a rational immunization strategy. The results show that the second dose of the inactivated COVID-19 vaccine significantly increases antibody titers and strengthens specific T cell responses compared to the first dose. Memory T cells specific to SARS-CoV-2 are also generated after the first vaccination. These findings provide scientific evidence for the vaccination strategy against COVID-19 in children.