infection-induced immunity

Background
To evaluate the duration of protection against reinfection conferred by a previous SARS-CoV-2 infection in children and adolescents.
Methods
We applied two complementary approaches: a matched test-negative, case-control design and a retrospective cohort design. 458,959 unvaccinated individuals aged 5-18 years were included. Analyses focused on July 1 to December 13, 2021, a period of Delta variant dominance in Israel. We evaluated three SARS-CoV-2-related outcomes: documented PCR confirmed infection or reinfection, symptomatic infection or reinfection, and SARS-CoV-2-related hospitalization or death.
Findings
Overall, children and adolescents who were previously infected acquired durable protection against reinfection with SARS-CoV-2 for at least 18 months. Importantly, no SARS-CoV-2-related deaths were recorded in either the SARS-CoV-2 naïve group or the previously infected group. Effectiveness of naturally-acquired immunity against a recurrent infection reached 89.2% (95% CI: 84.7%-92.4%) three to six months after first infection, mildly declining to 82.5% (95% CI, 79.1%-85.3%) 9-12 months after infection, with a slight non-significant waning trend up to 18 months after infection. Additionally, we found that ages 5-11 years exhibited no significant waning of naturally acquired protection throughout the outcome period, whereas waning protection in the 12-18 year-old age group was more prominent, but still mild.
Interpretation
Children and adolescents who were previously infected with SARS-CoV-2 remain protected to a high degree for 18 months. Further research is needed to examine naturally-acquired immunity against Omicron and newer emerging variants.