In a recent study posted to the medRxiv* preprint server, researchers evaluated the immunity dynamics following infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in children and adolescents.
Several studies have investigated the duration of protection conferred by previous SARS-CoV-2 infection, yet it remains inconclusive. Two challenges impede active research of infection-induced immune responses. The first impediment is the lack of a long-term evidence-based correlate of protection. The other is the difficulty in defining SARS-CoV-2 reinfection against the prolonged shedding of viral particles.
The Centers for Disease Control and Prevention (CDC) defined SARS-CoV-2 reinfection as two positive polymerase chain reaction (PCR) tests occurring ≥ 90 days apart. Reports suggest that adults exhibit age-depended protection from reinfection for 7 to 13 months, with people older than 65 being the least protected.
Further, it has been shown that infection-induced immunity in adults persists longer than vaccination-induced immunity. Still, the immunity dynamics following SARS-CoV-2 infection in children and adolescents remain poorly defined.
About the study
In the present study, researchers evaluated the duration of immune response conferred by prior SARS-CoV-2 infection in a cohort of Israeli children and adolescents aged between five and 18 years. The study population was not vaccinated against SARS-CoV-2 and included both convalescents and infection-naïve individuals.
They used data from the Maccabi Healthcare Services (MHS), which maintains electronic medical records (EMRs) covering over 26% of the Israeli population. Demographic data such as sex, age, socioeconomic status (SES), etc., were extracted. The authors investigated the following outcomes from July 1 to December 13, 2021, 1) documented reverse-transcription (RT)-PCR confirmed primary infection or reinfection, 2) symptomatic coronavirus disease 2019 (COVID-19), and 3) disease severity.
Similar to vaccination studies, researchers analyzed the outcomes by stratifying the time since infection into intervals of 90 days. Cases were defined as subjects with a positive PCR test, COVID-19-related symptoms, hospitalization, or death. for SARS-CoV-2 during the study period.
Only the first positive or negative test results for cases or controls were included. Negative test results of cases were excluded; therefore, a study subject was either a case or control but never both. A matched test-negative case-control design was included in additionally, they implemented a retrospective cohort design in secondary analysis, wherein the incidence rate of the three outcomes was compared between convalescents and naïve subjects.
More than 290,000 MHS members were eligible for the study and took at least one PCR test during the study period. Over 200,300 infections and 32,829 symptomatic cases were recorded and matched with controls. The peak incidence of SARS-CoV-2 cases was noted during August. –September 2021. Fifty cases were hospitalized, and no deaths were recorded.
The effectiveness of infection-induced immunity against reinfection was 89.2% three to six months after primary infection relative to SARS-CoV-2-naïve subjects, which declined to 82.5% in the nine to 12 months post-infection window. In the 5- to-11-year group, protection by prior infection against reinfection was similar across all 90-day intervals. In contrast, a rapid waning of protection was noted in individuals aged 12-18.
Further, the effectiveness of prior infection against the symptomatic disease was 93.6% three to six months after primary infection, which declined with time since infection. COVID-19 hospitalizations were fewer, with no deaths. In the secondary analysis, the effectiveness of previous COVID -19 to protect against reinfection and symptomatic disease was similar to the results obtained in the primary analysis.
The study estimated the dynamics of the immune responses induced by SARS-CoV-2 infection in the largest observational cohort constituting more than 293,000 individuals with 21 months of longitudinal data. The researchers observed that a prior COVID-19 infection conferred a robust protective effect against reinfection regardless of symptom status for at least 18 months in children and adolescents.
Notably, COVID-19-related fatalities were not recorded in convalescent or SARS-CoV-2-naïve groups. The primary analysis revealed that efficacy against reinfection was the highest (89.2%) three to six months post-primary infection with a marginal decline after a year but remained steady for up to 18 months.
Moreover, effectiveness against symptomatic COVID-19 reinfection was similarly high (> 93%). Interestingly, children in the younger age group (5 – 11 years) did not exhibit any significant waning of immune responses, therefore those aged 11-18 years showed These findings have important implications regarding the commencement and timing of vaccination of the younger population.
* Important notice
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice / health-related behavior, or treated as established information.