Age-Varying Susceptibility to the Delta Variant (B.1.617.2) of SARS-CoV-2

Abstract

Background: The Delta variant tends to be more transmissible than previous strains of SARS-CoV-2 especially among children. However, so far, there are no reports confirming this theory. Coronavirus disease 2019 (COVID-19) cases among children might be higher because (i) school-aged children have higher contact rates and (ii) the current COVID-19 vaccination strategy prioritizes the elderly in most countries. This study assesses the age-varying susceptibility to the Delta and pre-Delta strains.Methods: Combining age-specific contact matrices and observed periods between each stage of infection, we constructed a compartmental model (SEIQ) of the COVID-19 transmission. The strength of this model is that we know the transition time distribution to the next infection stage (E → I [Latent period], I given the Symptom onset, Symptom onset → Q [Diagnostic delay]), based on the robust contact tracing study in South Korea. We used a Bayesian inference method to estimate the force of infection (S → E), the remaining variable in this model. We compared the age-specific susceptibility to the Delta with those to the original virus, using the 3rd (pre-Delta) and 4th (driven by the Delta variant) waves in South Korea. As vaccine uptake increased, we excluded those who were immunized from the susceptible population in accordance with the vaccine effectiveness against the Delta variant.Findings: A significant difference between the age-specific susceptibility to the Delta and that to the pre-Delta variant was found in the younger age group. The fold rise in susceptibility to the Delta/pre-Delta variant was highest in the 10–15 years age group (1·92-fold rise), whereas in those aged 50 years or more, the susceptibility to the Delta/pre-Delta remained stable at approximately one-fold.Interpretation: Even after adjusting for both contact pattern and vaccination status, the Delta variant of SARS-CoV-2 tends to propagate more easily among children than the pre-Delta strains.Funding Information: This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIT) (No. 2020R1A2C3A01003550) and (No. 2021R1F1A1064473). TDeclaration of Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Ethics Approval Statement: All the data used in this study was publicly available, and therefore were regarded exempt from institutional review board assessment.