ABSTRACT
BackgroundsThis study analysed secondary attack rates (SARs), comparing alpha variants, delta variants and non-variants of concern (non-VOCs), using clinical and close-contact tracing data.MethodsWe analysed coronavirus disease 2019 (COVID-19) case data from a database and contact tracing data between July and October 2020 (Cohort 1) and April 2021 (Cohort 2) and between July and August 2021 (Cohort 3) in a city in Toyama prefecture, Japan. Real-time polymerase chain reaction (PCR) was used to detect the N501Y (alpha variant) and L452R (delta variant) mutations. We calculated the SARs considering close contact, index case and contact setting characteristics. Relative risks (RRs) of secondary attack were analysed using Poisson regression models.ResultsAmong 123 index cases and 530 close contacts in Cohort 1, 246 index cases and 988 close contacts in Cohort 2, and 304 index cases and 984 close contacts in Cohort 3, the SARs associated with alpha and delta variant index cases were 1.47 times and 1.89 times higher than those associated with non-VOC index cases. Delta variant index cases were associated with the highest SAR (29.2%) in the same household and a 2.40-fold (95% CI: 1.62-3.56) higher risk of transmission than non-VOC index cases. The age and symptoms of index cases were associated with the SAR.ConclusionsWe confirmed that VOC index cases were associated with increased transmissibility. Population longitudinal surveillance data linked with contact-tracing data provide valuable information for elucidation of the characteristics of newly emerging variants.