Most COVID-19 cases in Danish K-9 schools in fall 2021 didn't trigger clusters, but when they did, they were large, suggests a study published today in Epidemiology & Infection.
A team led by Serum Institut researchers in Copenhagen conducted a register-based, observational study linking student-level data on COVID-19 test results and vaccination status of children aged 6 to 15 years with grade-level and school-specific information from August 9 to December 19, 2021.
The SARS-CoV-2 Delta variant was dominant throughout the study, public health restrictions were minimal, and school COVID-19 testing was in place. The researchers defined clusters as three or more cases in a grade within 14 days of a primary case.
42% of cases tied to clusters
By study end, 2.5% of children in grades 0 to 4 and 63.1% in grades 5 to 9 were vaccinated. Almost all children (94.4%) were tested at least once.
A total of 75,225 COVID-19 infections were detected among 75,168 children (12.1% of all children). Overall, 5.7% of children were infected before the study. One or more infections were detected in 96.2% of schools, and 76.5% of 1,300 schools reported at least one cluster.
Overall, 21,497 case introductions and 7,518 clusters consisting of 55,912 cases were identified. More cases and clusters were identified in lower (4,239) than in higher grades (3,281). Of 21,497 case introductions, 41.6% produced a cluster. Overall, 74.4% of cases were linked to clusters of 3 to 65 infections.
Overall, more cases and clusters had been seen in the lower class levels, where children were mainly unvaccinated.
A higher assumed within-grade immunity level due to vaccination or previous infection appeared to significantly lower the likelihood of a link between a case introduction and a cluster (eg, immunity of at least 80% vs less than 20%; odds ratio [OR], 0.28). A previous infection in the primary case protected against clusters (OR, 0.58).
"Overall, more cases and clusters had been seen in the lower class levels, where children were mainly unvaccinated, than in higher class levels," the study authors wrote. "It further indicates that vaccination of children markedly reduces the risk of secondary infections."