COVID-19 transmission in educational institutions August to December 2020 in Germany: a study of index cases and close contact cohorts
Authors: Schoeps Anja , Hoffmann Dietmar , Tamm Claudia , Vollmer Bianca , Haag Sabine , Kaffenberger Tina , Ferguson-Beiser Kimberly , Kohlhase-Griebel Berit , Basenach Silke , Missal Andrea , Höfling Katja , Michels Harald , Schall Anett , Kappes Holger , Vogt Manfred , Jahn Klaus , Bärnighausen Till , Zanger Philipp 
Published: February 2021

Background

The lack of precise estimates on transmission risk hampers rational decisions on closure of educational institutions during the COVID-19 pandemic.

Methods

Secondary attack rates (SARs) for schools and day-care centres were calculated using data from state-wide mandatory notification of SARS-CoV-2 index cases in educational institutions and information on routine contact tracing and PCR-testing.

Findings

From August to December 2020, every sixth of overall 784 independent index cases caused a transmission in educational institutions (risk 0·17, 95% CI 0·14–0·19). In a subgroup, monitoring of 14,594 institutional high-risk contacts (89% PCR-tested) of 441 index cases revealed 196 secondary cases (SAR 1·34%, 1·16–1·54). Transmission was more likely from teachers than from students/children (incidence risk ratio [IRR] 3·17, 1·79–5·59), and from index cases in day-care centres (IRR 3·23, 1·76–5·91) than from those in secondary schools. In 748 index cases, teachers caused four times more secondary cases than children (1·08 vs. 0·25 secondary cases per index, IRR 4·39, 2·67–7·21). This difference was mainly due to a large number of teacher-to-teacher transmissions in day-care centres (mean number of secondary cases 0.66) and a very low number of student/child-to-teacher transmissions in schools (mean number of secondary cases 0.004).

Interpretation

In educational institutions, the risk of infection for contacts to a confirmed COVID-19 case is one percent, but varies depending on type of institution and index case. Hygiene measures and vaccination targeting the day-care setting and teacher-to-teacher transmission are priorities in reducing the burden of infection and may promote educational justice during the pandemic.

Funding

No particular funding was received for this study.

Research in context

Evidence before this study

We searched PubMed on Jan 27, 2021, without any language restrictions for all articles in which the title or abstract contained the search terms “COVID 19” or “corona”, and “school”, “education*”, or “daycare”, and “transmission”, and “risk”, “attack rate”, or “SAR”, and screened 175 results for original research or reviews on COVID-19 transmission risk in the educational setting. Following a similar strategy, we also searched Google Scholar, SSRN, medRxiv, and the reference lists of identified literature. We found five cohort studies on transmission risk looking at overall 171 index cases and their 6,910 contact persons in Australian, Italian, Irish, Singaporean, and German schools and reporting attack rates between 0% and 3% percent. These five studies were conducted before October 2020 and thus looked at COVID-19 transmission risk in schools before the second wave in Europe.

A number of modelling studies from the first wave of COVID-19 provide inconclusive guidance to policy makers. While two publications, one from several countries and one from Switzerland, concluded that school closures contributed markedly to the reduction of SARS-CoV-2 transmission and individual mobility, two other studies, one using cross-country data and one from Japan rated school closures among the least effective measures to reduce COVID-19 incidence rates.

Added value of this study

Based on a large data set that emerged from the current public health practice in Germany, which incorporates routine PCR-testing during active follow-up of asymptomatic high-risk contacts to index cases, this study provides a precise estimate of the true underlying SARS-CoV-2 transmission risk in schools and day-care centres. Its analysis also allows for a meaningful examination of differences in the risk of transmission with respect to the characteristics of the index case. We found that the individual risk of acquiring SARS-CoV-2 among high-risk contacts in the educational setting is 1.3%, but that this risk rises to 3.2% when the index case is a teacher and to 2.5% when the index case occurs in a day-care centre. Furthermore, we could show that, on average, teacher index cases produced about four times as many secondary cases as student/child index cases. Despite the relatively small proportion of teachers among index cases (20%), our study of transmission pathways revealed that the majority of all secondary cases (54%), and the overwhelming majority of secondary cases in teachers (78%) were caused by teacher index cases. Of note, most cases of teacher-to-teacher transmission (85%) occurred in day-care centres.

Implications of all the available evidence

In this setting, where preventative measures are in place and COVID-19 incidence rates were rising sharply in the population, we found a low and stable transmission risk in educational institutions over time, which provides evidence for the effectiveness of current preventative measures to control the spread of COVID-19 in schools. The identification of a substantial teacher-to-teacher transmission risk in day-care, but a clearly mitigated child/student–to-teacher transmission risk in schools, indicates the need to shift the focus to hygiene among day-care teachers, including infection prevention during staff-meetings and in break rooms. These findings also strongly support the re-prioritization of vaccination against SARS-CoV-2 to educational staff in day-care.