SARS-CoV-2 PCR Positivity in Elementary School Classes in Bavaria in the School Year 2021/22

After the school closures in the academic years 2019–20 and 2020/21, all pupils at elementary schools in Bavaria were to have regular lessons in the academic year 2021/22, with optimal protection against SARS-CoV-2 infections. When in the autumn of 2021 a nationwide vaccination offer was open to 12–17 year olds, protecting younger pupils became the priority. In Bavaria, pool tests were used in all elementary schools from September 2021 to April 2022, which were intended to promote adherence to framework hygiene plans (1) and keep schools open. This short report presents initial results regarding PCR positive rates at Bavarian elementary schools from the accompanying evaluation of the projects.

During the COVID-19 pandemic, pool tests proved a resource saving option for the timely confirmation of SARS-CoV-2 infections in groups with a low incidence (2). In this approach, specimens from several persons are analyzed together (pooled). Members of negative pools are considered negative. Positive pools were separated and their members underwent PCR testing individually. North Rhine Westphalia was the first federal state to introduce pool tests in all elementary and special schools in May 2021 (3) and showed their effectiveness in assessing the infection process (3, 4).

Method

Our evaluations relate to the pool tests carried out from 20 September 2021 to 10 April 2022 (start of the Easter break). The presented analysis investigated 26 030 classes at 2794 schools, with 476 478 children in grades 1–4. In each class, tests were undertaken twice per week, with two specimens per child: one for the pool test and one for individual testing in case the pools had to be dispersed. Information on the process and on data protection are on the homepage of the Institute for Medical Information Processing, Biometry, and Epidemiology (IBE) (5). From 29 November 2021, the children additionally tested themselves, under supervision but undocumented, by taking rapid antigen tests every Monday morning. Those who tested positive did not participate in the pool test and were sent home immediately. We divided the analysis into three time periods: from 20 September 2021 to 26 November 2021 (Delta wave), from 29 November 2021 to 23 December 2021 (Delta + self test), and from 9 January 2022 to 10 April 2022 (Omicron wave with self test) and report key indicators for the three periods. Within a calendar week, a class was considered PCR positive if at least one individual PCR test was positive. For a time period of interest, the positivity rate for pool tests per school was calculated from the proportion of pupils who tested positive among all those tested. Similarly, the individual positivity rate is calculated from the proportion of students who tested positive among all those tested. The data center of the project (www.novid20.at) submitted data for the purpose of evaluation to the IBE every day. The statistics software package R (version 4.1.0, www.r-project.org) was used for the analyses. We did not use any statistical inference methods as the survey was a complete survey. The evaluation was approved by the ethics committee of the medical faculty of the of the Ludwig Maximilian University of Munich (LMU) (application 21–0957).

Results

A total of 1,880,458 tests were carried out. Of these, 1 156 146 were pool tests (of which 46 436 were positive) and 724 312 were individual tests (of which 48 976 were positive). The average class size (school size) was 18.3 children (170.54 children). The table shows relevant key indicators regarding PCR positivity across all three project phases under study. The Figure shows the comparison of 7-day incidence rates (elementary school pupils, child population, Bavaria).

Figure.

Information on the infection process per calendar week between 20 September 2021 and 10 April 2022

7-day incidence per 100 000 pupils (blue), in the Bavarian child population (6–12 years, red) and in the Bavarian total population (green).

Holiday periods: Autumn break 2021 (beige), Christmas break 2021/2022 (green), and carnival break 2022 (purple).

Discussion

During the pool test project, no elementary or special school in Bavaria was closed because of coronavirus. Even if the class positivity rate was up to 14% (week 5/22, data not shown), more than two simultaneous cases of infection per class were rare, even in times of high incidence rates. The class positivity rates were comparable for all four grades (data not shown). If in the following week tests were lacking for a class, quarantine was assumed. This was a rare event over the time period (table). The positive rate of the pool tests barely changed after rapid antigen tests had been introduced on Mondays (in spite of simultaneously rising incidence rates [Figure]). Only the occurrence of the Omicron variant led to a massive increase in the rate. This was also the case for the positive rate of the individual tests (table). Incidence rates of infection in the elementary schools (figure) were by an order of multitude lower than the simultaneously notified incidence rates according to age and generally in the total population (figure). The data of the evaluation do not allow any conclusion about which mechanisms keep infected children away from lessons. The—compared with the background incidence rates according to age—low positive rates in school PCR tests might be showing that infected children mostly stayed at home and that suitable hygiene concepts prevented infections in schools. Furthermore they might confirm that the test strategies detect infections rapidly and in a targeted manner and therefore interrupt chains of infection. Even a cost effective monitoring mechanism for the infection process is an important support for teaching operations. Our results imply that the combination of testing and hygiene measures largely ensured face-to-face teaching for elementary schools in Bavaria. The testing concept also enabled for children to be included in an active social environment, as the school tests yielded the proof of testing that many leisure activities required.

Table. Key indicators of PCR positivity at elementary schools in Bavaria.

	Delta wave without self test 20 September 2021 to 26 November 2021	Delta wave with self test 29 November 2021 to 23 December 2021	Omicron wave with self test 9 January 2022 to 10 April 2022
Number of weeks	10	4	13
Class weeks under observation*1	203 088	101 188	331 994
Number of outbreaks in class*2 Incidence per 1 000 classes per week	132 0.65	53 0.52	1 328 4.0
Number of individuals testing positive	6 656	3 196	39 068
Class weeks not documented*1 Incidence per 1000 classes per week	473 2.33	562 5.55	321 0.97
School weeks not documented*3	28	28	28
Positivity rate for individual tests*4 per 10 000 pupils - Total - Schools with max. 5 classes - Schools with 6–8 classes - Schools with 9–12 classes - Schools with more than 12 classes	9.97 9.85 9.50 8.90 10.91	9.80 10.71 8.95 9.94 9.89	45.96 41.95 46.15 44.96 47.52
Positivity rate for pool tests*5 (%) – Total – Schools with max. 5 classes – Schools with 6–8 classes – Schools with 9–12 classes – Schools with more than 12 classes	1.41 1.33 1.37 1.32 1.63	1.44 1.52 1.35 1.44 1.45	5.25 4.84 5.29 5.30 5.63

*¹ Class weeks: the sum of the weeks of all classes that participated in the project during a period.

*² Class outbreak: more than 2 infected pupils per week in one class

*³ Weeks during which no testing was documented for an entire school

*⁴ For a time period of interest, the individual positivity rate was calculated from the proportion of pupils testing positive among all tested persons.

*⁵ For a time period of interest the positivity rate for pool tests per school was calculated from the proportion of positive pools among all pools under study.