in all three outbreaks, we documented case-clustering within approximately a week’s time, which is compatible with a one-or two-day exposure for the entire kindergarten. Remarkably, transmission occurred in similar proportions to children and adults alike. It occurred to eleven out of twelve groups, including the rather separated daycare groups. We have gained a good overview of attitudes and practices of educators and the adherence to hygiene and infection control measures in the kindergartens. For example, it enabled us to group the two kindergarten groups of kindergarten 3 together as “close contact” because they took their meals together. Furthermore, having visited the kindergartens one-by-one we observed that room sizesand ventilation differed significantly and could have contributed as a possible transmission factor. Kindergarten 1 had the smallest volume per person, kindergarten 2 had a ventilation system and two groups in kindergarten 3 were spread out over two floors. In addition, the third kindergarten was shut down entirely more quickly than the other kindergartens which might explain its smaller outbreak size.With the exception of the first kindergarten, susceptibility of children, as illustrated by the SAR in the other kindergartens and the household outbreaks, was similar as in adults and substantially higher than described for the pre-VOC strains. Household SAR among children and adults in this study (32% and 39%, respectively) were substantially higher compared to non-VOC household SAR as assessed in a meta-analysis (17% and 28%, respectively)(3). Anecdotal evidence on B.1.1.7-outbreaks in United States daycare centers indicate that SAR may occur at surprisingly high rates (12, 13). Data from the United Kingdom on AR among B.1.1.7 “contacts” (not further defined) stratified in 10-year age groups indicate a similar relative SAR increase among most age groups (14).
Important questions of this investigation were, if children –after having been infected in kindergarten -lead to further infections in their household, and if so to what extent do children infect adults in their households. The above mentioned meta-analysis found a (non-significantly) lower SAR when children were the PC (7.9%) compared to SAR when adults were the household PC (15.2%;) (3).
In our study, we found corresponding SAR of 39% (95% CI: 28-52%) for child PCs and 33% for adult PCs (95% CI: 20-50%).
In addition, the data presented suggest that both susceptibility and infectiousness of children 1-5-years-old is substantially higher compared to the pre-VOC era, and may be converging to those among adults. To prevent individual daycare center outbreaks, or at least limit outbreak size, measures need to be revisited, including non-pharmaceutical measures, early closure should be considered when cases are occurring and vaccination of educators should be promoted.