Bus Riding as Amplification Mechanism for SARS-CoV-2 Transmission, Germany, 20211

Abstract

To examine the risk associated with bus riding and identify transmission chains, we investigated a COVID-19 outbreak in Germany in 2021 that involved index case-patients among bus-riding students. We used routine surveillance data, performed laboratory analyses, interviewed case-patients, and conducted a cohort study. We identified 191 case-patients, 65 (34%) of whom were elementary schoolchildren. A phylogenetically unique strain and epidemiologic analyses provided a link between air travelers and cases among bus company staff, schoolchildren, other bus passengers, and their respective household members. The attack rate among bus-riding children at 1 school was ≈4 times higher than among children not taking a bus to that school. The outbreak exemplifies how an airborne agent may be transmitted effectively through (multiple) short (<20 minutes) public transport journeys and may rapidly affect many persons.

Keywords: 2019 novel coronavirus disease; COVID-19; Germany; SARS-CoV-2; bus; coronavirus disease; outbreak; respiratory infections; schools; severe acute respiratory syndrome coronavirus 2; travel; viruses; whole-genome sequencing; zoonoses.

Figure 1

Epidemic curves for 191 case-patients in COVID-19 outbreak, Hesse, Germany, 2021. Dates are for symptom onset or first positive test result, whichever was earlier. A) By probable transmission setting; B) by age group.

Figure 2

Plausible chains of transmission in COVID-19 outbreak in Hesse, Germany, 2021 (n = 191), Solid boxes indicate that whole-genome sequencing evidence of the outbreak strain was available for >1 case; dotted boxes indicate cases with no whole-genome sequencing evidence available.

Figure 4

The subtree of the global SARS-CoV-2 phylogenetic tree in Figure 3 that contains all outbreak patient sequences contains few nonoutbreak sequences, showing that the outbreak in Hesse, Germany, 2021, was effectively contained. Three international sequences are included (2 from the United States, 1 from Austria), as well as 8 sequences that are from outside of the defined outbreak time period and 16 from outside the geographic region but still within Germany.

Figure 3

Sequences from samples from case-patients in COVID-19 outbreak, Hesse, Germany, 2021 (red circles). Sequences cluster tightly together when placed in a global phylogenetic tree, with the exception of 1 outlier. The outlier sequence from 1 air traveler was removed from the outbreak case-patients on the basis of lack of sequence similarity compared with other outbreak sequences. The global phylogenetic tree includes all SARS-CoV-2 sequences GISAID (https://gisaid.org), GenBank, COVID-19 Genomics UK Consortium (https://www.cogconsortium.uk), and the China National Center for Bioinformation (https://www.cncb.ac.cn) databases as of July 16, 2023 (total ≈15 million sequences) and was downsampled to ≈2,000 sequences for easier visualization.