Team and workspace
The team consisted of 13 individuals with a median age of 49 years (IQR, 44-55). The workplace is located in a 700m2 sized open-plan office, approximately one quarter of the office is occupied with the concerned team (Figure 1). A service area is located in the middle of the office for individual consumption. Restrooms are shared with other teams on the same floor. Each team member owns a personal workstation with desk and computer, and workstations are a minimum of 0.8 m apart. A 30m2 conference room and several smaller meeting rooms of 7m2 or 9m2 are available. The ventilation system provides air renewal within one hour in the open office space, and within 15 minutes in the conference room, respectively. There is no recirculation of air or strong directed airflow. Windows can be tilted, but not completely opened.
Description of COVID-19 Outbreak
During 13 interviews with a median duration of 37 minutes we obtained the following information: On March 10, employee E1 experienced symptoms of fatigue and minimal cough at noon and left the office early in the afternoon. The following day, he informed the company’s medical officer about his condition and was isolated at home. He was not tested as he did not fulfill the testing criteria for SARS-CoV-2 at that time. On March 13 – by then his respiratory symptoms had worsened and he reported rhinorrhea, sore throat, and fever – he was informed that his friend, with whom he had close contact on a party on March 7, had tested positive for SARS-CoV-2.
Two days after E1’s last presence in the office, in the late evening of March 12, his teammate E2 developed cough and did not attend work the next day. On March 13, six other team members (E3, E4, E5, E6, E7, and E8) started feeling unwell; E3, E5 and E8 stayed home, E6 developed prodromal symptoms in the afternoon while at work, and E4 and E7 developed first symptoms after the end of their workday. An eighth team member, E9, fell ill on March 14. RT-PCRs for SARS-CoV-2, performed between March 14 and March 16, resulted positive for all employees E1-E9. Due to the sequence of events, E1 was considered the index patient for this outbreak. Home isolation and quarantine measures were imposed on March 15 for all sick and yet healthy team members, respectively.
On March 18, eight days after onset of symptoms of E1, team member E10 fell ill, and was tested positive for SARS-CoV-2 the next day. On March 19, team member E11 came down with fever and cough. E11 had already been tested negative on March 15 while being asymptomatic, and no SARS-CoV-2 PCR was performed after symptom onset. However, a serology taken from E11 23 days after symptom onset resulted positive for SARS-CoV-2-specific IgA, IgG and IgM. Therefore, 11 of 13 team-members were considered confirmed COVID-19 cases. Two team members (E12 and E13) remained asymptomatic throughout the two weeks after last exposure and were tested seronegative 9 weeks after exposure. Symptoms of the team members are shown in Table 1.
Table 1: Clinical presentation of COVID-19 in affected team members
Team interactions and adherence to hygiene measure
Physical presence of the individual team members in the office is depicted in Figure 2. Every morning, the team performs a five-minute ‘huddle’ team meeting, standing close to each other between their workstations. Working together at the same workstation, including sharing of mouse and keyboard, is frequent in this team. For this purpose, an extra chair is available at every desk. All team members reported to have had high-risk contacts with several other team-members in front of the computer throughout the week, but contacts could only rarely be reconstructed in detail, except that E1 specifically reported to have had frequent and close contact with E8 and E9 on March 10, while being introduced into new processes. Lunch breaks are usually spent among team members in the canteen, however, E1 did not join his teammates on March 9 and 10. Shared coffee breaks are not common, but E5 reported to have eaten nuts and dates from the same bowl as E1 on March 9 and 10.
On March 10, a one-hour team meeting took place in the conference room (Figure 1) shortly before first symptoms developed in E1. During the meeting, E2 remembered sitting next to the index case. Noteworthy, the participants sang “Happy Birthday” to celebrate a team members birthday (not E1’s birthday), but no hugs or kisses were exchanged. E5 joined the meeting later and was not present during singing. In summary, E1 spent approximately 3 hours at work while having first prodromal symptoms.
On Friday 13, E4 and E10 had a meeting in the late afternoon, and E11 reported high-risk contact to almost all present team members. At this day, E6 had first prodromal symptoms for approximately 2 hours before leaving work. From March 16, all members of the team stayed home for isolation or quarantine.
During working hours, none of the team members was wearing a face mask or consistently maintained a physical distance (>2 meters) during personal interactions and team meetings. Six (46%) employees disinfected their hands on occasional or frequent basis, and adherence to respiratory etiquette was self-reported by 62% of all team-members, including the index case.
Except E1, no other sick employee reported private contact with a person having symptoms compatible with COVID-19 within the 2 weeks before symptom onset or before quarantine.
Viral Co-Infections and estimation of SARS-CoV-2 viral loads
The Ct value of E1’s SARS-CoV-2 PCR in a swab 4 days after symptom onset was 20.7, corresponding to approximately 10E8 virions/ml. Median Ct values of the other team members was 21.5 (IQR: 19.9-24), collected a mean of 1.3 days after symptom onset. To assess the presence of viral co-infections, the available nasopharyngeal swabs of nine team members were re-analyzed by multiplex PCR. Co-infection with adenovirus was found in the index patient E1 and in E3, genotyping of E1’s isolate revealed adenovirus E Serotype 4 (AdV-4). Sequencing of E3’s isolate was not performed.
Phylogenetic analysis
For eight of the eleven individuals, the full-length genome could be sequenced (for E6, E7 and E11 no material was available). Phylogenetic analysis showed that all sequences form a cluster within lineage 20A of SARS-CoV-2 (nextstrain nomenclature) (31). Six of the sequences are identical, while E10 and E2 are each one nucleotide different from the others (Additional file 1). Sequences of several other individuals from Switzerland not related to this outbreak, sampled before March 9, 2020, have identical sequences.
Incubation period and secondary attack rate
Assuming that the index patient E1 passed the SARS-CoV-2 to all ten team members (scenario 1), the secondary office attack rate caused by E1 was 83% (95%CI: 52-98). Assuming that E10 and E11 were infected by other team members (scenario 2), the secondary office attack rate caused by E1 was 67% (95%CI: 35-90). The mean incubation period was 4.7 (95%CI: 3.2-6.2) and 4.1 (95%CI: 3.5-4.7) days for scenario 1 and 2, respectively. The eleven employees with COVID-19 had 18 household contacts, of whom 11 were adults (Table 1). Secondary household attack rate of all eleven team members was 39% (95%CI: 17-64) overall, or 55% (95%CI: 23-83) in adult household members. In the two employees co-infected with adenovirus household attack rate was 1/4 (25%), while it was 6/14 (43%) in households of non-co-infected employees.