The nursing home is located in the South-West of the Netherlands in a small town on the rural island of Goeree-Overflakkee (260 square km with 50,000 inhabitants). It is a skilled nursing home facility with 300 beds, giving long and short term residential care, divided over five buildings with two to four stories. There are 146 long-term residential care/assisted living apartments, 24 short-term residential care apartments (medical rehabilitation), and 17 residential groups of 7 – 8 residents each (15 psychogeriatric care groups and 2 somatic care groups). Most of the HCWs live on the same island.
On March 8, 39 persons attended a church service in the chapel of the nursing home: 26 were elderly non-resident community members including the reverend, and 13 residents of the nursing home. The service took approximately 50 minutes, in which people sang and shared supper by passing a serving bowl with pieces of bread. No hands were shaken. After the service, coffee was shared for about 20 minutes. The chapel had an air conditioning system, without recirculation. The few church members who did not develop any symptoms in the weeks after the service (5 of 39, of whom 2 residents) were seated at random in the chapel.
All 26 non-residents who visited the church service on March 8 responded to the online questionnaire, between March 27 and April 7. On the day of the church service, four non-resident visitors had symptoms fitting the case definition of COVID-19 and three reported a household member with symptoms (Table 1). Nobody reported contact with a confirmed COVID-19 case. In the 16 days following the church service, 19 of 26 (73%) non-resident church visitors developed symptoms indicative of COVID-19 (Table 1); 3 persons did not develop any symptoms, and 4 persons continued having symptoms. Seven persons reported a household member who also developed symptoms. Three church visitors with severe symptoms tested positive for SARS-CoV-2; two after admission to hospital/ICU and the third after admission of his spouse.
The epidemic curve of the outbreak in the nursing home and church visitors shows onset date of symptoms between March 1st and April 14th for 21 non-resident church visitors, 11 residents who did and 51 who did not attend the church service, as well as 30 HCWs (Figure 1). All residents and HCWs in the curve were RT-PCR-positive, and 21 residents died (of whom five had attended the church service). At the peak of the outbreak, 200 out of 300 residents were cared for in isolation or quarantine. The onset of symptoms ranged from 1 – 10 (median 4) days after March 8th in non-resident church visitors, and from 2 – 16 (median 10) days after March 8th in residents attending the church service (Figure 1). The reported symptom onset of other residents and HCWs who did not attend the church service ranged from 4 to 37 (median 16,5) days following the service. The outbreak started to decline after March 25th, about 8 – 12 days after first control measures were taken (see Table 2 for detailed timeline of infection prevention and control measures). Until April 14th (study period), 62 of 300 residents (21%) and 30 of 640 HCWs (5%) tested positive for COVID-19 (3 more residents and 12 more HCWs tested positive after the study period; 1 more resident died).
The outbreak started in the first and second floor of the nursing home, with most affected units housing at least one resident who attended the church service (Figure 2a). Residents on the ground floor (residential groups) were mostly infected later. Some residential groups were more affected than others (ranging from 0 – 7 of 8 residents affected). The air conditioning system did not recirculate used air.
We obtained complete genome sequences of 7 of 11 SARS-CoV-2 positive residents who attended the church service, 35 of 51 residents who did not attend, 20 of 30 HCWs and 21 inhabitants of the island (of whom one attended the church service). Sequences of viruses from residents and HCWs grouped in eight different clusters, and an additional eight residents and one HCW had unique sequences (Figure 3), indicating at least 17 separate introductions of SARS-CoV-2 into the nursing home. The seven resident church visitors were all infected with distinct viruses (≥3 nucleotides difference), making one common source of infection unlikely. However, it is possible that several people infected with different viruses visited the church service, which may have caused further transmission amongst visitors. Four church service visitors (three residents and one non-resident) were part of the large regional cluster A. Cluster A likely reflects widespread circulation in the region rather than direct transmission, in contrast to the other seven defined clusters which are likely transmission clusters.
Overall, the genetic clusters were scattered across the nursing home (figure 2b), with no clear pattern. Viruses belonging to several clusters were found on each floor. In some units there may have been local spread of one cluster type, such as cluster H in unit A0, cluster G in unit A1, and cluster B among residents of unit B2 and HCWs of unit B1 (both short term care units with some exchange of HCWs). The majority of the introductions seemed to have been controlled quickly, staying limited to 1 – 4 residents and/or HCWs (cluster C to H plus 9 unique sequences). Two clusters (A+B) grew bigger over time, resulting in 38/92 (41%) SARS-CoV-2 infections in the nursing home. All clusters started before March 28, which is 14 days after the start of testing and isolation of residents (on July 14), and 9 days after starting the total isolation of all units (on July 19). This shows the effectiveness of the measures in preventing new introductions into the nursing home, taking into account an incubation time of 14 days. However, these measures could not prevent all transmission within the nursing home.