The ECH consisted of three residential zones and the option for short term care with a total of 96 residents, mean age 84 years, 95% CI [81.8; 85.7], range (54-101), and 114 staff, mean age 46 years, 95% CI [43.3; 47.8], range (19-67), on 30 January 2021: 95 internal, 6 external care staff and 13 external therapists (Table 1). The staff is not described according to zones, since many staff members were active across zones. A first vaccination visit took place on 8 January and a second on 6 February, i.e. after/at possible exposure to the wild virus after introduction into the ECH.
Outbreak confirmation and staging
The morning of Wednesday 17 February, the day after notification, all residents and the present staff were tested with the rapid antigen test (Clinitest®, Siemens) under control of the management of the ECH. Twelve residents and one staff member were tested positive. In the afternoon a team of our CHD took swabs of all antigen-test positive persons and two residents with onset of symptoms in the meantime. Further exploration and interviews with the management, staff members and residents were carried out at the same time.
On Thursday 18 February morning, 10 of the 12 residents with positive antigen-tests, the one staff member and the two symptomatic residents were confirmed SARS-CoV2 positive by PCR. All antigen test positive persons had ct values below 26. This was proof that an outbreak was on-going and a comprehensive testing of all the 95 internal and the remaining 4 external (total 6) staff members whether on duty or not, was intended for the next day. The staff members not on duty during the entire period were excluded. External “staff members” such as therapeutic specialists were ordered to get a PCR test at their closest facility, if eligible due to exposure. Swabs of the residents for PCR testing were postponed for one day, since they were confined to the institution.
On Friday 19 February swabs from 87 available personnel were taken by our team. Additional 6 staff members were PCR-confirmed adding up to 8 total so far (Table 2).
On Saturday 20 February all not yet PCR-tested residents plus 7 further staff members not available at the previous day were PCR tested by the KV team. Ten additional residents particularly those with higher ct values were found positive, adding up to 22 (22.9%) positive residents and 8 (7.0%) staff. In zone 2 three residents had the highest ct values (31, 33 and 35, respectively) and therefore the most advanced infection. In all positive cases no variants of concern were diagnosed. Additional interviews based on these findings led to the primary case, a lady with a ct value of 31 who was in the local university hospital the entire day of 27 January, in the middle of an on-going outbreak there and most likely acquired the infection there. No better alternative source could be elucidated. She had no vaccination so far, showed no symptoms and had contact to her peers in the same zone, zone 2, the hot zone (Table 2). Given this new information, the set point for the case definition was reset to 30 January, the day of earliest transmission from the supposed primary case within the ECH. From zone 2 the infection was spread supposedly by junior staff and external staff to the other zones. An introduction of the infection by the external vaccination team on 6 February could be excluded by the testing and vaccination history.
Outbreak control measures
Immediately on Thursday 18 February, the morning when the outbreak was confirmed by the incoming PCR results, comprehensive measures were taken. The entire ECH was put under quarantine and visitors only allowed for moribund residents or a single visitor across the coming fortnight for psychological impaired residents. Visitors after 30 January and their families were put into quarantine, the so-called quarantine of contacts’ households (HhQ2°). For all but one family, quarantine could be lifted already on 21 February, the day of the PCR-results of the residents.
All PCR-test-negative staff was put under quarantine, but was allowed to work to keep the ECH functional under the premises that an antigen test is performed daily before starting duty on top of personal protective equipment (PPE). In the local jargon we call this “tunnel-quarantine”. Staff members could come to work by private transport or by the ECH shuttle, but were prohibited to use public transport. Their family members were put under quarantine, too (HhQ2°). When the swabs were taken on Friday 19 February by our own team, staff members could state on a questionnaire, whether they could separate themselves from their families at home from now onwards until the end of the quarantine period in case the PCR result was negative. If so, the quarantine of their family members was lifted. A junior staff member turned out positive and his entire family, who was under quarantine already by this measure since 18 February, turned out PCR-positive. So did one other co-worker and a family member. A spill over in the population was prevented to the best of our knowledge. The time interval from 18 February to 20 February protected by HhQ2° was of key relevance to prevent further spread from already incubating or shedding family members into the community given the long exposure history as off 30 January, i.e. three serial intervals.
On-going surveillance and maximum attack rate
In agreement with the management of the ECH the following measures were put in place for surveillance: daily antigen-testing of the three shifts before work and notification to the CHD Ploen, notification of symptom onset in residents and staff members, and notification of hospitalisations and deaths. One staff member was confirmed positive 3 days after the last swabbing by his own physician, and one resident showed symptoms and was confirmed positive, adding up to a total attack rate of 23 out of 96 residents (24.0%) and 9 out of 114 staff (7.9%). One of those vaccinated was admitted to hospital. Three residents died; two after two doses of vaccination and one, who refused vaccination in his final stage of cancer.
On 3 March another comprehensive PCR-test action was performed for residents and staff, again by the team of the KV and all PCRs were negative except one of a resident with an original ct value of 12, who now had a ct value of 27 and her isolation was extended for one further week.
Vaccination status and vaccine efficacy
With 84.4% in residents and 25.3 to 10.5 for permanent and external staff, respectively, vaccination coverage was very heterogeneous. This is also reflected in the attack rate per vaccination status (Table 2). It is important to note that all staff members but one with one vaccination only obtained their shot on 6 February and had no time to build up their immune response and can be considered unvaccinated. The two doses in residents and staff in fact only warrant a one dose situation since the second dose was unlikely effective at the time of exposure. A second dose given at the second visit could not yet guarantee full protection by a two dose regimen at the time of exposure in spite of a faster booster reaction by a second dose.