The outbreak occurred in an Internal Medicine ward of a 350-bed teaching hospital serving a population of about 200,000 inhabitants. The ward can hold 28 patients in 13 double rooms and two single rooms and is located on the seventh floor of an eleven-storey building that was opened in 1989 (Fig. 1).

Figure 1.

Hospitalization ward floor plan where the SARS-CoV-2 infection outbreak occurred.

PR: patient room; EE: emergency exit; NA: nursing area; MA: medical area; C: corridor; MR: meeting room; LO: lobby; RE: room for explorations; B: bathroom; S: secretariat; L: lift. V: room with faulty ventilation; Black dot: rooms where bronchodilator therapy by nebulisation was carried out.

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There is a shared air conditioner located in the hospital terrace for both the rooms and working areas, and this supplies primary air to the inductors located in the entrance area of each room. This inductor mixes the primary air under pressure with the room air. The air supply for the corridor and ward hallway is provided by another air conditioner. Extraction of air from the rooms occurs in the toilet of the rooms through a vertical box that ends on the terrace where the extractor fan is located.

When the outbreak took place, the incidence of SARS-CoV-2 infection in the previous 14 days was about 200 cases per 100,000 inhabitants in the area served by the hospital. Therefore, some measures had already been taken at the hospital before the outbreak occurred. Family visits had mostly been banned and limited to end-of-life situations or patients requiring special care.

Universal polymerase chain reaction (PCR) testing of patients prior to hospital admission was implemented. Protection of HCWs had also been stepped up with the use of FFP2 respirators and distancing measures had been adopted in shared workspaces and dining areas.

All SARS-CoV-2 infected patients were admitted to specific wards with enhanced personal protection measures. All HCWs received additional training on the use of personal protective equipment through videos and face-to-face sessions. The frequency of cleaning and disinfection of surfaces was increased.

Persons under investigation (PUI) were defined as patients hospitalized on the ward between 30 November and 6 December 2020 and HCWs working the ward during this same period. Permanent HCWs were defined as those who had worked at least one full working day on the ward, and occasional HCWs were defined as those who had worked less than a full working day on the ward where the outbreak occurred. A confirmed case was defined as a PUI with a positive PCR test for SARS-CoV-2 regardless of symptomatic status.

Subjects with a history of confirmed SARS-CoV-2 infection in the last three months were excluded from the study. SARS-CoV-2 reverse transcription PCR (RT-PCR) was performed on mucus obtained from nasopharyngeal swabs by a commercial kit (Allplex™ 2019-nCoV Assay, Seegene, Republic of Korea) according to the manufacturer’s instructions.

Descriptive analysis was performed using attack rates expressed as the percent of individuals with positive PCR among the persons at risk. Attack rates were compared using the chi-squared test or Fisher’s exact test as appropriate. A value of p < 0.05 was considered to indicate significance. The SPSS software package (version 15.0) was used for the statistical analysis.

The local ethics committee (reference number 02-21-101-045) approved this study. Both patients and HCWs gave verbal informed consent for PCR samples to be obtained. Written informed consent for participation was waived because all the interventions and collected data were included in the hospital infection control protocols for care purposes.

No identifying information about the participating patients or HCWs has been included. The study complied with the principles of the 1964 Helsinki Declaration and its later amendments.

On 6 December 2020, a PCR for SARS-CoV-2 was performed on a patient who was admitted to the ward on 2 December due to the presence of respiratory superinfection. The result was positive. The antigen test and PCR for SARS-CoV-2 performed on admission had been negative in this patient, as well as in all patients admitted to the ward. The patient was being treated with bronchodilators by nebulisation. During the same period another patient was receiving nebulisation in another room (Fig. 1). Three other patients presented with low-grade fever on the same day.

According to the hospital infection control protocols, all patients admitted to the ward were screened with PCR on 6 December and 13 patients tested positive. A second PCR performed 24 h later on all patients on the ward confirmed positivity in these 13 patients and was positive in 6 more patients. PCR was also performed on four patients discharged from the ward over the previous five days and was positive in two of them. In the following days, PCR screening was carried out on all HCWs who had worked the ward during the previous week at any time. Overall, 21 patients and 44 HCWs tested positive. None of the HCWs reported having recent community contact with a person with COVID-19.

The screening results and attack rates are shown in Table 1. The attack rate in patients was 80.8% and only five tested negative. Among HCWs, the attack rate was significantly higher in those who had worked at least one full working day on the ward (56.3%; 95% CI 44.1–68.4) than in those who had occasionally been on the ward (25.8%; 95% CI 10.4–41.2; p = 0.005). On the other hand, from December 4 to 13, only five (0.2%) of the approximately 2300 HCWs who worked in other areas of the hospital were diagnosed with COVID-19.

Overall, 17 cases (9 patients and 8 HCWs) were asymptomatic, and 46 cases (11 patients and 35 HCWs) had a low-grade fever; some of them also had headache and/or cough. One patient and one HCW developed severe pneumonia requiring admission to the intensive care unit. Both cases eventually recovered. Fig. 2 shows the epi curve of cases by PCR confirmation date and by date of onset in symptomatic cases. In all symptomatic cases, symptoms appeared between December 4 and 13.

Figure 2.

Epi curve of confirmed cases of SARS-CoV-2 infection by date of PCR confirmation (A), and symptomatic cases by date of onset (B), for COVID-19 outbreak in a medical ward.

HCW: healthcare workers.

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The Physical Resources Department reported that on 3 December (three days before the first positive case was detected) two extractor fans were found to be defective, affecting the ventilation of three rooms, and they had been replaced. Therefore, for a few days (the precise moment when they stopped working is unknown) the three affected rooms lacked an adequate number of air changes. In two of these rooms, patients were receiving bronchodilator therapy by nebulisation (Fig. 1).

Samples from one of the outbreak patients were sent to a reference centre (Hospital Universitari Germans Trias i Pujol, Badalona, Spain) for molecular study of SARS-CoV-2. This analysis identified variant B.1.177/20A.EU1, ruling out the possibility of variant B.1.1.7, with greater intrinsic transmission capacity, which at that time was starting to circulate in Spain.

Many actions were taken. The ward where the outbreak occurred was closed, the infected patients were transferred to a COVID ward and the three uninfected patients were quarantined. All affected HCWs were placed on sick leave and their contacts quarantined. The PCR screening protocol was modified by screening the HCWs throughout the hospital every two weeks. In addition, a second screening of all patients at five days post-admission was established. Personal protection was enhanced to perform aerosol-generating procedures. A nebulisation system, with vibrating mesh devices which keep the masks airtight, was implemented, and extra protection with surgical masks over the nebuliser masks was added.

Regarding ventilation, a horizontal air extraction system was installed in the window area of the rooms in order to pull all the air in the room from the door to the window, and the air extractors located on the terrace were replaced by more powerful ones.