Transmission of COVID-19 among healthcare workers - an epidemiological study during the first phase of the pandemic in Sweden

Abstract

During the first phase of the COVID-19 pandemic in 2020, concerns were raised that healthcare workers (HCWs) were at high risk of infection. The aim of this study was to explore the transmission of COVID-19 among HCWs during a staff outbreak at an inpatient ward in Sweden 1 March to 31 May 2020. A mixed-methods approach was applied using several data sources. In total, 152 of 176 HCWs participated. The incidence of COVID-19 among HCWs was 33%. Among cases, 48 (96%) performed activities involving direct contact with COVID-19 patients. Contact tracing connected 78% of cases to interaction with another contagious co-worker. Only a few HCW cases reported contact with a confirmed COVID-19 case at home (n = 6; 12%) or in the community (n = 3; 6%). Multiple logistic regression identified direct care of COVID-19 patients and positive COVID-19 family contact as risk factors for infection (adjusted OR 8.4 and 9.0 respectively). Main interventions to stop the outbreak were physical distancing between HCWs, reinforcement of personal hygiene routines and rigorous surface cleaning. The personal protective equipment used in contact with patients was not changed in response to the outbreak. We highlight HCW-to-HCW transmission of COVID-19 in a hospital environment and the importance of preventing droplet and contact transmission between co-workers.

Fig. 1.

Epidemic curve of the COVID-19 outbreak among HCWs at the infectious disease ward. Confirmed (black bars) and suspected cases (grey bars) of COVID-19 among HCWs at the ward. Total daily number of admitted patients with COVID-19 at the care unit is presented as an orange line, total bed occupancy is presented as a black line. Green arrows present time points for implementation of selected IPC measures: (A) increased frequency of disinfection of inanimate objects at the ward and reinforcement of personal hygiene practices; (B) ‘Friday tea’ cancelled from this day; (C) daily staff meetings took place in two groups instead of one from this day; (D) shared face shields were changed twice daily from this date; (E) increased frequencies of cleaning of shared areas such as lunch room and staff toilets. HCWs, health care workers; IPC, infection prevention and control.

Fig. 2.

Duration of symptoms for confirmed and suspected COVID-19 cases among HCW. Confirmed cases (C1–C36) are presented with blue bars and suspected cases (S3–S12) with yellow bars. The date marked in orange represents the date when tested positive for SARS-CoV-2 PCR. The letter T represents the date of a PCR test; while LD denotes the last date worked at the infectious disease ward; and T + LD shows that the persons worked on the same day they tested positive. The figure presents data for all HCWs for which we have data on duration of symptoms (47 of 50). Two suspected cases did not experience symptoms of COVID-19 whilst one suspected case had missing data; thus they were excluded in this illustration. The x-axis presents dates and the vertical lines between some dates indicate dates excluded for a more compact layout.

Fig. 3.

COVID-19 transmission tree based on contact tracing. Arrows pointing in the direction of assumed transmission according to the date of onset of symptoms. Confirmed COVID-19 cases are presented as C1–C36 (blue bubbles). The numbers 1–36 indicate the order in which the cases tested positive for SARS-CoV-2. Suspected COVID-19 cases are presented as S1–S14 (yellow bubbles). The numbers 1–14 indicate the order in which these suspected cases developed symptoms of COVID-19 or were identified as suspect cases based on contact tracing. The label ‘pc C1’ refers to a positive family contact of participant C1; ‘opcC6’ refers to a positive contact of C6 outside home and work. Only contacts with a confirmed infection based on PCR testing were included. Solid arrows represent confirmed close contact with a known case during the period the case was considered infectious. Broken arrows present possible contact between the cases – they were in the same place at the same day but we cannot establish that they had close physical contact with each other. Bi-directional arrows indicate that we cannot accurately establish who was infected first. Time is presented as calendar weeks for the year 2020; week 12 beginning at 16 of March and week 24 ending at 14 of May. C, confirmed case; S, suspected case; pc, positive family contact; opc, positive contact outside home and work.

Fig. 4.

Reasons given by HCWs on why they thought there was an outbreak of COVID-19 among HCWs at the infectious disease ward. The graph summaries the answers in categories; more than one reason to the outbreak could be given. Altogether 131 of 152 HCWs answered to this open-ended question in the questionnaire.