The contribution of hospital-acquired infections to the COVID-19 epidemic in England in the first half of 2020
Authors: Knight Gwenan M. , Pham Thi Mui , Stimson James , Funk Sebastian , Jafari Yalda , Pople Diane , Evans Stephanie , Yin Mo , Brown Colin S. , Bhattacharya Alex , Hope Russell , Semple Malcolm G. ,   ISARIC4C Investigators,   CMMID COVID-19 Working Group, Abbott Sam , Gimma Amy , Gibbs Hamish P. , Abbas Kaja , Barnard Rosanna C. , Sandmann Frank G. , Bosse Nikos I. , Mee Paul , McCarthy Ciara V. , Quaife Matthew , Kucharski Adam J. , Jarvis Christopher I. , Hellewell Joel , Finch Emilie , Rosello Alicia , Jit Mark , Pung Rachael , Eggo Rosalind M. , Endo Akira , Medley Graham , Tully Damien C. , Wong Kerry L. M. , Liu Yang , Sherratt Katharine , Munday James D. , Chapman Lloyd A. C. , Hué Stéphane , O’Reilly Kathleen , Davies Nicholas G. , Meakin Sophie R. , Sun Fiona Yueqian , Brady Oliver , Villabona-Arenas C. Julian , Atkins Katherine E. , Prem Kiesha , Hodgson David , Koltai Mihaly , Pearson Carl A. B. , Waites William , Procter Simon R. , Lowe Rachel , Read Jonathan M. , Cooper Ben S. , Robotham Julie V. 
Published: June 2022

Abstract
Background
SARS-CoV-2 is known to transmit in hospital settings, but the contribution of infections acquired in hospitals to the epidemic at a national scale is unknown.

Methods
We used comprehensive national English datasets to determine the number of COVID-19 patients with identified hospital-acquired infections (with symptom onset > 7 days after admission and before discharge) in acute English hospitals up to August 2020. As patients may leave the hospital prior to detection of infection or have rapid symptom onset, we combined measures of the length of stay and the incubation period distribution to estimate how many hospital-acquired infections may have been missed. We used simulations to estimate the total number (identified and unidentified) of symptomatic hospital-acquired infections, as well as infections due to onward community transmission from missed hospital-acquired infections, to 31st July 2020.

Results
In our dataset of hospitalised COVID-19 patients in acute English hospitals with a recorded symptom onset date (n = 65,028), 7% were classified as hospital-acquired. We estimated that only 30% (range across weeks and 200 simulations: 20–41%) of symptomatic hospital-acquired infections would be identified, with up to 15% (mean, 95% range over 200 simulations: 14.1–15.8%) of cases currently classified as community-acquired COVID-19 potentially linked to hospital transmission. We estimated that 26,600 (25,900 to 27,700) individuals acquired a symptomatic SARS-CoV-2 infection in an acute Trust in England before 31st July 2020, resulting in 15,900 (15,200–16,400) or 20.1% (19.2–20.7%) of all identified hospitalised COVID-19 cases.

Conclusions
Transmission of SARS-CoV-2 to hospitalised patients likely caused approximately a fifth of identified cases of hospitalised COVID-19 in the “first wave” in England, but less than 1% of all infections in England. Using time to symptom onset from admission for inpatients as a detection method likely misses a substantial proportion (> 60%) of hospital-acquired infections.