SARS-CoV-2 surface and air contamination in an acute healthcare setting in London

SARS-CoV-2 surface and air contamination in an acute healthcare setting in London.  Jefferson T, Heneghan C

Published on September 1, 2020

Reference Zhou J, Otter JA, Price JR et al. Investigating SARS-CoV-2 surface and air contamination in an acute healthcare setting during the peak of the COVID-19 pandemic in London Clinical Infectious Diseases, , ciaa905 Linked to COVID-19: INVESTIGATING SARS-COV-2 SURFACE AND AIR CONTAMINATION IN AN ACUTE HEALTHCARE SETTING IN LONDON 2020
Study type
Country UK
Setting Hospital
Funding Details NIHR Imperial Biomedical Research Centre, the NIHR Health Research Health Protection Research Unit (NIHR HPRU) in HCAI and AMR and the HPRU in Respiratory Infections at Imperial College.
Transmission mode Viral load, Droplets, Aerosol

Bottom Line

Despite apparent extensive air and surface contamination of  hospital environment, no infectious samples were grown.  A cutoff of RT-PCR of ct 30 or less is likely to be associated with infectious specimens.

Evidence Summary

91 of the 218 surface samples were suspect (41.7%) or 23 positive (10.6%) for SARS-CoV-2 RNA but no virus was cultured. The proportion of surface samples with suspected or positive RNA varied by item, including >80% of computer keyboards/mice, alcohol gel dispensers, and chairs, and >50% of toilet seats, sink taps, and patient bedrails.

For air samples, 2/31 (6.4%) were positive and 12/31 (39%) suspect for SARS-CoV-2 RNA but no virus was cultured (copy numbers reported in Table 1).

As no virus could be cultured the authors performed laboratory experiments to determine the limit of detection of SARS-CoV-2 dried onto surfaces. Four different dilutions of virus deposited onto two non-porous surfaces determined that dried inocula with a Ct value <30 (corresponding to an E gene copy number of ≥105 per mL) yielded SARS-CoV-2 that could be cultured (Table 2). In the study, all surface and air samples from the hospital environment had a Ct value >30.

What did they do?

The study is a cross sectional survey carried out in 7 areas (emergency department, an admissions ward, two COVID-19 cohort wards, theatres during tracheostomy procedures, an admissions ward, an intensive care unit, 6-bedded bay converted into a negative pressure area for management of continuous positive airway pressure (CPAP) and a public area of a 1200 bedded hospital in London during 2-20 April 2020. Each area was disinfected daily with additional twice daily disinfection of high touch surfaces using chlorine-based product. 3-5  air (with conical vials) and high touch area  and instrument (with swabs) samples were collected from each area and during 3 tracheostomy procedures.  

RT-PCR was performed with specific primers and probes targeting the envelope (E) gene.
Duplicate PCR was carried out and samples were considered positive if both duplicates had Ct< 40.4, or suspect if one of the two have Ct<40.4 (equivalent to one genome copy. 

For culture Vero E6 and Caco2 cells were used from air and environmental samples using a method adapted from one previously used to culture influenza virus. On day 0 and after 5-7 days, cell supernatants were collected, and RT-qPCR to detect SARS-CoV-2 performed as described above. Samples with at least one log increase in copy numbers for the E gene (reduced Ct values relative to the original samples) after 5-7 days propagation in cells compared with the starting value were considered positive by viral culture. 

A modified procedure was used to determine liability on surfaces after 7 days. Samples with an increase in copy numbers for the E gene compared to the original samples were considered positive by viral culture.

Study reliability

The paper is a very well reported study with clear designated ct cut offs. No description of the patients is given.

Clearly defined setting Demographic characteristics described Follow-up length was sufficient Transmission outcomes assessed Main biases are taken into consideration
Yes No Yes Yes Partly

About the authors

Carl Heneghan

Carl is Professor of EBM & Director of CEBM at the University of Oxford. He is also a GP and tweets @carlheneghan. He has an active interest in discovering the truth behind health research findings

Tom Jefferson

Tom Jefferson, epidemiologist.