With good reason, we are all a lot more interested than usual in the mechanisms of transmission of infectious diseases. I have written at length about aerosols and droplets. I have also discussed the very limited evidence that we have when it comes to aerosol generating procedures. However, almost all the available evidence was from influenza and SARS, which may or may not extrapolate well to the current COVID-19 pandemic. This post discusses a new paper looking at the distribution of SARS-CoV-2 in two units in a hospital in Wuhan, China.
Guo ZD, Wang ZY, Zhang SF, et al. Aerosol and Surface Distribution of Severe Acute Respiratory Syndrome Coronavirus 2 in Hospital Wards, Wuhan, China, 2020 [published online ahead of print, 2020 Apr 10]. Emerg Infect Dis. 2020;26(7):10.3201/eid2607.200885. doi:10.3201/eid2607.200885 PMID: 32275497 [article]
This is an observational study from Huoshenshan Hospital in Wuhan, China. They studied two wards – an ICU looking after 15 COVID patients and a general ward looking after 24 COVID patients – between February 19 and March 2, 2020. In those wards, they took swabs from various objects that could become contaminated and also took air samples at multiple locations.
In the designated clean areas, all swabs were negative. Three of eight (37.5%) floor swabs were positive in one dressing room, but all other swabs in their “semi-contaminated” or buffer areas were negative. Air samples in both of these areas were also all negative.
More samples were positive in the ICU (43.5%) than the general ward (7.9%), which could be explained either by sicker patients having high viral loads, or by aerosol generating procedures.
On the general ward, almost all the positive samples were found within the patient room. The single exception was a positive swab from a computer mouse or keyboard in the patient corridor. None of the PPE swabs were positive. The samples from the patient rooms that were positive had much lower viral concentrations than the samples from the ICU. The swabs from the air outlets were negative. None of the 5 air samples taken at 1 meter were positive, but 2 of the 11 (15%) taken at 2.5 meters were positive, and with higher viral concentrations than were found on the floor.
Contamination was found almost everywhere in the ICU. The floor is a high risk contamination zone. 70% of floor swabs were positive, as were 3 of 6 staff shoes that they sampled. Despite being an area that patients never entered, all 3 samples taken from the floor of the ICU medication room (pharmacy) were contaminated with high viral loads. Items frequently touched, such as computer mice, trash cans, and doorknobs had high rates of contamination.
In terms of airborne spread, 8 of 12 swabs from the air outlet filter were positive, indicating aerosol spread, and the viral concentrations on the air outlets were higher than in any other sample. 13 of 32 (41%) air samples taken at 1 meter from the patient were positive. Potentially more concerning, 1 of 8 air samples in the doctor’s office, which was more than 4 meters from the patient, was positive.
Somewhat miraculously, none of the staff members at this hospital were infected by SARS-CoV-2 as of March 30th.
This study provides us with some very valuable information. In large part, I am reassured. Good PPE and infection control practices are likely to prevent staff infection. Less sick patients, which represent the majority of patients we will see in the emergency department, are much lower risk for disease transmission that sicker ICU level patients. That being said, these results also emphasize the potential for aerosol spread of COVID-19, as the virus was found in the air 2.5 meters from the patient, even in less sick ward patients.
Conversly, sicker patients clearly produce a lot of virus, and it gets spread everywhere. It isn’t clear whether the increased contamination is the direct result of aerosol generating procedures, or whether sicker patients simply produce more droplets and aerosols containing more virus. If I had to guess, I would guess that latter, consider that even non aerosol generating procedures, such as placing a foley catheter or being in the room during an ECG, were associated with an increased risk of disease transmission during SARS. (Loeb 2004; Raboud 2010)
It is reassuring that none of the swabs taken in the clean areas of this hospital were positive, but the results will not extrapolate to most hospitals (or at least not to the hospitals that I have worked in). They had incredible separation between the clean areas and the contaminated areas of these wards, with multiple buffer rooms, and rooms dedicated to donning and doffing PPE. (See image.)
This separation is not present in any emergency department I have seen, so we should anticipate that there will be much greater contamination of the common “clean” areas where we work. In fact, I would just assume that there is no such thing as a clean area in any emergency department. We are currently very careful in patient rooms, but in the midst of busy shifts, it is easy to let your guard down. I have seen many things dropped on the floor and returned to counters, but the numbers in this study tell us that the floor is very likely to be contaminated. Also, I spend a lot of time cleaning my computer, but it isn’t easy when you are constantly forced to relocate to a new working space because there aren’t enough computer terminals to go around. These results also highlight the need for dedicated “clean areas” for breaks, where PPE and clinical tools (like pens and stethoscopes, if anyone still uses such as thing) are banned, and cleaning is frequent.
Implications for airborne/aerosol spread
This is some of the strongest data yet indicating a risk of airborne transmission of COVID-19. Air ducts had high concentrations of virus, and airborne samples up to 4 meters from the patient tested positive. Both of these units had excellent ventilation, with 16 air exchanges an hour in the ICU and 12 on the general ward, so the results might be worse in other settings. That being said, finding the virus by PCR doesn’t mean the virus is viable or capable of causing infection. Furthermore, this manuscript does not provide us with any details about what aerosol generating procedures might have been occurring in these settings. I assume procedures would be common in the ICU, but would have not been performed on the general ward.
As I discussed in the main post, I think that we have to assume that COVID-19 can be transmitted through airborne transmission. However, the risk of transmission is still much higher from larger droplets and direct contact with the patient. In an ideal world, I think everyone caring for a COVID-19 patient should be wearing an N95 mask, at a minimum, unless we are able to definitively prove surgical masks are non-inferior. However, that approach only works if we have an adequate supply of N95s to use for all COVID-19 encounters. Although airborne transmission of COVID-19 seems possible, it is still unlikely. For the average COVID patient, I am happy wearing a surgical mask (and a face shield). However, the sicker a patient is, the higher the risk of transmission. For any patient that looks like they are headed towards an ICU, I am wearing an N95, even if I am not in the midst of an “aerosol generating procedure”.
In this observational study from a single hospital in Wuhan, SARS-CoV-2 was found to contaminate many surfaces in patient care areas, especially in the ICU. More importantly, there is evidence of the potential for aerosol spread of the disease, with virus being detected in the air up to 4 meters away from the patient.
Guo ZD, Wang ZY, Zhang SF, et al. Aerosol and Surface Distribution of Severe Acute Respiratory Syndrome Coronavirus 2 in Hospital Wards, Wuhan, China, 2020 [published online ahead of print, 2020 Apr 10]. Emerg Infect Dis. 2020;26(7):10.3201/eid2607.200885. doi:10.3201/eid2607.200885 PMID: 32275497
Loeb M, McGeer A, Henry B, et al. SARS among critical care nurses, Toronto. Emerg Infect Dis. 2004;10(2):251–255. doi:10.3201/eid1002.030838 PMID: 15030692
Raboud J, Shigayeva A, McGeer A, et al. Risk factors for SARS transmission from patients requiring intubation: a multicentre investigation in Toronto, Canada. PLoS One. 2010;5(5):e10717. Published 2010 May 19. doi:10.1371/journal.pone.0010717 PMID: 20502660
Morgenstern, J. Aerosol and Surface Distribution of SARS-CoV-2 (COVID-19), First10EM, April 19, 2020. Available at: