DHM COVID-19 Clinical Dispatch
Bite-sized, weekly clinical updates
Issue #10: SARS-CoV2 Transmission
Welcome to our Tenth edition of the dispatch! This week we talk transmission, getting into the nitty gritty of the spread of COVID-19 and why, in the case of respiratory droplets, bigger is better.
Our Big Takeaways from the week? SARS-CoV2, like other viruses, is transmitted primarily by large respiratory droplets. There is evidence for detectable virus in aerosols and significant debate about whether this is an important mode of transmission, which has significant implications for the use of PPE in healthcare settings. We (as healthcare workers) are at higher risk of infection but vigilant PPE use appears to be extremely effective in prevent HCW transmission.
Finally, contrary to what some former baseball players would tell you... wearing masks in public reduces the spread of disease, and you’re probably better off outside!
Special thanks this week to contributor Cindy Fenton!
- COVID Clinical Working Group
Spaced Learning Corner: Quiz Yourself!
In the COVID-19 Mission Screening Study reviewed last week, the majority of people who tested positive for COVID-19 were:
1) Unable to work from home
2) Earning less than $50K/year
3) Latinx
4) Asymptomatic
5) All of the above
Size Matters - Methods of Transmission
How is SARS-CoV2 transmitted?
Let’s start with the basics. Most communicable respiratory infections are thought to primarily transmit via large droplets (>5 um) from the upper respiratory tract (figure below, Wei et al. 2020) either through direct inoculation or indirectly through contact with surfaces (fomites). Small droplets (aerosols, < 5µm) from the lower respiratory tract that travel in the air much farther are the source of airborne transmission (Scary!).
Like other viruses SARS-CoV2 is likely also transmitted on surfaces through indirect contact, and extensive contamination of surfaces has been documented. Additionally, an experimental study detected the virus on some surfaces for 72h and in the air for 3h but under highly artificial conditions. (Note: virus is also detected in stool, but the clinical significance of this as a method of transmission is unclear)
What do we know from other viruses?
Most respiratory viruses (influenza, RSV, SARS) are thought to transmit predominantly through large droplets and fomites. Some opportunistic airborne transmission is likely but this is controversial. A NEJM study on the 2003 outbreak of SARS at the Amoy Gardens housing complex did find evidence of airborne transmission but noted it may have been related to specific plumbing issues at the facility.
What’s the evidence for airborne transmission in
SARS-CoV2?
At the moment there is no definitive evidence that SARS-CoV2 is transmitted significantly via small aerosoles. In hospital settings SARS-CoV2 has been detected on surfaces and air vents far from patients, suggesting the virus can travel in small aerosols. However, evidence of infective virus over these distances is lacking.
The strongest evidence for the possibility of airborne transmission comes from several outbreak investigations with very high attack rates and infections over a longer distance, suggesting that some airborne transmission occurs, particularly in enclosed spaces. (Avoid choir practice!)
On the Job - Transmission in the Workplace
Rigorous studies quantifying the risk to HCWs of hospital-acquired COVID-19 are limited, but there are numerous lines of evidence supporting excess risk. It’s widely acknowledged that eliminating hospital-acquired infections is key to managing the pandemic:
- HCW infections: Wuhan: 29% of hospitalized patients were HCW and 41% of all hospitalized patients were hospital acquired. In Spain, 20% of cases of COVID-19 were in HCWs and in Italy 9%.
- Clusters found in hospitals: e.g. Portland, Hayward, South Africa
- Other viruses: Hospital transmission a major factor in SARS/MERS
- Epidemiologic studies: HCWs in Wuhan in January had a 6X increased risk vs. local prevalence. More recent studies estimate that HCWs have a 3-fold increased risk of COVID-19 (after adjusting for the increased testing in HCWs). Reduction in risk versus the early days in Wuhan is likely explained by universal masking, testing all patients admitted to the hospital, and improved PPE practices.
But what about... the numerous case reports of N= single digit patients, exposing numerous HCWs often without proper PPE? And we've spent 4,000 hospitalist-days taking care of 50+ COVID with no hospitalist infections??
Let's talk Overdispersion, Superspreaders, Prevalence and Scale:
The R0 for SARS-CoV-2 is estimated as 2-3, meaning on average, without social distancing and masking, an infected patient infects 2-3 others. But the R0 for an individual person ranges from 0-50+ (overdispersion of R0).
Recent studies estimate that 80% of transmission originates from a minority of cases (1-10%, 10%, 20%) and in Hong Kong, 70% of people infected did not infect anyone else. These minority of
patients are superspreaders - who readily infect 6 or more other people. The higher the prevalence of COVID-19 in the community (and among patients and HCWs in the hospital) the greater the risk of running into the rare extra-super-spreader!
Thanks to our local leaders and some luck, the prevalence of COVID-19 in SF has been low, around 0.3%. An excess risk of 3.4 X means we would expect 1% of DHM to have had COVID-19 during this time. If in the fall or winter we experience a small surge with 1-5% community prevalence, using current PPE approaches, the evidence to date would predict that 3-15% of us working in the hospital could acquire COVID-19.
On an inspirational note, Gov. Cuomo reported that HCWs in NYC have a lower prevalence of antibodies for COVID-19 than the general population (unadjusted). A peer-reviewed study from Wuhan (after their initial failures with HCW infections) found that with careful PPE approaches (not too far off from our approach) and environmental controls, they were able to get to zero infections in HCWs (confirmed by antibody testing).
Quick Lit - One-Page Literature Review
Do interventions like distancing and face masks lower your risk for contracting COVID? A new meta-analysis looks at that question.
Opt Outside - Transmission in the Community
What is the optimal distance for physical distancing?
The best available evidence suggests that a distance of at least 3 feet is associated with a reduction in infection transmission and that 6 feet might be even more effective.
Are the cloth masks used in the community helpful in reducing transmission?
Masks are helpful in preventing the outward spread of droplet and aerosol viral particles that typically occur with speaking, coughing, sneezing and eating. Surgical masks appear to be superior to cloth masks in terms of filtration efficiency, however most cloth masks are still useful in reducing viral transmission. There is no clear data about the best type of material to use in non-medical masks but we do know that the more cloth layers, the higher the filtration efficiency.
Is the risk of COVID transmission the same indoors and outdoors?
Transmission is dependent on viral particle dose and exposure time. Some suggest that less than 15 minutes spent with an exposed person makes transmission less likely. Preliminary studies suggest that transmission rates for SARS-CoV2 are higher indoors due to poor air flow and less consistent physical distancing.
So what about running outside...
Physical activity is vital during these uncertain times (and maybe even boosts our immune system?). It is thought that runners may release more viral particles due to increased and heavier breathing when running, however exposure time tends to be low in those exposed to runners.
What We’re Reading
Short summaries of articles our team finds interesting this week.
“Airborne Spread of SARS-CoV-2 and a Potential Role for Air Disinfection” (Nardell et al, JAMA, June 2020): This viewpoint piece discusses the likelihood and concern for airborne transmission of SARS-CoV-2 by normal breathing, and the potential use of air disinfection (either with a room air cleaner or upper-room germicidal ultraviolet fixtures) to help prevent asymptomatic airborne spread of the virus.
“The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission” (Stadnytskyi et al, PNAS, May 2020): This brief report discusses the results of a study which showed SARS-CoV-2 droplets generated by regular speech can be infectious and remain in the air for roughly 8 to 14 minutes and “confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments.”
“Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1” (van Doremalen et al., NJEM, April 2020): This Letter to the Editor assessed the stability of SARS-CoV-2 as aerosolized droplets and on surfaces as compared to SARS-CoV-1, and found similar stability with viable virus detected as aerosolized particles for up to 3 hours and on some surfaces for up to 72 hours.
“Visualizing Speech-Generated Oral Fluid Droplets with Laser Light Scattering” (Anfinrud et al., NJEM, May 2020): In this Letter to the Editor, the authors asked whether droplets generated during speech are implicated in the transmission of SARS-CoV-2, and performed a light-scattering experiment to visualize the trajectories of droplets generated by speech, including during the production of different sounds and at different volumes, and found that the number of droplets increases with volume and that the number of transmitted droplets is reduced with a cloth over the speaker’s face.
“Transmission of COVID-19 to Health Care Personnel During Exposures to a Hospitalized Patient” (Heinzerling et al, CDC MMWR, February 2020): This CDC report describes a single COVID-positive patient who was not tested and exposed to 121 HCWs in a hospital in Solano County. Risk of HCW exposure was stratified into high (n=14), medium (n=80), or low (n=27) risk. Of these exposed HCW, 3 (7%) subsequently tested positive - two with extensive, high-risk exposure (aerosolizing procedures, no PPE) and one with close contact (mask, gloves, no aerosolizing procedures).
“Risk of nosocomial transmission of coronavirus disease 2019” (Wong et al, J Hosp Infection, April 2020). This single-center, observational study from Hong Kong describes the contact tracing for a single, COVID-positive patient who was not properly isolated, remaining in an open hospital ward “cubicle” (shared by 10 patients) prior to diagnosis. Of 120 exposed individuals (71 HCW, 49 patients), 52 became symptomatic and were subsequently tested. Despite her active cough without a facemask, all tests returned negative (including those from patients in the same cohorted “cubicle”). The authors conclude that SARS-CoV-2 likely “is not spread by airborne route,” arguing for “vigilant basic infection control measures.”
Links
Questions, thoughts, insights? Share them on our Clinical Knowledge Portal!
Prior Dispatches:
Issue 1: GI symptoms in COVID
Issue 2: Predictors of Severe COVID Disease
Issue 3: Hypercoagulability in COVID-19
Issue 4: SARS-CoV-2 and the Inflammatory Response
Issue 5: Remdesivir for COVID-19
Issue 6: Convalescent Plasma
Issue 7: ARDS
Issue 8: Post-infectious Immunity
Issue 9: Racial Disparities in COVID-19
