Learn about the latest in COVID-19: Do masks prevent infection?, SARS-CoV-2 home testing, vaccine advice, and how to prevent and treat VTE in COVID-19. Join us as our tzar-of-clinical-appraisal Curbsider/internist/epidemiologist Rahul Ganatra MD MPH (@rbganatra) talks us through the controversial DANMASK-19 trial, plus Watto reports back on his conversation with Dr Parth Rali @parthrali (Temple Lung Center) who helped write the CHEST anticoagulation guidelines for COVID-19.
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Bundgard H, Bundgaard JS, et al: Effectiveness of Adding a Mask Recommendation to Other Public Health Measures to Prevent SARS-CoV-2 Infection in Danish Mask Wearers. Annals of Internal Medicine 2020.
Question: The DANMASK-19 Trial aimed to assess whether wearing a mask confers the mask wearer a 50% reduction in SARS-CoV-2 infection.
Primary Outcome: SARS-CoV-2 infection measured by antibody positivity, PCR, or hospital diagnosis.
Duration of Followup: One month.
Comparison: This randomized controlled trial compared encouragement to social distance versus encouragement to social distance and wear masks, along with a supply of 50 surgical masks.
Study Population: Participants were adults spending greater than 3 hours outside the home in Denmark during April and May 2020.
Results: Out of the 4862 participants who completed the trial, 95 total SARS-CoV2 infections were recorded, 42 (1.8%) participants recommended masks and 53 (2.1%) control participants (-0.3 percentage points [95% CI, -1.2 to 0.4; p = 038]). Of the participants recommended masks, 46% reported adherence.
Bottom Line: This negative study failed to show that wearing a mask leads to a 50% relative risk reduction.
Evidence Based Medicine Sidebar: Changes in Test Specificity Have a Large Effect on the Positive Predictive Value of Low Prevalence Diseases
The false-positive rate of any test increases rapidly when outcomes are rare and specificity is much lower than 100%. At a prevalence of 2% and a sensitivity/specificity of 82.5% / 99.5% for the antibody assay used in this study, the positive predictive value (PPV; the proportion of tests that if positive truly mean someone has the infection) is 77%. However, using the specificity reported by the FDA of 97.5%, just 2% lower, the PPV falls to 40%, a reduction of 37%! (The low PPV of a test when there is low prevalence is why we do series testing for diseases like HIV.) This is important for our interpretation of this study, because a PPV of <50% means the majority of all infections diagnosed by serology are likely to be false positives, distributed evenly between groups, thus biasing towards the null hypothesis.
While this study failed to show that the recommendation to wear a mask confers a risk reduction for SARS-CoV-2 infection to the wearer, to contextualize the DANMASK-19 results, it is important to consider how the study was powered and the effect of low incidence of COVID-19 during the study period. Powering the study for a 50% reduction in infection, an effect more in line with what can be expected from a vaccine, meant that this work would fail to identify a more subtle effect. In addition to the discussion above regarding how incidence/specificity affects PPV, studying rare events is notoriously difficult. A (theoretical) study looking at the efficacy of bulletproof vests will not show a mortality benefit if the incidence of gunshots is low, but that does not mean that bulletproof vests do not protect against gun shots.
Masks work. Looking at county-to-county differences in Kentucky, the CDC reported that mask mandates confer a sizable reduction in COVID-19 incidence (Van Dyke et al 2020). Furthermore, a study modeling the effect of masking on mortality predicts over 100,000 deaths prevented in the US by Feb 2021 if universal masking is achieved (IHME COVID-19 Forecasting Team). See this site for continuously updated IHME projections.
Check out Dr Ganatra’s tweetorial breaking it all down: Link to Rahul’s Thread
Background: Early reports of increased incidence venous thromboembolism (VTE) in patients hospitalized w/COVID-19; even if on anticoagulant prophylaxis. Both microthrombosis and symptomatic VTE occur (Daniel Kaul, NEJM JWatch article June 2, 2020 and Jiménez et al 2020).
A panel of experts decided on 13 PICO questions, conducted systematic reviews and made 22 recommendations (based on evidence when available, but ultimately used expert consensus) via a modified Delphi process. NOTE: The following recommendations refer to acutely ill (non-critically ill) patients with COVID-19.
These are in strong agreement with the CHEST guideline
Bottom line: Current recommendations can be broadly interpreted using the same prophylactic anticoagulation for patients hospitalized with COVID-19 as you would for any hospitalized patient.
What’s next?: Multiple large RCTs for VTE treatment and prevention are ongoing (NCT04498273, NCT04409834 aka COVID-PACT, NCT04373707 aka COVID-DOSE, and NCT04394377) with results expected in the next few months. The CHEST guidelines will be updated again at that time.
Listeners will learn about the DANMASK-19 trial, the latest on SARS-CoV-2 testing, and current anticoagulation recommendations for patients suffering from COVID-19.
After listening to this episode listeners will…
Dr Ganatra and The Curbsiders report no relevant financial disclosures
Ganatra RB, Gorth DJ, Williams PN, Watto MF. “#245 COVID Cakes: Masks, Home Testing, Anticoagulation”. The Curbsiders Internal Medicine Podcast. http://thecurbsiders.com/episode-list Final publishing date December 7, 2020.
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