COVID-19 research briefs: asymptomatic carriage during pregnancy is common, and unclear efficacy evidence for homemade masks

Daily POEMs

Published: 2020-05-24 © 2020 John Wiley & Sons, Inc.

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Clinical question
What is the rate of COVID-19 among asymptomatic pregnant women? Can homemade face masks prevent transmission of the virus?

Bottom line
The high rate of asymptomatic carriage of COVID-19 among pregnant women admitted to hospitals in New York suggests that COVID-19 spread will be difficult to contain with medical isolation, quarantine, and contact tracing only. There is equivocal evidence of effectiveness of face masks for preventing transmission. (LOE = 4)

Sutton D, Fuchs K, D'Alton M, Goffman D. Universal screening for SARS-CoV-2 in women admitted for delivery [letter]. N Engl J Med. Published online April 13, 2020. doi:10.1056/NEJMc2009316. National Academies of Sciences, Engineering, and Medicine. Rapid expert consultation on the effectiveness of fabric masks for the COVID-19 pandemic. Published April 8, 2020. Washington, DC: The National Academies Press.

Study design: Other

Setting: Other

Research Brief #24: Two obstetric services in New York City instituted universal COVID-19 testing in March, 2020. This report summarizes the screening of the first 215 women who presented in labor. Four women had symptoms consistent with COVID-19 and all 4 tested positive. Of the remaining 211 asymptomatic patients, 29 tested positive. Therefore, 29 of the 33 patients (88%) who had a positive test result were asymptomatic on admission to the hospital. The high rate of asymptomatic carriage of COVID-19 may have been due to the high prevalence of COVID-19 infections in New York City at the time of testing, though other surveillance reports also found a high proportion of asymptomatic COVID-19 infections. This and other studies suggest that COVID-19 will be difficult to contain with medical isolation, quarantine, and contact tracing only. Research Brief #25: The National Academies of Sciences, Engineering, and Medicine reviewed the evidence for the efficacy of homemade fabric masks to prevent the spread of influenza and SAR-CoV-2 (COVID-19). These viruses can be spread by visible and invisible droplets as small as 5 microns, and by even smaller bioaerosols. We don't know which size is most dangerous. A mask's effectiveness depends on how it is made and how well it is made. Leakage around the mask is a problem, so fit must be as tight as possible. Filtration efficiency and how much the mask impedes breathing (wearability) must also be considered. These authors found 7 studies that evaluated either the ability of the mask to protect the wearer from infection or to prevent the spread of infection by a wearer. Performance in reducing the wearer's exposure ranged from 'very poor' to approximately 60% depending on the material used. One study found a filtration efficiency of only 0.7% for 0.3 micron-sized particles with a 4-layer woven handkerchief fabric, 35.3% for 5-layered woven brushed fabric, and 50% for 4 layers of polyester knitted cut-pile fabric. A study from 2008 and another from 2010 of cotton mask wearers suggest moderate protection against inhalation of infectious-sized particles. A recent study of patients with COVID-19, however, found that surgical and cotton masks were not effective at blocking the virus from disseminating from coughs. In the only randomized trial, 2-layer cotton masks worn by healthcare workers were much worse than medical masks (3 layers of nonwoven material) at preventing the transmission of a respiratory infection (relative risk = 13).

John Hickner, MD, MS
Professor Emeritus
Dept of Family Medicine
Michigan State University
East Lansing, MI

Copyright © 2020 John Wiley & Sons, Inc.