June 9, 2020 — Editor’s note: The study on which this news story is based has been retracted by the journal. The retraction notice can be found here.

Cotton and surgical masks were not effective at filtering SARS-CoV-2 during coughs from patients infected with the virus, according to Seongman Bae, MD, of the University of Ulsan College of Medicine in Seoul, South Korea, and associates.

The report was published in Annals of Internal Medicine.

Because the COVID-19 pandemic has caused a shortage of N95 and surgical masks, cotton masks have gained interest as a substitute, as surgical masks have been shown to effectively filter influenza virus, the researchers wrote. However, the size of and concentrations of SARS-CoV-2 in aerosols generated during coughing are unknown.

To compare the effectiveness of cotton and surgical masks, a group of patients infected with SARS-CoV-2 coughed into petri dishes while wearing no mask, a surgical mask, and a cotton mask. The mask surfaces were swabbed afterward to assess viral positivity on the mask itself.

The median nasopharyngeal and saliva viral load was 5.66 log copies/mL and 4.00 log copies/mL, respectively. The median viral loads after coughing was 2.56 log copies/mL without a mask, 2.42 log copies/mL with a surgical mask, and 1.85 log copies/mL with a cotton mask. All outer surfaces of the mask were positive for SARS-CoV-2, while most inner surfaces were negative.

The investigators acknowledged that the test did not include N95 masks and does not reflect the actual infection transmission, and that they didn’t know whether cotton or surgical masks shorten the travel distance of droplets while coughing.

“Further study is needed to recommend whether face masks decrease transmission of virus from asymptomatic individuals or those with suspected COVID-19 who are not coughing,” they added.

The study was funded by a grant from the government-wide R&D Fund Project for Infectious Disease Research. The investigators reported that they had no conflicts of interest.

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SOURCE: Bae S et al. Ann Intern Med. 2020 Apr 6. doi: 10.7326/M20-1342.

Correction, 4/9/20: The headline of an earlier version of this article misstated a finding of this study. Whether cotton and surgical masks can block transmission was not investigated.