From Loeb M, Dafoe N, Mahony J, John M, Sarabia A, Glavin V, Webby R, Smieja M, Earn DJ, Chong S, Webb A, Walter SD. Surgical mask vs N95 respirator for preventing influenza among health care workers: a randomized trial. JAMA. 2009 Nov 4;302(17):1865-71. doi: 10.1001/jama.2009.1466. Epub 2009 Oct 1. PMID: 19797474. https://pubmed.ncbi.nlm.nih.gov/19797474/
Results: Between September 23, 2008, and December 8, 2008, 478 nurses were assessed for eligibility and 446 nurses were enrolled and randomly assigned the intervention; 225 were allocated to receive surgical masks and 221 to N95 respirators. Influenza infection occurred in 50 nurses (23.6%) in the surgical mask group and in 48 (22.9%) in the N95 respirator group (absolute risk difference, -0.73%; 95% CI, -8.8% to 7.3%; P = .86), the lower confidence limit being inside the noninferiority limit of -9%.
Percent infected wearing…
Surgical masks: 23.6%
N95 masks: 22.9%
Conclusion: N95 masks offer no more significant protection than a surgical mask. This assumes the surgical masks are quality masks, not made in China.
From Makison Booth C, Clayton M, Crook B, Gawn JM. Effectiveness of surgical masks against influenza bioaerosols. J Hosp Infect. 2013 May;84(1):22-6. doi: 10.1016/j.jhin.2013.02.007. Epub 2013 Mar 14. PMID: 23498357. https://pubmed.ncbi.nlm.nih.gov/23498357/
Methods: A dummy test head attached to a breathing simulator was used to test the performance of surgical masks against a viral challenge. Several designs of surgical masks commonly used in the UK healthcare sector were evaluated by measuring levels of inert particles and live aerosolised influenza virus in the air, from in front of and behind each mask.
IMPORTANT NOTE: aerosolised virus particles are microscopic water droplets the virus travels in and one source of these is an infected person coughing. A dry virus is MUCH smaller than a microscopic water droplet. Understanding the difference between which particles a mask stops is critical. And so is understanding what percent of that specific particle the mask stops. A cloth mask may only stop 60% of aerosolized micro water droplets. See image below which is from a study on mask types.
What size are dry virus particles?
Upon analysis of negative-stained SARS-CoV-2 articles by electron microscopy, researchers have determined the diameter of this virus to range between 60 nanometers (nm) to a maximum diameter of 140 nanometers (nm)… To date, research has shown that the viruses that have been identified and isolated can range in diameter size from 20 nm to as large as 500 nm.
https://www.news-medical.net/health/The-Size-of-SARS-CoV-2-Compared-to-Other-Things.aspx
The study findings:
Live influenza virus was measurable from the air behind all surgical masks tested. The data indicate that a surgical mask will reduce exposure to aerosolised infectious influenza virus; reductions ranged from 1.1- to 55-fold (average 6-fold), depending on the design of the mask.
Translation: virus exposure is reduced but not eliminated by surgical masks.
The next study is from 2017 and is about CDC recommendations during a flu pandemic. Some elements are changed from the 2007 CDC guidelines. This study has some interesting graphs. This study talks about when to wear masks, when to social distance, when to close schools and businesses. See Table 6 for severity assessment, i.e. what percent of people have the illness.
From Qualls N, Levitt A, Kanade N, Wright-Jegede N, Dopson S, Biggerstaff M, Reed C, Uzicanin A; CDC Community Mitigation Guidelines Work Group. Community Mitigation Guidelines to Prevent Pandemic Influenza – United States, 2017. MMWR Recomm Rep. 2017 Apr 21;66(1):1-34. doi: 10.15585/mmwr.rr6601a1. PMID: 28426646; PMCID: PMC5837128. Summary: https://pubmed.ncbi.nlm.nih.gov/28426646/ Full article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5837128/
NPI = non-pharmaceutical intervention (that is, no drugs are used)
The guidelines that resulted from this study are here: Community Mitigation Guidelines to Prevent Pandemic Influenza –United States, 2017Technical Report 1: Chapters 1-4. https://stacks.cdc.gov/view/cdc/44313
This study also has some good definitions on “mask” vs “N95 mask” vs “respirator” along with limitations of different types of masks. This also has a table of case fatality rate of COVID-19 vs other similar diseases like SARS and MERS.
Quintana-Díaz MA, Aguilar-Salinas CA. UNIVERSAL MASKING DURING COVID-19 PANDEMIC – CURRENT EVIDENCE AND CONTROVERSIES. Rev Invest Clin. 2020;72(3):144-150. doi: 10.24875/RIC.20000196. PMID: 32584329. https://pubmed.ncbi.nlm.nih.gov/32584329/ and https://www.clinicalandtranslationalinvestigation.com/frame_esp.php?id=283
Table 1 of this study comparing mortality rate.
