Not Covid-19, but here is a comparitive test of medical mask effectiveness against the influenza virus in 2009:
Surgical Mask vs N95 Respirator for Preventing Influenza Among Health Care Workers: A Randomized Trial
https://pubmed.ncbi.nlm.nih.gov/19797474/
Context:
Data about the effectiveness of the surgical mask compared with the N95 respirator for protecting health care workers against influenza are sparse. Given the likelihood that N95 respirators will be in short supply during a pandemic and not available in many countries, knowing the effectiveness of the surgical mask is of public health importance.
Objective:
To compare the surgical mask with the N95 respirator in protecting health care workers against influenza.
Design, setting, and participants:
Noninferiority randomized controlled trial of 446 nurses in emergency departments, medical units, and pediatric units in 8 tertiary care Ontario hospitals.
Intervention:
Assignment to either a fit-tested N95 respirator or a surgical mask when providing care to patients with febrile respiratory illness during the 2008-2009 influenza season.
Main outcome measures:
The primary outcome was laboratory-confirmed influenza measured by polymerase chain reaction or a 4-fold rise in hemagglutinin titers. Effectiveness of the surgical mask was assessed as noninferiority of the surgical mask compared with the N95 respirator. The criterion for noninferiority was met if the lower limit of the 95% confidence interval (CI) for the reduction in incidence (N95 respirator minus surgical group) was greater than -9%.
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%.
Conclusion:
Among nurses in Ontario tertiary care hospitals, use of a surgical mask compared with an N95 respirator resulted in noninferior rates of laboratory-confirmed influenza.
So exactly what does this scientific study mean in the context of our currently ongoing Covid-19 battle over mandatory use of masks to prevent (or even reduce rates of) infection?
Of 446 nurses working in tertiary care Ontario hospitals, half wearing surgical masks and the other half wearing N95 respirators, 98 (22%) of the nurses were infected with influenza. There was less than 1% difference in effectiveness of the N95 mask and the surgical mask. (neither worked!)
And they won't work for Covid-19, either.