美国流行病专家和传染病专家之所以反对用口罩预防COVID-19是因为他们无证可循。 根据以往防呼吸道病毒的经验， 他们反对使用口罩， 下面就是他们的一些理由：
（转载）• Air samples yielded small-particle viral RNA as far as 6 feet from infected patients....almost all of which was contained in particles small enough to elude standard surgical masks (J Infect Dis 2013;207:1037).....The air samples obtained 1 foot from emitters' heads had the highest concentrations of viral RNA.
• Wearing a mask outside health care facilities offers little, if any, protection from infection. Public health authorities define a significant exposure to Covid-19 as face-to-face contact within 6 feet with a patient with symptomatic Covid-19 that is sustained for at least a few minutes (and some say more than 10 minutes or even 30 minutes). The chance of catching Covid-19 from a passing interaction in a public space is therefore minimal. In many cases, the desire for widespread masking is a reflexive reaction to anxiety over the pandemic (NEJM. 2020;online April 1 DOI: 10.1056/NEJMp2006372).
• A study found no benefit with either surgical or cotton masks at blocking SARS–CoV-2 at a distance of 20 cm (Ann Intern Med. 2020 Apr 6;e-pub). They recruited 4 patients with SARS-CoV-2 infection to cough five times onto petri dishes containing viral transport media approximately 20 cm from their face while wearing either no mask, a surgical face mask, or a two-ply cotton mask. The median nasopharyngeal viral load was 5.66 log copies/mL, and the cough samples found viral loads of 1.4 to 3.5 logs/mL whether or not a mask was present for three of the four patients. Swabs of the outer surfaces of both types of masks were positive for all four patients.
• A RCT of cloth mask vs medical masks in 1607 healthcare workers (Hanoi, Vietnam) "cautions against the use of cloth masks" (BMJ Open. 2015;5(4):e006577). The rates of all infection outcomes in the cloth mask arm (relative risk (RR)=13.00, 95% CI 1.69 to 100.07) compared with the medical mask arm. Cloth masks also had significantly higher rates of ILI compared with the control arm. An analysis by mask use showed ILI (RR=6.64, 95% CI 1.45 to 28.65) and laboratory-confirmed virus (RR=1.72, 95% CI 1.01 to 2.94) were significantly higher in the cloth masks group compared with the medical masks group. Penetration of cloth masks by particles was almost 97% and medical masks 44%. "Moisture retention, reuse of cloth masks and poor filtration may result in increased risk of infection. Further research is needed to inform the widespread use of cloth masks globally. However, as a precautionary measure, cloth masks should not be recommended for HCWs, particularly in high-risk situations"
• Compared with cloth masks, disposable surgical masks are more effective in reducing particulate exposure; cloth masks are only marginally beneficial in protecting individuals from particles <2.5 μm (Evaluating the efficacy of cloth facemasks in reducing particulate matter exposure. J Expo Sci Environ Epidemiol. 2017;27:52–357).
• A study shows that common improvised mask fabric materials (sweatshirts, T-shirts, towels, scarves, and cloth masks) may provide marginal protection against nanoparticles including those in the size ranges of virus-containing particles in exhaled breath (Ann Occup Hygiene. 2010;54:789–798). Most had 40–90% instantaneous penetration levels with a some having a filtration efficiency comparable to some commonly used FDA-cleared surgical masks and unapproved dust masks. "The use of fabric materials may provide only minimal levels of respiratory protection to a wearer against virus-size submicron aerosol particles (e.g. droplet nuclei). This is partly because fabric materials show only marginal filtration performance against virus-size particles when sealed around the edges. Face seal leakage will further decrease the respiratory protection offered by fabric materials."
• The surgical mask is a bad fit for risk reduction in pandemics (CMAJ. 2016;188(8):606–607).
• There has been a consistent recommendation by various health authorities that symptomatic individuals and those in health-care settings should use face masks while evidence that face masks can provide effective protection against respiratory infections in the community is scarce (Rational use of face masks in the COVID-19 pandemic. Lancet Resp Med. 2020;Published:March 20).
• The World Health Organization guidance on using face masks during COVID-19 emphasizes that "the use of a mask alone is insufficient to provide an adequate level of protection, and other measures should also be adopted," such as hand hygiene and physical distancing (WHO/2019-nCov/IPC_Masks/2020.3. 26 April 2020). The WHO notes that medical masks should be reserved for healthcare workers and for people with symptoms, as well as caregivers in the same room as an infected person.
不过现在这种状况有所改观， Dr.Brix 刚刚说现在有足够证据说明口罩可以防止飞沫传播而降低感染率（ 5/24/2020）