The Science of Masks 5: Randomized Controlled Trials
Randomized controlled trials: you know, that thing where we see who gets sick and who doesn't.
Welcome to Part 5 of Too Much Analysis of Studies About Masks. Please, someone make me stop.
For those just joining us, we’re reviewed a bunch of references the CDC used when they decided to prank us about masks, while also watching an old TV show from back in the days when I thought agencies like the CDC and the FDA existed to benefit our health rather than the health of Pfizer’s stock price.
In the first set of posts we looked at the evidence compiled by the CDC intern to justify the use of masks to prevent the spread of a viral respiratory infection.
This time we will be looking at useful studies instead.
Also, Dharma & Greg Episode 5: “The Ex-Files” half of which takes place in the same café.
This is the meat of the matter (the masks, not the show), and I’m not going to sugar coat it for you - it’s long. There were 18 randomized controlled trials on masks published between about 2008 and 2020, and we will look at some (but not all) of them. Part 6 will look at some published summaries of those studies to see what professional researchers had to say about them.
Update from THE FUTURE:
I wrote a single post listing all 18 RCTs with a very short summary of each (Enough with the masks already). The post includes links to all 18, so if you want to read them yourself then go crazy.
If face masks are truly effective then it should be possible to demonstrate it through randomized controlled trials, and several were performed prior to 2020. And although not as common there are also studies looking at the possible negative effects from mask wearing.
[Quick author’s note on randomized: The American spelling is randomized, however the standard British spelling is randomised. Both spellings are used in the studies listed, neither is incorrect. Please be generous when criticizing, or criticising, the spelling of non native speakers. Americans and Brits are often inconsistent - Brits, for example, like to point out that silly Americans abbreviate mathematics as math and not maths, insisting it should be plural. The standard American response to this is to point out that if not for America, the Brits would probably be speaking German right now anyway so get off our backs.]
The randomized controlled trial (RCT), when well conducted, is generally viewed as the highest level of medical evidence. There are not very many RCTs on this compared to other subjects so it is not difficult to become acquainted with a significant portion of the relevant scientific literature.
One type of RCT is the cluster randomized controlled trial, in which randomization is of groups of subjects rather than of individual subjects. On the use of clustering (MacIntyre et al. 20111):
In infectious diseases which can spread from person to person, the ‘herd effect’ is a real and documented phenomenon where protecting some individuals with an intervention (most commonly vaccination, but also applicable to other interventions) can also protect individuals who were not protected by the intervention. Therefore, if some individuals are randomized to masks on a ward, the individuals who do not wear masks may also be protected because of the effect the masks have on interrupting the transmission of disease from person to person. This is why it is preferable to use cluster design, where everyone in the cluster gets the same intervention. (MacIntyre 2011)
So instead of completely randomizing who wears masks and who doesn’t, a clustered trial might, for example, have everyone in one hospital or one ward all wear masks or all not wear masks.
C. R. MacIntyre has designed several cluster randomized trials of face masks (which is a significant fraction of the total), and the three listed below were referenced in one or more of the papers cited by the CDC.
[The show is a little too much on point for today:
Larry: “So Ed, how goes the exploitation of the working classes?”
Edward: “It’s actually ahead of schedule.”]
1. Face mask use and control of respiratory virus transmission in households (MacIntyre et al. 20092)
Published in Emerging Infectious Diseases (a journal published by the CDC), this study is one of the references from a paper discussed previously (Davies et al. 20133), which itself is one of the references used by the CDC. The authors recruited 290 adults from 145 families who had been exposed to a child with clinical respiratory illness.
Although our study suggests that community use of face masks is unlikely to be an effective control policy for seasonal respiratory diseases, adherent mask users had a significant reduction in the risk for clinical infection. Another recent study that examined the use of surgical masks and handwashing for the prevention of influenza transmission also found no significant difference between the intervention arms. (MacIntyre 2009)
The authors state that the results may be better if people wear their masks more. In this study the subjects were adults caring for a sick child so the potential source of infection (and therefore when to wear masks) was clear.
We emphasize that this level of risk reduction is dependent on the context, namely, adults in the household caring for a sick child after exposure to a single index case. (MacIntyre 2009)
Due to the specific circumstances the authors caution drawing too many conclusions from this study.
We urge caution in extrapolating our results to school, workplace, or community contexts, or where multiple, repeated exposures may occur, such as in healthcare settings. (MacIntyre 2009)
They are not willing to give up on masks, however, stating they may reduce transmission.
Results of our study have global relevance to respiratory disease control planning, especially with regard to home care. During an influenza pandemic, supplies of antiviral drugs may be limited, and there will be unavoidable delays in the production of a matched pandemic vaccine. For new or emerging respiratory virus infections, no pharmaceutical interventions may be available. Even with seasonal influenza, widespread oseltamivir resistance in influenza virus A (H1N1) strains have recently been reported. Masks may therefore play an important role in reducing transmission. (MacIntyre 2009)
2. A Cluster Randomised Trial of Cloth Masks Compared With Medical Masks in Healthcare Workers (MacIntyre et al. 20154)
This paper was referenced by Konda et al. (20205) which is also one of the references used by the CDC, and was published in BMJ Open. This is an important study in the current context because it was using cloth masks and medical masks.
The aim of this study was to compare the efficacy of cloth masks to medical masks in hospital healthcare workers (HCWs). (MacIntyre 2016)
Although only completed a few years ago, this study was the first cloth mask RCT. The study included 1,607 participants in 14 hospitals, and based on their results the authors specifically state that cloth masks should not be recommended for health care workers.
This study is the first RCT of cloth masks, and the results caution against the use of cloth masks. This is an important finding to inform occupational health and safety. 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, and guidelines need to be updated. (MacIntyre 2016)
The study lasted four weeks, and the cloth mask and surgical mask test groups wore masks for their entire 8 hour shifts. The control arm participants were instructed to follow normal hospital procedure, so they would for example be wearing surgical masks when required by standard hospital procedure.
Participants wore the mask on every shift for four consecutive weeks. Participants in the medical mask arm were supplied with two masks daily for each 8 h shift, while participants in the cloth mask arm were provided with five masks in total for the study duration, which they were asked to wash and rotate over the study period. (MacIntyre 2016)
The cloth mask group did worse than the surgical mask group and the control arm. (Hey, look! Evidence that wearing masks might actually result in worse outcomes! I wonder why the CDC never mentioned this? Yes, I’m being sarcastic, but in my defense a lot of people have suggested that there is no downside to wearing masks. Those people have clearly not read any of these studies.)
We have provided the first clinical efficacy data of cloth masks, which suggest HCWs should not use cloth masks as protection against respiratory infection. Cloth masks resulted in significantly higher rates of infection than medical masks, and also performed worse than the control arm. (MacIntyre 2016)
The authors state to not recommend cloth masks until more effective designs have been developed and tested.
In the interest of providing safe, low-cost options in low income countries, there is scope for research into more effectively designed cloth masks, but until such research is carried out, cloth masks should not be recommended. (MacIntyre 2016)
3. A cluster randomized clinical trial comparing fit-tested and non-fit-tested N95 respirators to medical masks to prevent respiratory virus infection in health care workers. Influenza Other Respir Viruses (MacIntyre et al. 20116)
This is the third paper with MacIntyre as the principal investigator to be referenced by one of the papers cited by the CDC, this time by Ma et al. (20207). First they give us some background.
There are several prospective, randomized controlled trials on the use of handwashing, but only two trials on the use of medical masks ⁄ respirators in households. (MacIntyre 2011)
The authors point out that medical masks are not designed to protect the wearer but rather the patient from infection, but even that was not demonstrated by the three RCTs the authors looked at.
To date, there is one small randomized controlled trial (RCT) of medical masks compared to respirators in HCWs which found no difference, but lacked a control arm. Medical masks are not designed to provide respiratory protection.
Medical masks were designed to prevent wound contamination when worn by the surgeon; however, three RCTs failed to show efficacy against their intended design. (MacIntyre 2011)
Did anyone read this one? Did they see the part about how medical masks aren’t designed to protect from respiratory diseases? Yet wearing these masks will meet most government mandates requiring masks. In all fairness, the people writing the mandates may have simply been taking dictation from other people who can’t actually read.
This study was focused on comparing surgical masks to N95 respirators, and some of the N95 wearers were not fit tested. This would be more in line with general population use where proper fitting is not always expected.
The aim of this study was to determine the efficacy of medical masks compared to fit-tested and non-fit-tested N95 respirators in HCWs in the prevention of disease because of influenza and other respiratory viruses. (MacIntyre 2011)
The study had a convenience group (that’s a group not part of the RCT so their data may or may not be useful) of health care workers not wearing masks, but the authors state that conditions under which the convenience group worked may be different than those for the test groups so they do not make any conclusion about masks versus no masks.
The rates of all outcomes were higher in the convenience no-mask group than in the masks groups. By adjusted intention-to-treat analysis, N95 respirators but not medical masks had significantly lower rates of infection compared to no masks. However, the convenience no-mask group was not a randomized control arm and hospitals in this group were actually selected on the basis that most of their staff did not wear masks (which is not the norm in hospitals in Beijing), suggesting that conditions in those hospitals were different than those in hospitals from the masks groups. As a consequence, it is not possible to make any definitive judgement on the efficacy of masks on this basis. (MacIntyre 2011)
[This is taking so long I’m now on Episode 6. Dharma is trying to convince Greg to take her yoga class.
D: “Yoga would change your life.”
G: “So would breast implants.”]
4. Preliminary findings of a randomized trial of non-pharmaceutical interventions to prevent influenza transmission in households (Cowling et al. 20088)
This paper reports on the preliminary results of a RCT on hand hygiene and surgical masks to reduce the spread of influenza in households.
Our study is the first reported community-based randomized trial of these interventions specifically against influenza, with laboratory-confirmed outcomes. (Cowling 2008)
This study did not find the interventions to be effective, with the secondary attack ratio of laboratory confirmed influenza being the same for all three groups (no mask, mask only, hand hygiene only). The secondary attack ratio (SAR) is the percent of people who become infected after the first (index) person, and the SAR in this group was lower than the authors anticipated based on previous studies. As a result, they suggest that a larger study might still find a difference.
Although we found little effect of the interventions in preventing household transmission, our study was underpowered.
Overall, the SAR was lower than we had expected. Only 6% of household contacts developed laboratory-confirmed influenza, whereas 5%–18% of contacts developed clinical influenza, depending on case definitions. (Cowling 2008)
Clinical influenza means a finding based on symptoms only (not lab confirmed) and the authors use three different definitions for clinical influenza, which is why they report 5% to 18%. The definition being used can be important in this context, for example by the CDC definition of clinical influenza*. the mask wearing group had twice as many cases as the control group (8% versus 4%).
(*The CDC definition of a clinical diagnosis of influenza is a fever ≥ 37.8 degrees C (about 100 degrees F) plus either a cough or a sore throat.)
Let’s repeat that statement about subjects with a clinical diagnosis of influenza. The mask wearing group had twice as many people who had a mild fever and a cough or sore throat - so they were diagnosed by a medical doctor as being sick. Maybe they didn’t have influenza, but something was making them feel not so great and it was tied to the masks. Once again, evidence that mask wearing might have negative consequences.
The authors believe their study design is good and planned a larger study based on this one.
Whereas the present study was not powered to assess the relative efficacy of the interventions, it has proved successful in demonstrating the feasibility of our study design and the local characteristics of influenza transmission. The present findings have facilitated the planning of a subsequent larger study, described in more detail in Protocol S1. (Cowling 2008)
And further research like that study is important because so little data is available. Also, the organization that funded the study might sound familiar - it was the CDC. Somebody please tell the intern.
In conclusion, there remains a serious deficit in the evidence base of the efficacy of non-pharmaceutical interventions. The US Centers for Disease Control and Prevention have awarded grants to study non pharmaceutical interventions in community settings, including this study. (Cowling 2008) (emphasis added)
It is interesting (by which I mean unethical) that the CDC made no mention of this study or of any other RCTs of non-pharmaceutical interventions that they funded.
5. Facemasks and hand hygiene to prevent influenza transmission in households: a cluster randomized trial (Cowling et al. 20099)
This is the larger study referenced in the previous paper (Cowling 2008), and the primary funding source again was the CDC. This study compared a control group with a group practicing hand hygiene and another group using face masks plus hand hygiene. There was no face mask only group.
Hand hygiene with or without facemasks seemed to reduce influenza transmission, but the differences compared with the control group were not significant. (Cowling 2009)
It’s not clear that face masks by themselves were doing any good, since the rates of laboratory confirmed infection were the same for the two hand hygiene groups (one with and one without the inclusion of face masks).
No significant difference was found between the facemask plus hand hygiene group and the hand hygiene group in RT-PCR–confirmed influenza virus infections in household contacts. (Cowling 2009)
The overall results of the study did not show a reduction in transmission. Looking at just a subset of the data the authors saw a short term benefit but not over the whole duration of the study.
Overall, the interventions did not lead to statistically significant reductions in household transmission, although we did observe statistically significant reductions where interventions were applied early after symptom onset in the index patient. (Cowling 2009)
Just like in the preliminary study, the group with masks actually had a slightly higher incidence of clinical influenza based on the CDC definition, although for all groups the differences are not statistically significant. Despite this being a large, multi-year trial being conducted after a preliminary trial, the results are still very weak.
Although our results suggest a benefit of hand hygiene and facemasks in combination if applied early, our study cannot precisely distinguish the relative contributions of the 2 interventions. (Cowling 2009)
The results only suggest a benefit, depending on when interventions are started, and it is not clear if it was the result of hand hygiene, masks, or both. And once again the masked group had slightly more people diagnosed as being sick.
I wonder, is breathing through artificial fibers all day maybe not good for you?
6. N95 Respirators vs Medical Masks for Preventing Influenza Among Health Care Personnel: A Randomized Clinical Trial (Radonovich et al. 201910)
This study is another one of the RCTs funded by the CDC, along with funding from the Veterans Health Administration and the Biodefense Advanced Research and Development Agency. The study involved 2,862 participants in 137 sites at 7 U.S. medical centers and lasted from September 2011 until May 2015.
In this pragmatic, cluster randomized clinical trial involving 2862 health care personnel, there was no significant difference in the incidence of laboratory-confirmed influenza among health care personnel with the use of N95 respirators (8.2%) vs medical masks (7.2%). (Radonovich 2019)
So the N95 wearers had a slightly higher incidence of influenza, but the difference was small so it might just be the result of chance (which is essentially what “no statistically significant difference” means).
The authors explain the logic behind the idea that N95 respirators might be more effective than medical (surgical) masks in preventing infection based on the transmission of respiratory viruses, based on droplets being the primary method of transmission.
Medical masks fit the face loosely and do not reliably prevent inhalation of small airborne particles.
Respiratory viruses are primarily transmitted by large droplets. Because a fraction of respiratory viruses may be transmitted by aerosol, N95 respirators have been presumed to provide better protection than medical masks against viral respiratory infections in health care settings. (Radonovich 2019)
The reference given for droplet transmission is a report published by the National Academy of Science in 2008 titled Preparing for an Influenza Pandemic: Personal Protective Equipment for Healthcare Workers11. This document says the primary transmission route of influenza is still unknown (Chapter 2, Understanding the Risk of Influenza to Healthcare Workers).
Although it has been 70 years since the influenza A virus was discovered and despite the recognition that it can cause yearly epidemics worldwide resulting in severe illness and death, little is known about the mechanisms by which influenza A is transmitted or its viability and infectivity outside the host. Debate continues about whether influenza transmission is primarily via the airborne or droplet routes and the extent of the contribution of the contact route (including contact with blood, fecal matter, or contaminated surfaces. Further, the aerosol-droplet continuum needs to be clarified as soon as possible in order to develop and implement effective prevention strategies. (Radonovich 2019) (emphasis added)
The authors in the paper are referring to respiratory viruses in general, and the NAS report is specifically about influenza so this may explain the discrepancy. Whatever the actual mechanism of transmission, this study reported no difference in the likelihood of health care providers (HCP) being infected with influenza based on the type of mask.
Among outpatient HCP, N95 respirators vs medical masks as worn by participants in this trial resulted in no significant difference in the incidence of laboratory-confirmed influenza. (Radonovich 2019)
Of course one reason for finding no reported difference could be that neither intervention had any impact on the rate of influenza. This study didn’t include an unmasked control group so we have no way of knowing.
[Episode 6 is also channeling 2021. Dharma asks Pete why one of the assistants in the office is so mean. Pete responds “she’s a federal employee.”]
7. Findings from a household randomized controlled trial of hand washing and face masks to reduce influenza transmission in Bangkok, Thailand. (Simmerman et al. 201112)
This is another study comparing hand hygiene against hand hygiene plus face masks in households, with a total of 442 index children and 1,147 household members enrolled.
We report the largest study to date of the efficacy of interventions to promote hand washing and hand washing plus face mask use to reduce influenza transmission. Influenza transmission among household members of a confirmed index case was not reduced by promotion of hand washing and face mask use. (Simmerman 2011)
The intervention groups (hand wash, hand wash plus face mask) had higher rates of laboratory confirmed influenza and clinical influenza, although the difference for laboratory confirmed influenza did not rise to the level of statistical significance. For clinical influenza the differences appear quite large.
Relative to the control group, the ORs for ILI* among household members in the hand-washing arm (2.09; 95% CI 1.25, 3.50; P = 0.005) and hand washing plus face mask arm (2.15; 95% CI: 1.27, 3.62; P = 0.004) were twofold in the opposite direction from the hypothesized protective effect (Table 3). (Simmerman 2011)
(*ILI: influenza like illness)
The OR is odds ratio. The control group is defined as having an odds ratio of 1.00, so an OR of 2.15 is more than twice the chance of something happening as in the control group. Each of these numbers has an error range associated with it, and in the case of the hand washing plus face mask arm the range given by the authors is 1.27 to 3.62 for the actual OR, meaning it could be anywhere between those two numbers.
This study appears to show that the interventions increased the likelihood of secondary infections (at least when measured based on symptoms), but the authors caution that this study does not prove these interventions are useless (scientists have a saying: “absence of evidence is not evidence of absence”) since there could have been other factors involved.
Our findings should not be interpreted to conclude that hand washing or face mask use are not potentially useful public health measure [sic] to prevent infections other than influenza, but they do provide a potent example of the importance of understanding the dynamic and complex relationship between public health recommendations, local social customs and individual behavior, and their application for preventing transmission of specific pathogens. (Simmerman 2011)
The authors of this study state the interventions did not reduce influenza transmission, but do not conclude that hand hygiene and masks increased the chance of infection. They discuss possible reasons for the outcome but these would require further study before making a determination of the true cause or causes.
Now it’s time for some serious ranting. Once again we have a study in which there is no significant difference in laboratory confirmed influenza, but a lot more people in the mask group are diagnosed as being sick. These people are being diagnosed based on having an actual fever and other symptoms, so clearly something is bothering them.
What is the actual issue? Who knows - no one seems interested in finding out. Maybe they are inhaling fumes from the artificial fabrics used to make the masks. Maybe the change in humidity in their lungs is an issue.
We have evidence that wearing masks all the time for no good reason might actually be harmful, and no one in the medical industry seems to care.
8. Surgical mask to prevent influenza transmission in households: a cluster randomized trial (Canini et al. 201013)
This trial included 105 households, and in the households with masks illness was reported in 24 out of 148 subjects while in the households without masks illness was reported in 25 out of 158 subjects. The authors conclude that no difference was found between the groups. They specifically mention that adherence to the use of masks was high, therefore this is likely not a factor in the result.
We observed a good adherence to the intervention. In various sensitivity analyses, we did not identify any trend in the results suggesting effectiveness of facemasks. (Canini 2010)
The authors also mention some issues reported by the participants.
Thirty-eight (75%) patients from the intervention arm reported discomfort with mask use (Table 4). The three main causes of discomfort were warmth (45%), respiratory difficulties (33%) and humidity (33%). Children wearing children facemasks reported feeling pain more frequently (3/12) than other participants wearing adult facemasks (1/39) (p = 0.036). No difference was detected concerning the other cause of discomfort depending on the facemask type. (Canini 2010)
The study was originally intended to include 372 households, but in the end only included 105. As a result the authors caution drawing conclusions from the result.
This study should be interpreted with caution since the lack of statistical power prevents us to draw formal conclusion regarding effectiveness of facemasks in the context of a seasonal epidemic. (Canini 2010)
Summary for “journalists” who were laid off by Buzzfeed, hired by Vice, laid off by Vice, hired by Salon, laid off by Salon, and now have three roommates none of whom know how to do their own laundry.
Randomized controlled trials, designed and conducted by scientists hoping to demonstrate that masks work, have consistently failed to show that masks work.
Sometimes the mask group does a little better than the control, but it rarely meets the level of statistical significance when laboratory-confirmed infection is being measured. Sometimes the non-mask group does a little better but with the same caveat.
And sometimes the mask group has twice as many people who are mildly sick for some reason other than what is being tested, and no on seems interested in trying to investigate why. These people aren’t faking it - they really have mild fevers and sore throats.
Based on these results the most likely thing we can expect from forcing people to wear masks is that more people will feel sick. Yet smug midwits on Twitter continue to insist there’s no reason not to wear a mask, even if they don’t work, just to make other people feel safe.
This took so long we finished Episode 7 as well, and Chuck Lorre’s vanity card this time is correcting a mistake he made previously:
“I believe that in my earlier statement of beliefs, I erroneously believed that beer was a gateway drug that led to vodka. After intensive consultation with ABC executives, I now believe I was very, very wrong. Beer is good. Especially beer brewed by major manufacturers, and enjoyed in a responsible manner.”
See, they started with the entertainment industry and now they own the CDC. At this point I really don’t have a better explanation.
Next time: “Scientific consensus” is literally a meaningless phrase.
MacIntyre CR, Wang Q, Cauchemez S, et al. A cluster randomized clinical trial comparing fit-tested and non-fit-tested N95 respirators to medical masks to prevent respiratory virus infection in health care workers. Influenza Other Respir Viruses. 2011;5(3):170-179. doi:10.1111/j.1750-2659.2011.00198.x
https://pubmed.ncbi.nlm.nih.gov/21477136/
MacIntyre CR, Cauchemez S, Dwyer DE, et al. Face mask use and control of respiratory virus transmission in households. Emerg Infect Dis. 2009;15:233-241.
https://pubmed.ncbi.nlm.nih.gov/25903751/
Davies A, Thompson KA, Giri K, Kafatos G, Walker J, Bennett A. Testing the efficacy of homemade masks: would they protect in an influenza pandemic?. Disaster Med Public Health Prep. 2013;7(4):413-418. doi:10.1017/dmp.2013.43
https://pubmed.ncbi.nlm.nih.gov/24229526/
MacIntyre, C. R.; Seale, H.; Dung, T. C.; Hien, N. T.; Nga, P. T.; Chughtai, A. A.; Rahman, B.; Dwyer, D. E.; Wang, Q. A Cluster Randomised Trial of Cloth Masks Compared With Medical Masks in Healthcare Workers. BMJ. Open 2015, 5, No. e006577.
https://pubmed.ncbi.nlm.nih.gov/23413265/
Konda A, Prakash A, Moss GA, Schmoldt M, Grant GD, Guha S. Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks [published correction appears in ACS Nano. 2020 Jun 18;:]. ACS Nano. 2020;14(5):6339-6347. doi:10.1021/acsnano.0c03252
https://pubmed.ncbi.nlm.nih.gov/32329337/
MacIntyre CR, Wang Q, Cauchemez S, et al. A cluster randomized clinical trial comparing fit-tested and non-fit-tested N95 respirators to medical masks to prevent respiratory virus infection in health care workers. Influenza Other Respir Viruses. 2011;5(3):170-179. doi:10.1111/j.1750-2659.2011.00198.x
https://pubmed.ncbi.nlm.nih.gov/21477136/
Ma QX, Shan H, Zhang HL, Li GM, Yang RM, Chen JM. Potential utilities of mask-wearing and instant hand hygiene for fighting SARS-CoV-2 [published online ahead of print, 2020 Mar 31]. J Med Virol. 2020;10.1002/jmv.25805. doi:10.1002/jmv.25805
https://pubmed.ncbi.nlm.nih.gov/32232986/
Cowling BJ, Fung RO, Cheng CK, et al. Preliminary findings of a randomized trial of non-pharmaceutical interventions to prevent influenza transmission in households. PLoS One. 2008;3(5):e2101. Published 2008 May 7. doi:10.1371/journal.pone.0002101
https://pubmed.ncbi.nlm.nih.gov/18461182/
Cowling BJ, Chan KH, Fang VJ, et al. Facemasks and hand hygiene to prevent influenza transmission in households: a cluster randomized trial. Ann Intern Med. 2009;151(7):437-446. doi:10.7326/0003-4819-151-7-200910060-00142
https://pubmed.ncbi.nlm.nih.gov/19652172/
Radonovich LJ Jr, Simberkoff MS, Bessesen MT, et al. N95 Respirators vs Medical Masks for Preventing Influenza Among Health Care Personnel: A Randomized Clinical Trial. JAMA. 2019;322(9):824-833. doi:10.1001/jama.2019.11645
https://pubmed.ncbi.nlm.nih.gov/31479137/
Institute of Medicine 2008. Preparing for an Influenza Pandemic: Personal Protective Equipment for Healthcare Workers. Washington, DC: The National Academies Press. https://doi.org/10.17226/11980.
https://www.nap.edu/download/11980
Simmerman JM, Suntarattiwong P, Levy J, et al. Findings from a household randomized controlled trial of hand washing and face masks to reduce influenza transmission in Bangkok, Thailand. Influenza Other Respir Viruses. 2011;5(4):256-267. doi:10.1111/j.1750-2659.2011.00205.x
https://pubmed.ncbi.nlm.nih.gov/21651736/
Canini L, Andréoletti L, Ferrari P, et al. Surgical mask to prevent influenza transmission in households: a cluster randomized trial. PLoS One. 2010;5(11):e13998. Published 2010 Nov 17. doi:10.1371/journal.pone.0013998
https://pubmed.ncbi.nlm.nih.gov/21103330/