Studies on mouth protection

Today, there are well over a hundred studies that, in an overall assessment, provide convincing scientific support for the use of face masks to reduce the spread of COVID-19 in society. On the basis of this new knowledge, the major international infection control organizations, such as the WHO, ECDC and CDC, have changed their recommendations, and they now all advocate the general use of oral protection - in healthcare, care for the elderly, in public indoor environments, and where insufficient distance cannot be maintained outside. In line with these new findings, more than 170 countries in the world have introduced oral protection recommendations or oral protection requirements. We address here some of the most important. Most of these are already published in scientific journals or in reports from public health authorities, some are awaiting review:

1. In the German city of Jena, on April 6, face masks were introduced in shops and public transport three weeks before they were introduced throughout Germany. One could therefore compare the development. It was found that the spread of covid-19 in Jena in principle ceased after April 6, but continued to increase in the other cities. The authors write: "We believe that a reduction of the daily increase in infections by 40 to 60 percent is our most reliable estimate of the effects of mouth protection."

A new publication on this work is now in Proceedings of the National Academy of Sciences (PNAS)(”

2. There has also been an overall look at mask use and increased mortality in covid-19 in different countries. A study of 198 countries found that they were effective. ”In countries with cultural norms or government policies supporting public mask-wearing, per-capita coronavirus mortality increased on average by just 8.0% each week, as compared with 54% each week in remaining countries.”(

3. In a CDC study, researchers looked at 999 schools in Maricopa and Pima counties in Arizona, where schools started in July. Of those schools, 21% had a mask requirement at the start of the school year, around 30% added a mandate later, and 48% did not mandate masks. There were 191 schools that experienced an outbreak between July 15 and August 31. After adjusting for potential described confounders, the odds of a school-associated Covid-19 outbreak in schools without a mask requirement were 3.5 times higher than in schools with an early mask requirement.
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4. A study in the British Medical Journal from 25 August analyzes how the virus moves in the air between individuals. It is found that the distance of 1-2 meters in many contexts is insufficient. It also presents a graphic illustration of how the use of mouth guards affects the risk of infection from asymptomatic, but infectious, individuals.

5. On 10 June, the esteemed Proceedings of the Royal Society published a mathematical model of the effect of mouth guards, which takes into account the properties of both the shield and the wearers (the risk of wearing the mouth guard, wearing it incorrectly, etc.). It was found that if mouth guards are also worn by people without symptoms, the spread of infection is significantly reduced, and that this can contribute to the epidemic being extinguished, even without shutting down communities

6. On june 1 a meta analysis was published in The Lancet covering 44 studies of the incidence of the disease in groups that wore and did not wear face masks. Although the association was not considered statistically significant, the authors' conclusion from this review was that the use of face masks "may result in a sharp reduction in the risk of infection", especially when using more advanced face masks. But, they write, even "face masks in general lead to a large reduction in the risk of infection".

7. Another meta-analysis was published on the 26th of June by the British Royal Society and The British Academy. Their conclusions include: Cloth face coverings are effective in reducing source virus transmission, i.e., outward protection of others, when they are of optimal material and construction (high grade cotton, hybrid and multilayer) and fitted correctly and for source protection of the wearer”. It also points out the importance of the authorities giving clear and unambiguous advice: “Consistent and effective public messaging is vital to public adherence of wearing face masks and coverings. Conflicting policy advice generates confusion and lack of compliance.”

8. Meta-analyzes of studies on other viruses and the effect of oral protection have given varying results: A meta-analysis of the effect in influenza found some support for general use of oral protection, but if the oral protection was used without other infection control measures, the finding was not statistically significant.

9. Another meta-analysis, which assessed the effect of mouth protection during the 2003 SARS epidemic (also caused by a coronavirus), found a clear protective effect of mouth protection

10. One of the studies where one has directly looked at the filtering effect of mouth guards is from this year. A coronavirus was studied, but not the one that causes covid-19. In the air in front of those infected who did not wear mouth guards, both airborne and drip-borne viruses were found. When patients wore simple mouth guards, no virus was found in front of a single patient. 

11. This study looked specifically at the filtering effect of fabric masks of different types. It was found that fabric in several layers significantly increased the mask's filtering effect on particles the size of the virus. One writes, among other things: ”We find that cotton, natural silk, and chiffon can provide good protection, typically above 50% in the entire 10 nm to 6.0 μm range, provided they have a tight weave.”

12. A study on outward emissions of micron-scale aerosol particles by healthy humans performing various expiratory activities while wearing different types of medical-grade or homemade masks. Both surgical masks and unvented KN95 respirators, even without fit-testing, reduced the outward particle emission rates by 90% during speaking and 74% during coughing.

13. The Centers for Disease Control have demonstrated that double masking – using for instance a cloth mask and a surgical mask – significantly reduces the risk of contagion. Double masking blocked 85 percent of produced cough particles. If both parties were double masked, the recipient’s exposure was reduced by 96 percent.

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14. A Swedish study compared the production of particles during singing and speaking, with and without face protection. The authors found that loud singing produced 3.6 times as many particles per second as normal speaking. When the singer wore a simple surgical mask, the number of particles in the air was reduced to a level comparable to that during speech without a mask.


15. In a study of 124 Beijing households with > 1 laboratory-confirmed case of SARS-CoV-2 infection, mask use by the index patient and family contacts before the index patient developed symptoms reduced secondary transmission within the households by 79%.

16. A retrospective case-control study from Thailand documented that, among more than 1,000 persons interviewed as part of contact tracing investigations, those who reported having always worn a mask during high-risk exposures experienced a greater than 70% reduced risk of acquiring infection compared with persons who did not wear masks under these circumstances.

17. A study looking at different US states, where the reopening after shelter-in-place was or was not combined with a face mask mandate. The study looks at how this mandate affected the subsequent spread of the virus. The study found: "On average, the number of excess cases per 100,000 residents in states reopening without masks is ten times the number in states reopening with masks after 8 weeks ... Over 50,000 excess deaths were prevented within 6 weeks in 13 states that implemented mask mandates prior to reopening."

18. In a CDC study, researchers looked at 999 schools in Maricopa and Pima counties in Arizona, where schools started in July. Of those schools, 21% had a mask requirement at the start of the school year, around 30% added a mandate later, and 48% did not mandate masks. There were 191 schools that experienced an outbreak between July 15 and August 31. After adjusting for potential described confounders, the odds of a school-associated Covid-19 outbreak in schools without a mask requirement were 3.5 times higher than in schools with an early mask requirement.
191 av dessa skolor fick covid-19- utbrott mellan 15 juli och 31 augusti.  Man fann att risken för skolutbrott var 3,5 gånger högre i skolor utan munskyddstvång jämfört med i skolor med tidigt munskyddstvång.(

19. Another CDC report of 520 US counties found that the increase in pediatric Covid-19 cases after the beginning of the school year was more than twice as high in counties without school mask requirements as in counties with school mask requirements (an increase of 34,85 per 100,000 per day vs 16.32 per 100.000 per day).

20. In the Swiss town of Graubünden, an outbreak of hotel staff took place recently. Everyone on the staff wore protection, but some wore visors, while others wore mouth guards. It turned out that only those who wore visors were affected, not in a single case those who wore mouth guards.

21. At the end of March, the largest healthcare organization in the US state of Massachusetts - with 75,000 employees - introduced a mouth guard for all staff and all patients. Prior to that, the spread of infection within healthcare facilities increased exponentially, from 0 to 21.3% (on average by 1.16% per day). After the introduction of the oral protection requirement, the proportion of infections decreased to first 14.7%, then 11.5% (a decrease of on average 0.49% per day)

22. The governor of Kansas issued an executive order requiring wearing masks in public spaces, effective July 3, 2020, which was subject to county authority to opt out. After July 3, COVID-19 incidence decreased in 24 counties with mask mandates but continued to increase in 81 counties without mask mandates.

23. A similar study was conducted in the entire United States of America comparing the prevalence of covid-19 in different states, related to whether and when those states introduced oral contraceptives. The authors' conclusion:(

24. In Springfield, Missouri, two hairdressers at a salon developed respiratory symptoms. But they continued to work, with mouth guards (which was mandatory), for a week, until word came that they had covid-19 - and that one of them had infected four family members. Then the 139 customers that the hairdressers worked with were contacted. Customers were isolated for fourteen days. No symptoms developed, and of the 67 tested, all remained negative. All of them, hairdressers and customers, had worn mouth guards.

25. On the American aircraft carrier USS Theodore Roosevelt, a major eruption occurred in April 2020 in covid-19. When on board they were later tested for antibodies, 60% were positive. It turned out that among those who had used mouth guards, 55.8% had become infected. Among those who did not use a mouth guard, 80.8% had become infected. Avoiding common areas and observing social distancing had also had a positive effect, but not to the same extent as the use of mouth guards.

26. It has often been pointed out that "the countries that have been most effective in reducing the spread of covid-19 have been those that introduced general use of mouthguards, such as Taiwan, Hong Kong, Singapore and South Korea".
Since the beginning of April, 13 million mouth guards have been manufactured in Taiwan since the beginning of April.

27. Investigations involving infected passengers aboard flights longer than 10 hours strongly suggest that masking prevented in-flight transmissions, as demonstrated by the absence of infection developing in other passengers and crew in the 14 days following exposure. Three mass transmission flights without masking are contrasted to 5 with strict masking and 58 cases with zero transmission.

28. This danish study has been used as an argument that face masks do not protect the wearer. Therefore, here is a detailed review of the study. It included 4862 individuals who were drawn in two groups, 2392 people were allowed to wear mouth guards for one month and 2470 people were drawn to the control group without mouth guards. Patients were considered to have been infected during the study period if they responded positively in either (1) a rapid test, which detected antibodies to the virus and which the participants performed themselves at the end of the study, (2) a PCR test, which indicated ongoing viral infection, also at the end of the study or (3) was diagnosed with covid-19 disease in hospital (presumably by PCR) during the month of the study. The result was that 42 in the mouth protection group and 53 in the control group were judged to have been infected. In the oral protection group, 0 were PCR-positive at the end of the study and 5 were diagnosed with covid-19. In the control group, 5 were positive by medical analysis and 10 were diagnosed with covid-19 in the group without masks. Given that PCR positivity at the end of the study does not overlap with diagnosis in hospital, which they are unlikely to do, this means that 5 in the group wearing masks became positive against 15 in the control group, ie a protective effect of 67%! Here, however, people in both groups may have been infected at the beginning of the study but had time to become virus-free at the end of the study.
So we have the antibody test where at least 37 in the oral protection group and 38 in the control group were positive. Here we have three sources of error. The first is that the test has a false positivity of 0.5 to 2.5%. This means that between 12 and 60 people in both groups get a positive result in the test even though they do not have antibodies to SARS-CoV-2, which gives rise to covid-19. The second source of error is that it takes about 14 days from the time you become infected until you develop antibodies. This means that people in the study may have been infected almost two weeks before the study begins and still be negative in the test taken at the beginning of the study (the positives in this test were excluded from the study). This also means that if a person is infected in the second half of the study period, he or she will not have had time to develop antibodies at the end of the study. The third source of error is that 20% of the participants did not submit a test result from the test that was taken at the beginning of the study but were still allowed to participate. With the study approach used, the antibody test can thus not be used as a measure of how many have been infected in each group. 
How many could be expected to become infected in each group if oral protection had no effect given the spread of covid-19 in Denmark at the time of the study? As not all Danes who became infected during the study period (April 3 to June 2) had tested themselves, one can try to make an estimate by looking at the number who died in Denmark of covid-19 from infection during the study period. As it takes an average of 19 days from the time you become infected until you die, you must examine deaths in the period 22 April to 21 June. If one in 200 infected people dies, it can then be calculated that in the mask protection group 19 people should be infected and in the control group 20 people. To compare with the outcome of the study, if one disregards the antibody values, namely 5 in the oral protection group against 15 in the control group. Given that some who became infected at the beginning of the study period had time to become PCR-negative at the end of the period, this is a reasonable outcome. 

29. It has also been studied how much people peck in the face with and without mouth guards. Videos in public places taken in Southeast Asia, Western Europe and the United States before and after the Covid-19 pandemic show that the number of people with mouth guards has increased radically, also in Southeast Asia (from, for example, in China 1.1% before the pandemic, to 99, 4% during the pandemic). In a study of 4699 individuals before the pandemic and 2887 individuals during the pandemic, it was found from these videos that facial contact decreased by between 50 and 80% in the different countries.

30. With regard to an occasional argument that oral hygiene would 'lull people into false security', a comprehensive study has been carried out on this very issue, published in the British Medical Journal. The review ends with the words: "In 2016, Pless argued that risk compensation theory 'is a dead horse that no longer needs to be beaten.' We would add that this dead horse now needs burying to try to prevent the continued threat it poses through slowing the adoption of effective public health interventions. ”

This is just a selection of the many studies that have been published so far on how the use of face masks affects the spread of covid-19. There are many more. There are articles that show that some forms of face masks are better than others (see e.g., but to our knowledge no study shows that the general use of mouth guards increases the risk of spreading covid-19.

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Science forum Covid-19 works extensively with information and information about the ongoing pandemic and related topics and areas. Our goals are to reduce morbidity and mortality in Covid-19, to reduce the chronic disease states of those who survive Covid-19. In the association, we have expertise in areas such as virology, biology, molecular biology, epidemiology, infectious diseases, lung diseases, mathematics, political science, psychology, ethics and risk research. The chairman of the association is Professor Emeritus Anders Vahlne, more information at