Study offers more evidence that face masks can prevent SARS-CoV-2 transmission

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) – the causative pathogen of coronavirus disease 2019 (COVID-19) –  spreads primarily through the respiratory route, especially respiratory droplets and aerosols expelled from infected individuals when they speak, cough or sneeze.

In response, the world plunged into a frenzy of non-pharmaceutical interventions (NPIs), such as border closures, social distancing, school and business closures and face mask use. The latter has been controversial in a few countries, notably the USA.

A new study released as a preprint on the medRxiv* server suggests that the use of face masks, of various designs, successfully prevents SARS-CoV-2 transmission. This supports the continued implementation of mask policies in public places until herd immunity is achieved in the region involved.


Masks are unique instruments of protection in that they protect not only the wearers but also the individuals around them. Epidemiological evidence suggests that masks effectively reduce viral particle emission into the surroundings.

With respect to the type of masks, the surgical mask of non-woven material filters out the virus better than cloth masks, but the N95 mask is the most efficient. With masks being out of reach for low-income individuals in many countries, homemade coverings have been used widely in the event of shortages.

The current paper from Brazil, where the virus has wrought great havoc, compared virus-blocking efficacy between different mask types, showing that all were useful in preventing viral transmission and were worth using in the current pandemic.

What were the study findings?

The researchers collected pieces from 45 masks, both woven and non-woven, worn by 45 suspected COVID-19 patients. The latter made up the majority (67%) of the sample. All samples were positive for the virus, but only on the inner surface of the masks.

Not a single mask showed the presence of the virus on the outer surface.

The median cycle threshold (Ct) values were approximately 28 for the swabs and 38 for the mask samples. Thus, the Ct values were 3 logs lower, corresponding to a difference of 1,000 copies/mL, for the mask samples compared to the swabs.

There was no significant difference in the use of cloth or surgical masks, with the median Ct value being 36 and 40, respectively, compared to 25 and 31 for the swab Ct values in the wearers of either type of mask.

The findings of this study contradict the anti-mask messaging that is currently circulating through the Americas, despite the surge in cases in many countries in these continents. Overall, the protective effect of masks in preventing the spread of the virus from an infected individual to another is obvious.

By blocking the passage of infectious viral particles in respiratory aerosols and droplets, mask use benefits the community by preventing viral transmission. This effect is independent of the type of mask used, supporting the use of homemade or commercially used cloth masks.

Relationship between severe acute respiratory syndrome coronavirus 2 viral loads (Ct values) of nasopharyngeal swabs and masks used by infected patients. CT value, cycle threshold value.

Relationship between severe acute respiratory syndrome coronavirus 2 viral loads (Ct values) of nasopharyngeal swabs and masks used by infected patients. CT value, cycle threshold value.

What are the implications?

These findings validate those of earlier studies showing that masks can be coupled with social distancing to significantly reduce the spread of viral particles – and hence the incidence of new cases. Models that simulated human breathing have shown that 40% to 95% of virus particles were blocked by either cloth or non-woven masks.

Interestingly, the current study showed that viral genetic material was present only on the inner aspect of the mask, and that the Ct values were significantly lower in mask samples. This supports the strong protective effect of using masks against viral transmission.

Despite the fact that some researchers have demonstrated that cloth masks are inferior in filtration efficiency compared to N95 masks, the team concluded that “woven, and non-woven cloth masks may be used to reduce the transmission of SARS-CoV-2 and to filter viral particles.”

Since social distancing is not always possible with large low-income families sharing cramped quarters, or in crowded slums, masks assume a larger role. They are also important when sharing public transport, even more so in the presence of a crowd.

In this study, the investigators also noted the difference in Ct values in samples and swabs from men and women, with the latter showing higher values, indicating lower viral loads. Further research is required to explore this phenomenon.

Finally, lower Ct values were found in symptomatic patients relative to asymptomatic ones, suggesting that viral load is proportional to the Ct.

The team concludes:

Our results provided real-life evidence regarding blocking of viral transmission by masks used by individuals infected by SARS-CoV-2. Furthermore, the results also reinforce the suggestion to use a mask by everyone, regardless of whether the individual is infected or not.”

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

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