Sat. Oct 1st, 2022
Image of a masked person on a commuter train.
enlarge Commuters wear face masks as they travel on the London Underground on June 12, 2020 as lockdown measures are eased amid the novel coronavirus COVID-19 pandemic.

TOLGA AKMEN/AFP

Advice on whether or not to use face masks to limit the spread of the pandemic varies from country to country, even by location within countries. This policy had to weigh up whether there were enough supplies for medical personnel to divert some of them to the general public. And the whole matter was decided without a clear idea of ​​whether face masks were actually effective against SARS-CoV-2.

But there’s reason to believe that masks would be at least somewhat effective, based on studies of the dispersion of droplets of material we emit when coughing or sneezing. And a recent analysis suggested that a large group of individual studies collectively pointed to their effectiveness. But that analysis left a great deal of uncertainty about how effective they would be at the population level and how face mask use would interact with other policy decisions.

The situation required us to have population-level modeling, which a group of British scientists has now provided. The group’s model indicates that face masks may not be particularly effective at slowing the spread of SARS-CoV-2 — as long as they limit the spread of the virus from infected people, they can limit the pandemic, even if they make masks carriers more prone to infections. But to really get the pandemic under control, masks will have to be combined with a lockdown if we want to see the total infected population shrink.

Masks vs Virus

Mask usage actually has two different functions. To some extent, it limits the ability of infected people to release infectious particles into the environment. And to a possibly different degree, it restricts access to two of the main routes by which those particles can reach new hosts: the mouth and nose. It is not a complete solution, as a person’s eyes are still uncovered and the degree of effectiveness will vary based on the number of potentially infectious particles filtered out. Still, even a rough mask will probably catch many of the largest particles we produce, and those are the ones that will carry the most viruses.

There is also some uncertainty about the behavior of the virus. We don’t know how much of it is present in a typical droplet emitted by an infected person or how long the virus remains infectious once those particles are released into the environment. There is also some residual uncertainty about when a person becomes contagious relative to the onset of symptoms. While some of these unanswered questions do not change the effectiveness of masks, they can affect the effectiveness of mask use policies.

To get around these issues, the researchers decided to model a wide variety of conditions, some where the face masks were only slightly effective and others where they blocked much of the virus’ spread. The authors even used models in which wearing a face mask was supposed to be wearing increase the chance of someone becoming infected by having people bring their hands in front of their face to adjust the fit of the mask.

To answer all of these questions, the researchers relied on two different models. The first was something called a “branching process model” that raised the question of how effective a face mask had to be before it could affect the transmission rate of SARS-CoV-2, based on the number of people wearing masks. They used this model with two different baseline rates of infectivity and looked at how different levels of effectiveness and use changed those rates.

Different scenarios test the different effectiveness of masks, as well as the frequency of their use.  The authors looked at situations with high (blue) and moderate (red) infectivity.
enlarge Different scenarios test the different effectiveness of masks, as well as the frequency of their use. The authors looked at situations with high (blue) and moderate (red) infectivity.

In the chart above, the blue bars represent a high degree of infectiousness (each infection results in four extra) and the second a more moderate degree (2.2 new infections for each infection). Even low levels of use of ineffective masks were able to reduce the rate of viral spread to a rate of 1, below which the pandemic would gradually stop spreading.

But for the most part, mask use alone isn’t able to get there. If, as in the top row, people start wearing masks after symptoms start, there are no scenarios where face masks alone can stop the pandemic — even if they’re 95 percent effective and everyone with symptoms is wearing them. On the other hand, if everyone wears them all the time, even a 75 percent effective mask could reduce the number of new infections on its own.

More Policies

But the use of face masks probably won’t take place in a vacuum; it will be part of a series of policy solutions being implemented in response to the pandemic. So the authors built a second model based on a standard epidemiological approach that divides a population into groups of susceptible, infected and recovered people. They added a fair amount of complexity to this approach, dividing the population into mask-wearers and those without masks, exposed, asymptomatic and symptomatic groups, and building an exposure process that took into account the formation and spread of virus-containing droplets.

This last part of the model was essential to consider the role of masks, as they influence both the dispersion of these droplets in the environment and the exposure of a susceptible individual to them. The model also assumes that anyone who reaches the recovered state will be immune to further infection — something that has yet to be confirmed.

Running the model without a facemask and lockdowns in response to high infection levels delivers what virtually every model has seen: a major infection spike triggering a lockdown, a recovery period during which lockdown is eased, followed by an additional spike. In this model, three individual peaks are seen before sufficient immunity is achieved to delay further waves of infection.

Assuming a face mask that is even less effective than cotton cloth at preventing the spread of droplets, a 50 percent percentage of mask use can delay further spikes. For example, the second peak would start at about the same time that the population is already in lockdown under the mask-less scenario. At 100 percent face mask use, there is only a single wave of infections and then the pandemic starts to subside. In this scenario, the infections will decrease even if face masks are only 50 percent effective. At 95 percent mask use, an N95 level of protection is sufficient to mitigate the pandemic.

As mentioned above, the researchers also considered a scenario where wearing masks makes people Lake prone to infection, as they touch their face more often due to the presence of the mask. While mask wearers suffer from this scenario, the general population still benefits under most circumstances where at least two-thirds of the population wears masks. That’s because there are so many fewer infectious particles around that it makes up for the increased sensitivity.

Models meet the real world

At this point, we simply don’t know enough about SARS-CoV-2 and protective clothing to judge which of these models best reflects reality. But the models do set reasonable limits to what we might aspire to. For example, they indicate that masks don’t have to be particularly good if we get enough people to wear them and link their use to other policy initiatives.

A few clues that mask use works in the real world are beginning to emerge. For example, an economic institute in Germany looked at the implementation of mask rules in the city of Jena and compared it with other areas in Germany. It concluded that the rules reduced infection rate growth by 40 percent. (Note that this is the growth rate, not the overall infection rate.) A non-peer-reviewed study published by PNAS found that mask use made a difference in China, Italy and the US, although some of the data isn’t quite compelling. (Looking specifically at Figure 3A, the rules for face masks appear to have been introduced after infections were already on a downward trend.) Nothing definite yet, but there is some suggestive evidence and no signs that face mask use is making things worse. makes things worse.

Proceedings of the Royal Society A2020. DOI: 10.1098/rspa.2020.0376 (About DOIs).

By akfire1

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