Did a recent CDC study show that 85% of people who wear masks get COVID?

Masks Uncertainty and Misinformation

A: No. The CDC study showed that close contact with people who had COVID-19 or visiting locations that offered on-site eating and drinking were associated with getting COVID.

This study does not offer any information about whether wearing a mask is a risk factor for getting COVID.

So where does this 85% figure come from?

In the study, they also reported that 85% of those who tested positive for COVID-19 said they had worn a mask “always” or “often” and that 88.7% of those who tested negative for COVID-19 said they had worn a mask either “always” or “often”.

This DOES NOT MEAN that 85% of people who wore masks got COVID-19. It actually means there wasn’t much difference in the proportion of cases and non-cases that reported wearing masks “always” or “often”. Or if anything, a slightly greater proportion of people who DIDN’T GET COVID-19 reported wearing masks frequently, compared to those who got COVID-19.

Importantly though, this study wasn’t intended to assess the effect of wearing face masks on getting infected with COVID-19 in the first place and several limitations to how the data was collected make it challenging to make conclusions about this.

First, while they reported how many cases and non-cases reported wearing masks “always” or “often”, we don’t actually know anything about how often they took them on or off, or whether they wore them over their nose and mouth, so individuals who reported “always” or “often” wearing a mask might not actually have done much mask wearing in practice. This means the level of mask use reported by individuals in the study might be misclassified (epidemiologists refer to this as measurement error).

Second, individuals were asked about their mask use AFTER finding out if they had COVID-19 or not. Knowing whether you have COVID-19 might influence your reporting of how often you wore masks in the past 14 days. For example, cases of COVID-19 may over-report mask use compared to non-cases because they are worried about being perceived as not taking the correct precautions (epidemiologists refer to this as reporting bias). Those who don’t have COVID-19 might also be less likely to remember their exact mask use because they aren’t thinking about it to the same extent as cases (epidemiologists refer to this as recall bias).

Third, cases of COVID-19 were more likely to report eating out in restaurants which are settings where masks are required, but people take them on and off to eat, and also more likely to report being in close contact with a case of COVID-19, where they may have been wearing a mask but were < 6ft from the individual for > 15 minutes. For these reasons, any assessment of the association between mask use and becoming a case of COVID-19, would need to take into account the extent to which mask wearers dined out or didn’t social distance from cases of COVID-19, which isn’t the case (epidemiologists refer to this as adjustment for confounding).

Overall, the way the data was collected and the analyses that were reported in this study makes it hard to say anything conclusive about whether mask use was associated with being a case of COVID-19, and importantly, that WASN’T THE POINT of the investigation.

What this study DOES confirm is that being in close contact with those infected with COVID-19 and spending time in settings where masks might be taken on and off frequently such as restaurants IS associated with being a case of COVID-19. This supports what we already know about the benefits of social distancing and wearing masks for reducing risk of transmission of COVID-19, so keep staying #SMART.

See a CNN article highlighting these points here.

For the original study report, see here.

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