COVID Risk Reduction

A systematic review and meta-analysis of how distance, masks, and eye protection reduce virus transmission was published in The Lancet on June 1, 2020. The scope of the review included 172 observational studies across 16 countries and six continents. There are three key findings involving over 25,000 participants in health-care and non-health-care settings.

  1. The Absolute Risk (AR) for transmission without distancing is 12.8%, versus 2.6% with distancing.
  2. The AR of virus transmission without masks is 17.4%, versus 3.1% with masks (surgical or N95).
  3. The AR of virus transmission without eye protection is 16%, versus 5.5% with eye protection.

NOTE: according to “A Dictionary of Epidemiology” put out by the the International Epidemiological Association, Absolute Risk is defined as “The probability of an event (usually adverse, but it may also be beneficial) in a closed population over a specified time interval. The number of events in a group divided by the total number of subjects in that group”

Those percentages might seem small, but under the assumption that each probability (AR) is independent of the other, the following things are true based on these findings.

  1. The combined Absolute Risk for virus transmission from NOT distancing and NOT wearing a mask is the product of each AR, or 12.8% times 17.4% = 2.2%
  2. The combined Absolute Risk for virus transmission from distancing and wearing a mask is the product of each AR, or 2.6% times 3.1% = 0.0806%

At first glance, that may not seem as dramatic as it is, so consider these interpretations of the decimal forms of these numbers.

  • AR of NOT distancing and NOT masking = 0.128 x 0.174 = 0.022272 = 22,272 per million.
  • AR of distancing and masking = 0.022 x 0.031 = 0.000806 = 806 per million.

On a per million basis, the risk is reduced by a factor of 28 for distance plus mask mitigation. Another way to see the difference in gross terms is 2 per 100 for NO mask+distance versus 8 per 10,000 for mask+distance. Now consider the fact that your probability of winning the Powerball Lottery is 1 in 175 million. I’d love to swap virus transmission probability with the Powerball for just a few weeks.


BUT wait! Shouldn’t the combined risk of NOT masking and NOT distancing be bigger than either risk factor? Yes, that makes sense. However, we DO have the combined risk of both masking and distancing being lower than either probability, which also makes sense. These musings are based on the acceptance that Absolute Risk is a probability and that these AR’s are independent of one another. That might be wrong.

Perhaps we can kick this idea around in the Comments section?

No matter what the final answer is on combined risk, the clear finding in the Lancet paper is a great deal of risk reduction by masks and distance.


Risk reduction is NOT the same as risk elimination. However, with respect to COVID-19, the risk reduction that accompanies the simple mitigations of distance plus masks is “the cake” flavored kick COVID to the curb that is only made complete by “the icing” known as widespread testing and contact tracing for isolation of the infected. Countries like Australia, New Zealand, Taiwan, and others have done just that. Masks plus distance come very close to stopping the transmission of a virus, but effective contact tracing leading to the isolation of the infected can’t occur unless most of the population gets tested. The USA is doing a terrible job with all 4 of these things.

One might ask about the fact that masks in this analysis are either surgical masks or N95 masks, things that most folks do not have. There is plenty of research showing that many combinations of various cloth materials are at least as effective as a surgical mask, with some exceeding the effectiveness of an N95. Moreover, there is barely one percent difference in effectiveness between surgical masks and N95, in spite of having entirely different functional purposes. In other words, you can have a mask other than what these studies referred to that is as good or better, which means that YOU can obtain the same risk reduction provided that WE comply along with you.

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