COVID: do masks work? they do in Kansas

 The question keeps coming up over whether masks are actually useful. i will present one study on mask use in Kansas in this blog, and another on a Danish study in the next blog.

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MMWR just published a county level survey in Kansas from June 1 to August 23, comparing Covid-19 incidence in counties with and without a mask mandate (see https://www.cdc.gov/mmwr/volumes/69/wr/mm6947e2.htm) 

 

Details: 

-- July 2, 2020: Kansas governor issued an executive order effective July 3 requiring people to wear a mask in public spaces, though the mandate allowed counties authority to opt out 

-- as of August 11: 24 of Kansas' 105 counties accepted the state mandate or had their own mask mandate; 81 counties opted out 

    -- counties accepting the mandate account for two thirds of the Kansas population (1,960,703 people, 67%), spread throughout the state, though they did tend to cluster together 

    -- 13 of the mandated counties (54%) and 7 non-mandated ones (9%) implemented at least one other public health mitigation strategy besides masks (e.g. limiting size of gatherings and occupancy of restaurants) 

-- the only other Covid state mandates in Kansas were for mitigation strategies for schools that reopened in mid-August 

 

Results: 

-- Covid-19 incidence, by 7-day rolling average number of new cases/100,000 population: 

    -- counties that had a mask mandate: decreased a mean of 0.08 cases/100K per day, net decrease of 6% 

    -- counties without a mask mandate: increased a mean of 0.11 cases/100K per day, net increase of 100% 

-- in terms of absolute overall Covid cases over baseline, 7-day rolling average of daily cases: 

    -- June 1-7 (4-5 weeks prior to intervention): 

        -- counties that ultimately had a mask mandate: 3 cases/100K 

        -- counties that ultimately had no mask mandate: 4 cases/100K 

    -- July 3-9 (the week of the executive order): 

        -- counties just accepting a mask mandate: 17/100K, a 467% increase 

        -- counties not accepting mask mandate: 6/100K, a 50% increase 

    -- August 17-23 (5-6 weeks after the mandate): 

        -- counties that had a mask mandate: 16/100K, a 6% decrease 

        -- counties that had no mask mandate: 12/100K, a 100% increase 

-- in the 13 counties with a mask mandate, some had other mandated mitigation strategies (limiting sizes of gatherings and occupancy for restaurants), but in sensitivity analyses there were similar decreases in Covid-19 whether there were these additional mitigation strategies or not 

 

Commentary: 

-- the findings in Kansas are consistent with those observed in 15 states and the District of Columbia, where masks were mandated, as compared to states without mask mandates 

-- the Kansas findings suggested that mask wearing itself was an important intervention, since those counties appeared to benefit from masks independent of some other mitigation strategies in the sensitivity analysis [though, a sensitivity analysis does not have the statistical validity of an actual study comparing those with or without other mitigation strategies]

    -- it was pretty striking that there was a dramatic increase in cases at the time the mask mandate was accepted, markedly so in those areas accepting the mandate (?more populous areas), but this huge increased trajectory was followed by a slight decrease 5-6 weeks later; however, in counties without mask mandates, their smaller increase one week after the mandate blossomed to a 2-fold increase 5-6 weeks later

-- but, were those counties that continued with the mandate different in important respects from the other counties: were the people there more likely to believe that Covid was real? Were they more likely to embrace other, unmeasured mitigation strategies (eg distancing)? were there differences in testing frequency or differences in asymptomtatic/symptomtatic people being tested that might account for at least some of the differences in case finding?

 

-- the CDC presented a scientific brief on the use of cloth masks to control the spread SARS-CoV-2 (see https://www.cdc.gov/coronavirus/2019-ncov/more/masking-science-sars-cov2.html?fbclid=IwAR28PppCa6x2uxwO8Z2baHM0KHS4JXx0inzzMQs3zRHV1qql_0a8mxZfpCw , updated 20Nov2020), making the following points: 

    --in terms of "source" protection (masks preventing those who are infected from infecting other people)

        -- multilayer cloth masks do block respiratory particles exhaled into the environment, both large droplets (20 to 30 µm or larger) and fine droplets/aerosols (smaller than 10 µm)  

        -- these particles do increase in number with the volume of speech, and with specific types of phonation 

        -- these masks block 50 to 70% of fine droplets and particles, and also limit their forward spread 

        -- human experiments have shown blockage on the order of 80%, similar to the barrier effects of surgical masks 

   -- in terms of the effectiveness of masks for personal protection (masks protecting the mask wearers): 

        -- multiple layers of cloth with higher thread counts work better than single layers of cloth with lower counts [ie, loose bandanas partially covering the nose may not help so much...]

        -- the better ones filter nearly 50% of fine particles less than 1 µm 

        -- some materials (e.g. polypropylene) enhance filtering effectiveness through a static electric charge that traps charged aerosolized particles; other fabrics may repel moist droplets (e.g. silk) reducing fiber wetness and be more comfortable to wear 

-- there are several human observational studies suggesting masks work: 

    -- there are a few smaller studies:  2 symptomatic, Covid-infected hairstylists who wore masks and interacted for 15 minutes with 139 clients found that none of these clients developed infection; another of infected passengers on flights longer than 10 hours found that in-flight transmissions did not occur to either other passengers or the crew over the next 14 days when the infected persons wore masks

    -- an outbreak on a ship with congregate living quarters and close working environments found that mask coverings were associated with a 70% decreased risk of transmission

    -- a study of 124 households with confirmed SARS-CoV-2 infection, masking the index patient and family contacts before the index patient developed symptoms reduced transmission by 79% 

    -- and a retrospective case-control study from Thailand with more than 1000 persons interviewed as part of contract tracing found that those who always wore a mask during high-risk exposures had more than a 70% reduced risk of acquiring infection 

 

Limitations of the Kansas study: 

-- no assessment that those people who actually wore the masks in the Kansas counties with mandates did better than those who did not wear masks (i.e., was this the ecological fallacy of attributing community findings to specific interventions on individual level?: ie, was it actually those people who wore masks who had decreased viral transmission, or another factor in the communities not related to masks?) 

-- there were mask ordinances in 6 cities in non-mask mandated counties, though accounting for this did not change results 

-- they did not have data on potential important factors such as mobility patterns, changes in other mitigation strategies implemented, access to testing, or even self-distancing from others

-- and generalization of these results to other states or counties may not be an appropriate, given substantial differences in different areas (demographics, cultural, types of exposures etc)

-- to verify the singular beneficial effect of masks, we would need a robust randomized controlled trial: having participants utilize all the other mitigation strategies similarly and randomize them to mask vs no mask wearing, make sure they were actually wearing the asks appropriately, and follow them a month or so.  this would probably be impossible to do, and we would further need to know if mask wearing was beneficial in different scenarios: as a sole mitigation strategy, as an add-on to one or more other mitigation strategies, etc

-- and remember that an especially important issue as we approach the exigencies of winter and increased indoor living: with less ventilation, the virus can remain in the air for 3 hours...  (see  http://gmodestmedblogs.blogspot.com/2020/11/covid-high-risk-tranmission-from.html and http://gmodestmedblogs.blogspot.com/2020/08/covid-transmission-and-distancing.html )

so, all these data do reinforce that masks are an important aspect of decreasing viral transmission. though we do not have a rock-solid answer on the role of masks, from this and other studies, it certainly seems reasonable and appropriate that mask wearing be an important cornerstone (pardon the rock analogies...) to controlling this horrendous virus. and, the stupidest thing imaginable is to attribute a potentially very important (and easy) public health initiative with a political party.....

geoff

 

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