covid: masks work, yet again

 A report by the CDC confirmed that indoor mask use decreased SARS-CoV-2 transmission, with the N95/KN95 outperforming surgical masks (see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8830622/ ).

Details:

-- test-negative case-controlled study of randomized California residents receiving SARS-CoV-2 tests between Feb 18 and Dec 1, 2021

-- face mask or respirator use was assessed in participants who were positive for SARS-CoV-2 vs matched control participants who had negative SARS-CoV-2 tests and had been in indoor settings (retail stores, restaurants and bars, recreational facilities, public transit, salons, movie theaters, religious services, schools, or museums) in the prior 14 days and had no known contact with anyone with confirmed or suspected SARS-CoV-2 infection during that time

-- 1528 case-participants (SARS-CoV-2 positive) were compared to 1511 control patients, matched by age group, sex, and state region; all this from a total of 11,387 case and 17,051 control participants

-- age 0-6yo 3%/7-12yo 4%/13-17yo 4%/18-29yo 31%/30-49yo 35%/50-64yo 16%/>65yo 7%; 51% female; annual income <$50K 25%; white 44%/Black 5%/ Hispanic 28%

-- unvaccinated: 78% of cases vs 58% controls/fully vaccinated 18% cases vs 32% controls

-- reasons for testing: symptomatic 78% cases/17% controls; required for medical procedure 6% cases/17% controls; routine testing thru work/school 11% cases/43% controls

-- patients were excluded if they had a previous positive test result of any sort, or clinical diagnosis of Covid

-- case-positive participants were randomly selected from those who were SARS-CoV-2 positive during the prior 48 hours and invited to be interviewed by phone

-- those enrolled from September 9 to December 1, 2021 were also asked type of face covering typically worn: (N95/KN95) respirator, surgical, or cloth mask in the indoor settings

-- primary analysis: self-reported facemask or respirator use in the indoor settings 14 days before SARS-CoV-2 testing between cases and controls; secondary analyses accounted for the consistency of facemask or respirator use

-- statistical adjustment for self-reported covid vaccination status and what type, household income, race/ethnicity, age, sex, state region, country population density

Results:

-- mask usage, comparing the 14-day period before covid testing:

    -- none: 44 (6.7%) positive cases/42 (3.6%) negative controls (referent group)

    -- any use: 608 (93%) vs 1134 (96%), adjusted OR 0.51 (0-29-0.93), p=0.03

    -- some of the time: 62 (10%) vs 76 (7%), adjusted OR 0.71 (0.35-1.46), nonsignificant

    -- most of the time: 153 (24%) vs 239 (20%), adjusted OR 0.55 (0.29-1.05), p=0.07 (almost statistically significant)

    -- all of the time: 393 (60%) vs 819 (70%), adjusted OR 0.44 (0.24-0.82), p<0.01

--534 people specified type of face covering, vs no mask:

    -- N95/KN95 use: 10 (4% of positive cases) vs 21 (8% of negative controls), aOR 0.17 (0.05-0.64), p <0.01

    -- surgical mask: 113 (44%) vs 139 (51%), aOR 0.34 (0.13-0.90), p=0.03

    -- cloth mask: 112 (43%) vs 104 (38%), aOR 0.44 (0.17-1.17), p=0.10 (trend to being significant)

Commentary:

-- this study did find that face coverings do help, that N95/KN95s are better than surgical masks,  which seemed tp be better than cloth masks. But the real public health issue is just wearing one... eg, if the N95/KN95 is too uncomfortable, wearing a well-fitting surgical mask regularly is certainly better than nothing

-- this study confirms others that face masks are beneficial for decreasing SARS-CoV-2 transmission, through different SARS-CoV-2 variants (see http://gmodestmedblogs.blogspot.com/2021/09/covid-importance-of-masks-and.html, or http://gmodestmedblogs.blogspot.com/2021/10/covid-aerosol-spread-worse-with.html for a few of the prior blogs), including both protection from getting infected as well as source control

Limitations:

-- there might be a selection bias here: were those using N95/KN95 more concerned about SARS-CoV-2 than those using surgical masks? if so, perhaps they were more adherent to other mitigation strategies (minimizing public exposures, using masks indoors with others, having HEPA filters or better air circulation at home??) which might have biased their findings. in this case, the actual benefit they attributed to the N95/KN95 masks may be a bit overstated since other mitigation strategies might have added to their results for "masks" (and they did not assess other mitigation strategies that might have been used)

-- using broad categories of age matching could obscure some important differences:  perhaps those in the 50-64 bracket, for example, were predominantly closer to 50 than 64 in one group but vice versa in the other; or, one group had patients in the >65 group who averaged 66yo but the other group averaged 86yo? could distort the results (this is a general criticism of using groupings instead of a continuum of ages)

-- this group overall used masks quite frequently (60-70% used them all the time), which might limit generalizability to other areas of the country, and also might reflect more assiduous use of other mitigation strategies, as noted above

-- no breakdown for mask utility in asymptomatic vs symptomatic persons, since they may have different outcomes. though of note above, 78% of cases vs 17% of controls were symptomatic, making it highly likely that there was significant asymptomatic transmission in the control group (and perhaps if they assessed only those who were symptomatic, masks might have afforded even more protection??)

-- this was not a random sample of the population, but those seeking testing. and there was no breakdown in the results between those being tested by requirement vs by symptoms

-- the data reflected self-reported activities which might not truly reflect what people did (and, those agreeing to participate might well have exaggerated their stated mask use, perhaps biased by their political proclivities...)

-- also, people might have used mixed types of masks and not fit neatly into the categories above. 

-- no objective data on correctness of mask use. it might well be that those with N95/KN95s did not use the mask throughout their contact with others because the masks were not comfortable (and the hot California weather might have led to even less comfort); alternatively, they might have used them more regularly since they chose to use N95/KN95s and were therefore more likely to be more keyed into self-protection

-- this study was pre-omicron and might not apply to more transmissible variants now and in the future (eg omicron variant BA.2)

-- no data on the specifics of the covid exposures people had. Were more of the non-mask users in venues with likely higher numbers of covid cases/probability of transmission (eg bars are probably worse than shopping in large store with tall ceilings and good ventilation...)?

so, a useful study presenting results of the real-world benefits of masking during the time of the delta variant galloping through the country. This publication was quite timely given the current country-wide shift to decreasing mitigation strategies (and, i personally do have lots of covid fatigue, like probably almost all of us). But the issue is that covid is not a benign disease, even with mild infections, since "long covid"/PASC can be so devastating even in young people who had mild or asymptomatic infection. And we are likely to have another variant emerging.  the jury is still out about the BA.2 omicron variant, but other variants will undoubtedly follow. So, we should all celebrate the dramatic improvements with covid now, and this is indeed a window of opportunity to do more "normal" things, like seeing friends/family, traveling. But, wearing a mask in a higher risk venue (concerts, densely populated venues, unvaccinated friends/family) seems like a pretty easy price to pay for avoiding a potentially devastating infection, given its potential long-term, life-changing possibilities.... And, of course, the ongoing mantra of vaccinate, vaccinate, vaccinate

geoff

 

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