I have briefly read through the part on masks and the studies it references. Below are my opinions on it, but the tl;dr is that the review is useless and the studies it cites are very weak or inapplicable. We wouldn't approve use of ivermectin because a survey of patients showed that those with better outcomes were more likely to have taken it, so we shouldn't accept it for other interventions either. Low quality studies, such as those which use self-reporting, should only be used as hints as to where to make proper investigations - not support for society-wide interventions.
36 Effectiveness of personal protective health behaviour against COVID-19.
doi:10.1186/s12889-021-10680-5
This is based on 24 patients with Covid-19 and 1113 people without.
The data on mask wearing is solely gathered through asking people
about what they did. It also shows that mask wearing is correlated
with other precautions.
43 The role of mask mandates, stay at home orders and school closure in curbing the COVID-19 pandemic prior to vaccination. doi:10.1016/j.ajic.2021.02.002
This does not study mask wearing at all - it compares legal measures
in the 50 states of the USA. It makes no allowance for people wearing
masks when not mandated or refusing to wear maks when mandated. Since
everything about Covid seems to be politicised (especially in the USA)
it is impossible to draw conclusions from such a simple study - even
about mask mandates rather than actual mask wearing.
57 Reduction of secondary transmission of SARS-CoV-2 in households by face mask use, disinfection and social distancing: a cohort study in Beijing, China.
https://www.bmj.com/lookup/ijlink/YTozO ... owOiIiO30=
This study looks a transmission with a household. There were 41
families where transmission occurred (infecting 77 people) and 83
families with no transmission. It relied on telephone interviews, so
suffers from the usual biases of that. Its conclusions are
interesting. The biggest risk factor was "Close contact at home >=
4 times" (OR=18.26 relative to no contact), followed by "Primary case
has diarrhoea" (OR=4.10) and Ventilation < 1 hour/day
(OR=2.55). Protective effects are found for "1 or more family members
(including primary case) wearing a mask at home before primary case's
illness onset date" (OR=0.21) and "Frequency of chlorine or ethanol based
disinfectant use for house cleaning" (OR=0.23 for once a day relative
to more rarely).
60 Relationship Between COVID-19 Infection and Risk Perception, Knowledge, Attitude, and Four Nonpharmaceutical Interventions During the Late Period of the COVID-19 Epidemic in China: Online Cross-Sectional Survey of 8158 Adults.
doi:10.2196/21372
Another self-reporting study. It also has this interesting bit:
"The reasons that we also included proper coughing habit as an end
point was that the habit may not only potentially reduce other
people’s risk of developing a COVID-19 infection but may also reduce a
person’s own risk through enhanced self-protection (eg, turning away
from those who did not practice proper coughing habit) or through
indirectly influencing other people’s coughing behavior as a role
mode". They don't define "proper coughing habit", but it seems obvious
that they have found evidence of some confounding (as the coughing and
infection refer to the same person).
63 Case-Control Study of Use of Personal Protective Measures and Risk for SARS-CoV 2 Infection
doi:10.3201/eid2611.203003
Yet another self-reporting study. This time 211 asymptonatic, infected
people and 839 uninfected people. It also includes correlation data
between different measures, showing that any relevant measure not
included would be quite likely to be correlated with the others, so
inflating their apparent effect.
66 Effectiveness of Adding a Mask Recommendation to Other Public Health Measures to Prevent SARS-CoV-2 Infection in Danish Mask Wearers: A Randomized Controlled Trial.
doi:10.7326/M20-6817
This one contains the magic word "randomised" which makes it better
than the others. It also does not rely on self-reporting.
Unfortunatly, it does not test mask wearing either. It tests the
difference between people told to wear a mask and people not told to
wear a mask. It then says:
"Results: A total of 3030 participants were randomly assigned
to the recommendation to wear masks, and 2994 were assigned
to control; 4862 completed the study. Infection with SARS-CoV-
2 occurred in 42 participants recommended masks (1.8%) and
53 control participants (2.1%). The between-group difference
was 0.3 percentage point (95% CI, 1.2 to 0.4 percentage
point; P = 0.38) (odds ratio, 0.82 [CI, 0.54 to 1.23]; P = 0.33).
Multiple imputation accounting for loss to follow-up yielded sim-
ilar results. Although the difference observed was not statistically
significant, the 95% CIs are compatible with a 46% reduction to
a 23% increase in infection."
I see I have been ninja'd by the BMJ as mentioned above, but their article has some different points, so well worth reading.