(1/16) However, I don't follow the argument that the guidance has changed because of new evidence. The evidence has been there all along. I have a few comments on the most recent review in the Lancet
(2/16) My team worked with WHO last year and came up with these pandemic influenza guidelines. This included a recommendation for face mask use in the community during severe or very severe influenza pandemics, despite a limited evidence base. who.int/influenza/publ…
(3/16) Our review of the evidence base for community face mask use in influenza pandemics was also published in EID wwwnc.cdc.gov/eid/article/26…. We found evidence against substantial effects of masks, but mechanistic plausibility that masks would have some effect on transmission. 
(4/16) We recommended masks while acknowledging they would not be sufficient to stop the spread of influenza, and other measures would also be needed. The same can be argued for COVID-19 thelancet.com/journals/lanre… bmj.com/content/369/bm…
(5/16) In the Lancet, a top tier medical journal with the highest standard of peer review and editorial processes, Chu et al recently published a review of physical distancing, face masks, and eye protection for COVID-19 thelancet.com/journals/lance…
(6/16) This particular paper, acknowledging funding by the WHO, draws some conclusions that I find unconvincing.
(7/16) I think mixing together observational data from healthcare and community studies is not a good idea. PPE such as masks and eye protection work very well in healthcare settings not only because of the equipment but because of the training, knowledge, and environment...
(8/16) ... it is inappropriate to use data from studies done in healthcare settings to make recommendations on control measures that could be used in the community, because these are very different settings.
(9/16) There is no strong reason to believe that COVID-19 transmission patterns are more like SARS and MERS than other respiratory viruses(?) SARS+MERS spread mainly in hospitals, whereas COVID-19 is mainly spreading in the community. Why ignore data on other respiratory viruses?
(10/16) One sentence jumps out in the discussion – “At the moment, although there is consensus that SARS-CoV-2 mainly spreads through large droplets and contact …” – I don’t think there is any such consensus. There is quite a lot of disagreement in fact! @Don_Milton @linseymarr
(11/16) On physical distancing, we are aware that risk of COVID-19 is increased by “close contact”. Duration and distance both matter. This review remarkably fails to take duration into account. So we can spend hours in a room with a COVID-19 case as long as we stay 2m away?!
(12/16) On face masks, only three community studies are included. And authors may not have realised that data on masks in Lau et al (ref 50) actually refer to mask use when visiting a family member with SARS in hospital, which is a healthcare exposure.
(13/16) The results on eye protection are the most fascinating, Chu et al estimate that eye protection can reduce the risk of infection by 75%. Would widespread distribution of goggles be enough to stop COVID-19 from spreading?!
(14/16) More likely, eye protection was associated with use of other PPE such as masks/respirators, and the massive reductions in risk shown here are not because of the eye protection. We might prefer to have estimates of the benefit of eye protection /on top of face masks/.
(15/16) The use of unadjusted odds ratios from case control studies is difficult to justify when the objective is to derive estimates of the causal effect of these measures in preventing transmission. There are many potential confounders. Not clear how risk ratios were derived.
(16/16) My conclusion – this review of mostly older studies has important findings for healthcare workers, indicating that airborne precautions and eye protection might be justified. But it is not appropriate to extrapolate these findings to the community.
