Thread by @MackayIM

We know masks/respirators mechanistically filter our particles–effectiveness varying with type, style, fit and use.

•They work in practice, not only in theory.

But that doesn't mean they work all the time.

The use of 'masks' (overarching term) 𝗰𝗮𝗻 reduce the risk of transmitting an airborne pathogen.
They are imperfect. They are not a panacea. They cannot be used alone. They are a leaky\"holey layer". This means that outbreaks & epidemics still progress.

If you define "work" as 100% effective, then yes, they absolutely fail. That’s a stupid definition by the way.

•There is entrenched mythology that only healthcare settings need/should use N95/P2s (or better) masks.

Sadly, in the middle of a respiratory virus epidemic..

..outside that setting, we can often be exposed to equivalent levels of risk due to prolonged periods sharing airspace with those shedding aerosols containing high viral loads.

•Time, virus load, not being worn or being worn poorly will all increase the risk that masks fail.

All of those things can be overcome with training and making better choices. We could improve the former (but haven't), but the latter is up to us.

•Wearing no mask 𝗴𝘂𝗮𝗿𝗮𝗻𝘁𝗲𝗲𝘀 no reduction of risk. Vaccines alone won't remove all risk. Neither will handwashing.

Some say masks are useless at the community level.

But 𝘱𝘭𝘦𝘢𝘴𝘦 ensure they provide you the trajectory of that epidemic 𝗶𝗻 𝘁𝗵𝗮𝘁 𝘀𝗮𝗺𝗲 𝗽𝗼𝗽𝘂𝗹𝗮𝘁𝗶𝗼𝗻 if they went without any masks at all: they should contrast both with & without masks

How many people were infected? How many got a reduced infectious dose? Did a reduced dose change their disease course? How many had serious disease? How many died?

•Don’t accept their “evidence” that masks don’t make 𝘢𝘯𝘺 difference – they can’t assure that.

We do have data that show masks help reduce some risk. But they need to be one layer among many – vaccines, air cleaning, (better, properly worn) masks and distance are top of the list of useful ways to interrupt transmission of many respiratory viruses.

And this is a problem unique to mask. What intervention to reduce risk that involves us flawed beings doing something, has ever been 100% perfect?

I don’t mean removing the risk source entirely – clearly, we can’t do that with SARS-CoV-2 right now.

We also can’t remove cars from the roads to prevent road deaths.

At the community level then, are there any other imperfect but functional risk reduction measures we don't get quite so angry about adopting?

🔵Food expiry dates-don't prevent foodborne disease but reduce risk compared to no such dates\regulation
🔵Handwashing-reduces transfer of faecal-oral pathogens but doesn't prevent outbreaks

🔵Vaccines-reduce severe disease& death but don't always prevent it (although some are much better than others)
🔵We treat our tap water-make sit safe but can still be affected by mistakes
🔵Chlorinate public swimming pools-but can still mess it up resulting in disease outbreaks

🔵We separate our sewage from us and treat our waste-can fail or overflow in floods
🔵Before this pandemic we isolated exotic diseases-can still have additional infections art times
🔵HCWs get trained, but that doesn’t prevent them from making mistakes

🔵Laboratory scientists train but that doesn’t mean research mistakes don’t happen or that lab tests aren’t wrong sometimes
🔵Condoms reduce the risk of unwanted pregnancy & transmission of sexually transmitted diseases – but bad choices, mistakes and failures happen

🔵Healthcare workers use infection control in hospitals to prevent nosocomial infections-but these still occur
🔵Seatbelts-don't prevent car accidents but reduce accident deaths

🔵Speeding laws and fines-reduce car accidents but don't prevent them entirely-we still make bad decisions, drive drunk
🔵Traffic lights- must be obeyed but sometimes humans don’t and fines/harm result

🔵We have to train and pay for a drivers license-some still drive without them and cause harm despite training
🔵𝗖𝗼𝗺𝗯𝗶𝗻𝗲 training, seatbelts, speed limits, laws and fines and you better reduce risk of harm due to driving than any one or two items alone. #layers

Masks reduce infection risk & better masks worn well have a better effect.
But it's not that simple. We need masks plus other layers-vaccines, distance & cleaned air really reduce transmission.
Nowhere has done everything.
Masks alone won't stop epidemics. But they'll help.