A few days ago, the World Health Organisation (WHO) updated its guidance on how COVID-19 transmitted. It now says:
“Current evidence suggests that the virus spreads mainly between people who are in close contact with each other, typically within 1 metre (short-range). A person can be infected when aerosols or droplets containing the virus are inhaled or come directly into contact with the eyes, nose, or mouth.
The virus can also spread in poorly ventilated and/or crowded indoor settings, where people tend to spend longer periods of time. This is because aerosols remain suspended in the air or travel farther than 1 metre (long-range).
People may also become infected by touching surfaces that have been contaminated by the virus when touching their eyes, nose or mouth without cleaning their hands.”
This is not too bad. I am not aware of any evidence that COVID-19 spreads “mainly” between people when they are close, as opposed to between people a few metres away but in the same indoor space. And the phrase “aerosols or droplets” when just “aerosols” would be simpler, is I think a final vestige of some medics using the term “respiratory droplets” to mean droplets that are somewhere so large they can’t go far but are simultaneously somehow responsible for most transmission. With successive revisions of the WHO’s guidance, this misleading “respiratory droplet” is thankfully fading from view.
So, there is progress with the WHO, what about Public Health England (PHE)? Here the picture is not as good. As of time of writing (3rd May 2021), to combat COVID-19 transmission PHE says:
“a) Contact precautions
Used to prevent and control infections that spread via direct contact with the patient or indirectly from the patient’s immediate care environment (including care equipment). This is the most common route of cross-infection transmission. COVID-19 can be spread via this route.b) Droplet precautions
Used to prevent and control infections spread over short distances (at least 3 feet/1metre) via droplets (>5µm) from the respiratory tract of individuals directly onto a mucosal surfaces or conjunctivae of another individual. Droplets penetrate the respiratory system to above the alveolar level. COVID-19 is predominantly spread via this route and the precautionary distance has been increased to 2 metres in care settings.c) Airborne precautions
Public Health England, COVID-19 infection prevention and control guidance: transmission based precautions (TBPs), accessed 3rd May 2021
Used to prevent and control infection spread without necessarily having close patient contact via aerosols (=5µm) from the respiratory tract of one individual directly onto a mucosal surface or conjunctivae of another individual. Aerosols penetrate the respiratory system to the alveolar level. COVID-19 can spread via this route when aerosol generating procedures (AGPs) are undertaken.”
Note that PHE appears to be implying that airborne transmission across distances greater than a metre only occurs when a type of medical procedure called an aerosol generating procedure (AGP) occurs*. This is wrong, the evidence is very strong that you can be infected even if you never get within metres of an infected person. And the division into droplets that are larger than 5 micrometres and aerosols that are smaller is also demonstrably nonsense.
And I think this matters, PHE guidance (as of 3rd May) still talks about the use of the professional FFP2/FFP3 masks as being: “used to prevent inhalation of small airborne particles arising from aerosol generating procedures (AGPs).” We have known since at the latest the 1940s that just breathing generates small aerosol particles**, most of which are less than 5 micrometres across. So equipping nurses and doctors treating COVID-19 patients with simple surgical masks, not the possibly ten times*** better FFP2/FFP3 masks, seems like letting them down.
Now that the WHO is moving with what I think/hope is becoming a scientific consensus, this leaves PHE behind. I think PHE should catch up.
* For this post just take it that some medical procedures are classified as AGPs – I will try and do a post on them as the evidence for AGP classification is just shockingly bad, but that is another story.
** See work of JP Duguid, published 1946.
*** Limitations in our knowledge of how COVID-19 spreads mean we only have rough estimates of how much protection surgical masks and the professional PDE FFP2/FFP3 masks offer. These estimates are that a reasonably well fitting surgical mask should reduce exposure to virus by maybe between 50 and 75%, while an FFP3 mask should reduce it by 99%.
Chance & Necessity 