Transmission of COVID-19 likely starts with production of a small droplet that contains one or more SARS-CoV-2 viruses. This occurs somewhere in the lungs, throat, or mouth of an infected person. We have almost no data on this process as it occurs inside the body of an infected person, but we do know how droplets are generally made. One mechanism is named after Lord Rayleigh and Joseph Plateau, and we have all seen it in action. We see it every time we turn a tap a little bit on, and it drips. The stream of water from the tap breaks up into droplets, and the same thing may be happening inside us every time we breath.
One of the studies that convinces many (including me) that COVID-19 is mostly transmitted directly across the air, is a “superspreader” event in a restaurant in Guangzhou, China. It looks like a single infected person infected five people at other tables, in addition to four people on the same table as them. The restaurant was very badly ventilated, but there was a system that just circulated air around the table the infected person sat at, and the other two tables where people become infected. This is all perfectly consistent with the infected person breathing out virus that is then carried by air currents across the restaurant, and breathed in by people at another table. However, we can’t rule out alternative mechanisms, for example what is called the fomite route.
Covid-19: Health staff in plea for better protection is a depressing headline to be reading at any time, let alone a year and half into a pandemic. NHS healthcare workers have less protection to infection than those in many other countries, for example, the USA. Totally understandably many in the NHS are not happy about this. The Royal College of Nursing is just one of the unhappy organisations. The official infection-prevention guidelines were updated on 1st June but are still pretty hopeless, with pages on ensuring masks are stored in a “clean, dry area” (page 19) – which is a nice I guess but hardly an essential point – but almost nothing on the fact that proper FFP3 PPE masks offer you much more protection than a surgical mask.
The government now tells us “hands, face, space, fresh air”* to reduce transmission of COVID-19, but the evidence supporting these four measures is very weak. By contrast the evidence showing that the Oxford AstraZeneca vaccine I had a few weeks ago is safe, is much much stronger, and assessed much more rigorously.
Let’s take them in order:
- “Hands”: There is essentially no good evidence that washing hands and surfaces reduces the likelihood of catching COVID-19.
- “Face”: by which they mean masks. We understand fairly well how mask work, colleagues at Bristol have a paper recently out in Physics of Fluids, and this is just a small part of a lot of work over the past year, that has assessed how effective masks (of different types) are. So we have a reasonable idea of how effective a given type of mask is at filtering out virus-containing droplets of a given size, but unfortunately, we have little idea of what size of droplet are transmitting COVID-19. Vaccines like those for COVID-19 are typically tested via a randomised controlled trial (RCT), we have almost no RCT data here.
- “Space”: by which they mean 1 or 2 m “social distancing”. As you can tell by the various limits of 1 m, 2 m, 6 feet, etc suggested by different countries there is no well justified distance beyond which you are safer. But basic geometry tells you that the concentration of virus will be highest right in front of an infected person, in their breath, so there is sound logic behind not getting too close, especially if you are facing someone. No RCT data here either.
- “Fresh air”: There a observations of “superspreader” events in which multiple people are infected, and they are strongly associated with people being together indoors, and in rooms with poor ventilation, i.e., air that is the opposite of fresh. We also have a reasonable understanding of the mechanism, and a simple model (Wells-Riley model).
The UK government of Boris Johnson has pledged to build back better. We can all form our opinions as to how much good will come of this pledge, but an international group of scientists have made a suggestion for how learn from the pandemic and improve all our health in this week’s Science magazine. Morawska et al‘s point is that since, the 19th nineteenth century in the UK’s case, we have taken decisive action to combat the spread of typhus, cholera and other diseases spread by infected water. Now in the 21st century it is past time to pull our fingers out and take comparable action against diseases spread by infected air, such as tuberculosis, and COVID-19.
Last week colleagues at Bristol and I published a paper looking at how surgical and cloth masks filter out droplets, and this week I read a recent UK Government SAGE (Scientific Advisory Group For Emergencies) report: Masks for healthcare workers to mitigate airborne transmission of SARS-CoV-2. The two publications are like the proverbial ships passing in the night. There is no overlap, let alone a meeting of minds. For example, in our paper we use “filter”/”filtered” etc 34 times – because masks are basically air filters we wear on our face – the SAGE report uses “filter” etc 0 times. There is a complete absence of curiosity as to what masks actually are, and how they work.
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.”
World Health Organization, Coronavirus disease (COVID-19): How is it transmitted? – updated 30th April 2021
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.
Today I had my first dose of the Oxford AstraZeneca vaccine. Waiting for it in the queue at the G Live vaccination centre felt a bit like Christmas morning when I was kid – the same anticipation of receiving an exciting gift, although in this case it is an 80% reduced chance of death in case of COVID-19 infection. I feel very grateful to all the people at Oxford University, AstraZeneca, the G Live vaccination centre, and in other organisations that together made it possible. The centre gave me some paperwork on the vaccine to read, with side effects etc. It also said that the dose is 0.5 ml, which contains 50 billion viral particles. This got me thinking. The world population is about 7.5 billion, and at two doses per person, we need about 10 million litres of vaccine, or about four Olympic swimming pools full of vaccine.
As someone who taught himself BASIC in the 1980s and was taught Fortran 77 in the early 1990s*, I do need to try and keep up with modern developments. One of the things I have being wanting to for a while now, is find a way of putting some simple plotting/data analysis on the web, so anyone can use it. There have been solutions for this problem for a while but until yesterday I had not come across one I liked. I want to go from Python code on my local machine to a running a web app in minutes, and I want to spend a couple of hours learning how to do this at a very basic level. I am kind of busy so can’t spend a day learning this. I think I have a solution that fits these criteria, and I am pretty pleased with it.
As numerous relatives have (delightedly!) pointed out, due to my recent 50th birthday, I am now sufficiently old to be in the next or next-but-one group to be vaccinated. Turns out the advantage of getting old is not wisdom, which I don’t appear to be acquiring, but moving up the vaccine queue. So I am following all news of vaccinations with interest. Currently in the news is the suspension of use of the Oxford-AstraZeneca vaccine in a number of European countries including Germany. There is information on the basis of this decision in Germany here.