There’s nothing we can do about the spread of Covid. 1: the argument from mechanism.

The null hypothesis
Once Covid has spread significantly among a population, normal lockdown actions have no significant effect on the spread of the disease.

Definitions of significant and normal
By ‘spread significantly’ I mean the sort of spread that was observable in most countries, the UK in particular, well before there was a general awareness of the presence of the virus, say in January 2020.
By ‘normal lockdown actions’ I mean those that were applied generally in the UK.
By ‘no significant effect on spread’ I mean no effect that could not reasonably be regarded as a statistical fluctuation.
Another way of stating the null hypothesis is ‘by the time we know we have Covid, there is nothing we can do about it.’

Reasons for the null hypothesis
There is an excellent paper, the Routes of Transmission of the Influenza Virus, published by the Department of Heath. Summarising relevant content and applying it to Covid one sees the following.

1. Scientists have tried to transmit viruses by contact with surfaces (fomites). But their best attempts to do so have failed. This accords with the lack of evidence for the fomite transmission of Covid. So all our washing supermarket trolleys, prohibiting trying clothes on in shops and cleaning chairs has likely done nothing to restrict the transmission of Covid and has been a waste of time. As the Nature article quoted is titled, Why are we still deep cleaning?

2. Respiratory viruses are transmitted by personal contact. The evidence for this is good. This is easy to stop. We’ve stopped it and Covid has still spread. From this we conclude that Covid spreads by means other than fomites and personal contact.

3. There is no doubt that respiratory viruses are transmitted by droplets. But droplets fall to the ground in a few feet. In any case, droplets are readily stopped by the upper respiratory tract and do not penetrate deeply into the lungs, where the body is more susceptible to infections. In any case, we’ve massively restricted the transmission by droplets by stopping stopped sneezing and coughing in public and the wearing of masks. Even so, Covid has spread. So there must be a mechanism other than fomite, personal contact and droplet that transmits Covid.

4. The final mechanism for transmission of respiratory viruses is aerosols, about which there is some knowledge, but there are clear limits to our knowledge. So far as the evidence pre-covid about influenza is concerned, the Department for Health paper referred above states:
i) There is good evidence for aerosol transmission between animals, despite lack of research about transmission between humans.
ii) Lower doses seem to be needed with aerosols into the lung compared with virus drops into the nose.
iii) Resulting illness from aerosol inoculation seems more severe.
iv) Though there is no good quality epidemiological data to support transmission via aerosols, even long-range transmission cannot be ruled out and and short-range aerosol transmission may be significant.

As the Nature article already quoted states, ‘As evidence has accumulated over the course of the pandemic, scientific understanding about the virus has changed. Studies and investigations of outbreaks all point to the majority of transmissions occurring as a result of infected people spewing out large droplets and small particles called aerosols when they cough, talk or breathe.

As already discussed, we are aware of droplets and our measures to control droplet spread are well understood and well implemented. What we now know is that aerosol transmission is an important mechanism for transmission and, since we have implemented methods that markedly reduce, possibly largely eliminate other forms of transmission, we realise that aerosol transmission is sufficiently able to spread Covid that, even if we eliminate other transmission methods, Covid still spreads.

Agreement about how Covid is spread.
The scientific consensus is now that Covid is airborne and spread effectively by aerosols. Here are some references.

Coronavirus drifts through the air in microscopic droplets – here’s the science of infectious aerosols

Aerosols, Droplets, and Airborne Spread: Everything you could possibly want to know

Aerosol transmission of SARS-CoV-2? Evidence, prevention and control

A quick google reveals many further sources showing that Covid is airborne.

How can we stop the spread of aerosols?
Those of us who live near farms on which various forms of muck is spread as fertiliser, know that everyone downwind smells the much that has been spread. Those who live near landfill sites are also used to the smells of decomposing which are produced. If you visit Burton, you smell Marmite, a bi-product of the brewing there. And there is no way to stop airborne smells.

In the same way it is not possible to stop airborne viruses. They spread around like smells and we all encounter the viruses.

Do masks help?
Likely not at all. See for example Our findings indicate that surgical masks can efficaciously reduce the emission of influenza virus particles into the environment in respiratory droplets, but not in aerosols and a host of other easily available references.

The null hypothesis
Once Covid has spread significantly among a population, normal lockdown actions have no significant effect on the spread of the disease.

So, once it’s around, there’s nothing we can do to stop the spread of Covid? That’s pretty much the case. California locked down hard: Florida didn’t. Death rates were similar in the two states. There is similar evidence from comparing a whole range of states in the US and seeing that their various lockdown policies made not difference.

But if you don’t like what I have written, prove me wrong. Show any situation in which lockdown action has had any significant effect. After all, that’s how science proceeds: one makes a null hypothesis and invites others to prove one wrong. One doesn’t bumble along saying ‘wearing masks helps’ if one has no evidence that they do so.