All models are wrong, but some are useful.

George Pelham Box

This is an update on the previous post about the virus. It is based on a slightly more sophisticated model which better captures the impact of different rates of infection and on the Chief Medical Officer’s estimate of death rate. The message is the same.

Other countries have adopted major lock-down policies to contain the corona virus. The UK has fewer measures in place and introduced them more slowly than most others.


And the evidence is that countries that act fast and drastically, have a far lower death rate.

But the UK’s chief scientific advisor has suggested that we should follow a different path: hoping for 60% of the UK population to get the virus to build herd immunity.

What are the pros and cons of the UK approach? And is there a better alternative?

The Pros are speed and subsequent immunity

We have been experiencing growth in number of cases growing from 13 on 26/2/2020 to 459 today (12/3/2020), which suggests that each infected person can infect around 0.25 others per day. On the basis of that rate of potential infection, the build-up of herd immunity could look like this.

Within about 15 weeks, the virus would largely have run its course and significantly more than 60% of the UK population would have developed herd immunity.

The population would be, of course, smaller than before but as the journalist Jeremy Warner wrote in the daily Telegraph,

“Not to put too fine a point on it, from an entirely disinterested economic perspective, the COVID-19 might even prove mildly beneficial in the long term by disproportionately culling elderly dependents.”

Rather more convincingly, once the herd immunity exists (assuming that it would, which is far from certain), the threat from the virus is over. Even if visitors from another country reintroduced it, too many people would be immune for it to spread. This is a possibly significant advantage over any strategy which successfully contains the virus.

The Cons are that half a million people could die

According to the John Hopkins dashboard, as of 13/3/2020, there are 135,382 identified cases of the virus globally. Of these, 4,981 have died and 69,645 have recovered. That is a death rate so far of 3.7% and a recovery rate so far of 51.4%. Over time, the recovery rate can be expected to grow; hopefully not the death rate. In the UK, the Chief Medical Officer has estimated the death rate to be 1%. At first glance, this seems in conflict with the facts, but since many of the infected will be asymptomatic or have very mild symptoms, we may never know their true numbers and the true death rate could indeed be well below 3.7%.

To get herd immunity in the UK, with 60% immune, would mean infecting over 37 million people. And assuming a death rate of 1%, over 350,000 deaths would result. If, as some others have suggested, herd immunity requires something more like 80% to be infected, the total number of deaths would be closer to half a million.

In addition, if this is achieved quickly, then the spike in workload for the NHS would look like this.

This is dramatically more than the NHS would be able to cope with. The practical reality is that few of those even with severe symptoms would be able to be hospitalised; they would have to be cared for at home – and their survival rate would probably be lower as a result.

To put this death toll into context, here are some other noted killers in the UK.

This would make the coronavirus significantly more deadly than the Spanish ‘flu – indeed it would be comparable in terms of mortality to World War II.

What is the alternative?

The principal alternative is to do what the rest of the world is doing – impose restrictions to reduce the rate of spread of the virus.

The chart below summarises the predicted number of deaths in the UK at different rates of infection (potential number of people infected by each infected person per day). As noted above, the evidence so far suggests that each infected person can infect around 0.25 others per day. The chart shows how critical it is to get that number down if we want to avoid hundreds of thousands of deaths.

If the potential infectivity rate does not change, we could easily have around half a million deaths in the UK. If it does, of course, the death toll could be very different. And there are two variants of this strategy:

  • Retard the progress of the virus through the population in order to prevent the healthcare system being overwhelmed, and conceivably to give time for a vaccine to be developed;
  • Lockdown the system to such an extent that the virus is eliminated before it kills significant numbers.


Retard the progress of the virus

If we could reduce the potential infectivity rate to under 0.15 per person daily, the spike is flattened dramatically. The peak number with serious symptoms if we follow this strategy is around 25,000 as against approximately 150,000 hospital beds in total – massively disruptive but not completely inconceivable.

The number of deaths, theoretically at least, is dramatically reduced to below something of the order of 100,000. But the overall process takes more than two years, during which significant restrictions on the economy and on individual freedom are necessary. The level of economic damage done in this strategy is hard to assess, but it is reasonable to suppose that in itself, it might cause a significant number of additional deaths.

Also, the number of people who have become immune is far short of any estimate of what is required for herd immunity. So if there were to be any relapse, as there was with the Spanish flu, a second wave of infections could sweep through the population. Permanent quarantine might be needed.

This strategy may simply be too difficult to implement.

Lockdown the system and eliminate the virus

But if we could get the potential infection rate down to 0.1, then the whole thing would be over in about 13 weeks.

In this scenario, the total number of deaths is negligible. Of course it shares with the Retard strategy the disadvantage of not having built herd immunity, so for these numbers to be meaningful once the lockdown ends, there would need to be quarantine for visitors from any country which had not eliminated the virus. This is the policy which Hong Kong has now adopted.

So what we need the government to do now is to put us on an emergency 13-week lockdown of all non-essential public contact. Critical National Infrastructure, of course, must continue to function, but most businesses should ask employees to work exclusively from home; and those which cannot do that – and which are not part of the Critical National Infrastructure (such as food and pharmacy retail) should close temporarily.

To make this possible, of course, government would need to provide financial support to citizens and business owners. Germany, for example, has just announced unlimited credit to support its businesses during their lockdown procedures.

And an emergency 13-week universal basic income might be a key plank of such an idea. Without something along these lines, many people could not afford to self-isolate, as existing provisions like universal credit run the risk of being both inadequate and too slow, leading to non-compliance with the lockdown measures. The cost, of course, would be enormous – maybe of the order of £200 billion. But that is a half of the Bank of England’s QE programme after the Global Financial Crisis. And it could save half a million lives.

If this matters to you, please share the message with your friends and family. And sign the petition:

If you have time, write to your MP.