by Tom Chivers
Friday, 13
March 2020
Explainer
11:05

Can we build a herd immunity to Covid-19?

The idea put forward by the Chief Scientist comes down to a simple formula
by Tom Chivers
A government adviser floated the idea that we ought to let lots of people get Covid-19 so that the country will develop herd immunity

There’s a lot of talk about “herd immunity” at the moment, since a government adviser has floated the idea, confirmed by the Chief Scientist on the Today programme this morning, that we ought to let lots of people get Covid-19 so that the country will develop herd immunity.

A very important note: NONE OF WHAT FOLLOWS SHOULD BE TAKEN AS ANY SORT OF HEALTH ADVICE OR POLICY RECOMMENDATION. This is just to explain the idea of herd immunity and how it (might) work.

The contagiousness of diseases is measured in something called R0. It is the average number of people each infected person spreads the disease to. An R0 of 1 means that each person gives the disease to an average of one person; an R0 of 100 means they give it to an average of 100 people, etc.

The key to stopping the spread of a disease is to get R0 below 1. If it’s above 1 it will exponentially increase throughout the population; if it’s below 1 it will dwindle to nothing. There are complicating factors but that’s basically it.

One way to reduce R0 is to reduce the number of susceptible people in the population, for instance by vaccinating. Take measles as an example. It’s very infectious: in a susceptible population it has an R0 of about 20. You want to get that below 1. To do that with vaccination alone, you need to vaccinate at least 19 out of every 20 people. Then of the 20 people it would have infected, 19 will be resistant, so the number of infected will not increase.

It’s a simple equation. If your vaccine is perfectly effective, then you need to vaccinate 100% of the population minus (100 divided by R0). For measles, that’s 100 – (100/20) = 95; so you need 95% vaccination for herd immunity from measles.

Vaccination is not the only way of achieving immunity. For some diseases — including measles, and chicken pox — you are usually immune for life once you have it. So you could get herd immunity for measles if >95% of the population had it (and no one new was born).

That’s basically what this part of the government plan seems to be. With Covid-19, the R0 is not well known (and depends on social factors), but Imperial College puts it between 1.5 and 3.5 and the WHO at between 1.4 and 2.5. Let’s take the highest estimate, 3.5. If having the disease once makes you immune forever, then 100-(100/3.5)=71%; herd immunity would be achieved when 71% of the population has had it.

Of course, that assumes perfect immunity. If — as with flu — that’s not the case, the maths is very different. In fact, if the percentage who are still vulnerable after vaccination or exposure is greater than the percentage required for herd immunity, then it becomes impossible to achieve herd immunity, even if 100% of the population is exposed or vaccinated.

At the moment it’s not clear whether Covid-19 will become seasonal, like flu, or will be a once-in-a-lifetime illness like measles. It makes a huge difference to whether herd immunity will work or not.

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Stephen Follows
Stephen Follows
2 years ago

Hang on. Doesn’t that mean that the last thing we need to do is self-isolate? By banning all those football matches and so on, aren’t we ensuring that herd immunity will never be possible, because we’re deliberately preventing all the low-risk people from catching it?

benbow01
benbow01
2 years ago

I should also have said that that is how vaccination works. Once enough of the herd has been immunised, the spread of a virus is limited, epidemics prevented, and in some cases the virus totally eradicated, like Smallpox.

Nick Podmore
Nick Podmore
2 years ago

The more I listen to the government the more I come to realise that they will be quite happy for all of us get Covid19. From an economic point of view it makes perfect sense. Over 75’s and those with chronic medical conditions cost us billions of pounds each year…from the medical and social support they need to clogging up hospitals and the net negative of being an unproductive drain on society it, mathematically, makes perfect sense…thin the herd, the old, the sick, the weak….I can actually imagine D Cummings doing the sums….THAT, I think is their COVID19 strategy and frankly, I think support for herd immunity proves it…..

Robert Forde
Robert Forde
2 years ago
Reply to  Nick Podmore

I share your obvious distaste for this government, but not your interpretation of its covid-19 policy, which seems to turn the UK into Auschwitz. Support for herd immunity is a perfectly respectable scientific position to take, and need not mean anything sinister at all. Delaying the worst until the summer is also a respectable strategy, as is the attempt to even out the impact of the virus on the NHS. My stepson is currently in nursing training, and tells me that the expectation is that ALL staff will be infected at some point and have to be isolated. They certainly don’t want all those staff to get it at the same time. Delaying and slowing the spread makes sense in that context.

They could take stronger steps now, but with what result? The spread of infection would be reduced. Not eliminated. But at some point the restrictions would have to be relaxed. We can’t live in lockdown indefinitely. Then what? A powerful resurgence is the most likely scenario, with maybe all NHS staff going down at once. This is the weakness of the fear-driven “shut it all down now” idea. About 70% of the population will get this virus whatever we do. It can be managed to some degree, but not prevented.

Despite my prejudices, I think the government has probably got it about right on this. I certainly hope so, as my wife and I are both in the vulnerable over-70 category, and she has chronic asthma as well. We won’t be mixing as lot in the next few weeks.

Nick Podmore
Nick Podmore
2 years ago
Reply to  Robert Forde

I actually don’t have distaste for this government at all but as someone who works on the frontline of such a potential catastrophe I don’t deal in statistics and numbers but actual people. For a start I have some 50 odd students that could well perish, a wife with severe asthma, a mother with copd and 25% kidney function and a father trapped in a care home with limited mobility and poor health. THAT is reality. IF the government had the slightest handle on this there would be no talk of herd immunity. They don’t, so using statistics, economics and science they seek to allay fears and assuage conscience. Ultimately we WILL all contract this but at the moment the disease is in full control and will display the same hockey stick characteristics as it has in every other country. Singapore and South Korea are the BEST examples on how to deal with this. Social distancing, social isolation and restrictions on movement WILL slow the spread and WILL flatten the curve…THAT is epidemiological fact. Standing back and letting it run it’s course will result in a shattered NHS and bodies stacked like driftwood in morgues.

Dave Weeden
Dave Weeden
2 years ago
Reply to  Nick Podmore

While I understand your concerns, the disease is not the only problem the government faces. It does seem, for now, that Singapore and South Korea have handled this best. But I’m not sure the UK economy can survive over three months of self-isolating. Cancelling the Premier League’s going to hit the economy, and that’s a lot of lost days for everyone who doesn’t work at a desk. Thing about closing schools is that children have a very very low death rate. Many may lose some education in exchange from being saved from a very low risk.
There are trade-offs here, and PERHAPS our government is the only one recognising them.

Stephen Follows
Stephen Follows
2 years ago
Reply to  Nick Podmore

I think you’ll find that the over-75s gave Boris much of his majority. They’re the last people he’d want to sacrifice, from a purely electoral point of view.

James Chilton
James Chilton
2 years ago

It’s probably true that the over-75s put Johnson in power, and it’s also probable that he will remain in office for another 5 years.

Johnson will be gambling that the “sacrifice” of his aged supporters now, will not be remembered at the next General Election. His cynicism usually pays off.

wolffeeney
wolffeeney
2 years ago

……..and still he’s doing it. Boris will NEVER be forgiven for what he has done and is doing now. You will see him back peddling and throwing blame about VERY soon.

Damian Chunilal
Damian Chunilal
2 years ago

This is INSANE!
1. What death rate are they assuming? Regardless it’s a lot of incremental deaths vs the alternative of effective prevention and containment. They haven’t even tried the latter strategies.
2. They are using an untested model (that probably hasn’t been properly independently validated) rather than the scientific and empirical evidence we now have from the experience of others. This is reckless.
3. What if the infections don’t play out as their model predicts but explode as per Italy and Hubei and they lose control. More draconian restrictions than would have been the case will then be needed.
4. The death per WHO is 3.4%. If 3 happens the UK death rate may be even higher as medical system is overwhelmed?
5. And their science is questionable. Does herd immunity persist if the virus mutates? What if their foolishness simply creates a reservoir of infections in the UK for the winter whereas others wipe this out by taking far stronger measures.

If there is a properly qualified epidemiologist or medic who can convincingly take this apart with empirical evidence to support please do. Otherwise we are witnessing the greatest error of UK government policy in modern times.

wolffeeney
wolffeeney
2 years ago

I agree with everything you have just said Damian. And the fact that there is NO explanation to the public about any of this untested, unverified model, is an explanation in itself !

benbow01
benbow01
2 years ago

Questions we can answer. Yes.

That is how immunity works which is why we are no longer killed or as seriously affected by diseases that killed our ancestors, and why diseases common-place and rarely life-threatening at home didn’t kill the colonists who took them to the New World, but which did kill the indigenous who lacked herd immunity.

It is also why bacteria/virus mutate because previous versions no longer work. If they did not mutate herd immunity would kill them off for good.

Given the by now evident low mortality rate, the best practice would be to let the virus infect the population as quickly and widely as possible, get that herd immunity and the virus would disappear.

The best precautions would be to quarantine those who are most at risk, closely monitoring them, rather than quarantining those infected. It would be less expensive, better for the economy, and far more effective because people with the virus can infect those at risk prior to showing symptoms or before symptoms become apparent.

Of course… Government.

Paul T
Paul T
2 years ago

Most people taking part in the Coronavirus debate are not epidemiologists or medical experts and haven’t got a clue. The BBC, and other mainstream media, has proven itself worthless, dangerous and pathetic, as it has on all the major stories of the last half century – remorselessly pumping out its globalist orthodox drivel, and fear-mongering speculative garbage. The BBC has 6 of 10 of its most read stories linked to CV today, in between a story about a pregnant illegal migrant who died falling off the US border wall (while never asking what she was doing there in the first place – needless to say).

After this has passed the only thing worth reading will be the lessons learned in relevant scientific reports. History should record how badly we were served by 24 hour news, political chancers and other wretched speculators. Until it does, we should listen only to those qualified to comment.

wolffeeney
wolffeeney
2 years ago
Reply to  Paul T

Great point Paul. So can you point me to them please ?

Paul T
Paul T
2 years ago
Reply to  wolffeeney

I’m certain that you have the intellect to tell the difference between Adam Boulton or Robert Peston and an actual scientist

R Fogh
R Fogh
2 years ago
Reply to  Paul T

What is *your* expertise in immunology, come to that?

Paul T
Paul T
2 years ago
Reply to  R Fogh

Please point me to where I’ve claimed to be an expert in immunology. Before you do though please review your contribution above and tell me why you have limited yourself to fueling the speculation engine while hiding any expertise you may have in the field under a bushel.

R Fogh
R Fogh
2 years ago
Reply to  Paul T

No one else has claimed to be an expert in immunology either. You cannot demand credentials from everybody else, and then refuse to provide your own.

I am not an immunologist, but for the few simple facts I quote I do not need to be:

– The UK is acting differently from all other countries, and has not given a reason why the UK situation should be different.

– Herd Immunity depends on enough people remaining immune after having been sick (or vaccinated).

– We do not yet know enough about Coronavirus to say if that is a reasonable assumption.

If you think that these simple statements are wrong, by all means tell us why. Let’s hear your arguments.

kdelafield
kdelafield
2 years ago

Herd immunity can only be achieved in a global pandemic when the whole world is following the herd immunity strategy. Current information is the UK is the only country publicly supporting such a strategy as it’s main defence. Where the UK to achieve herd immunity and 50,000 of us were dead we would need to close our borders, and never travel to non-herd countries, to retain our herd immunity.The Government strategy is poorly conceived and extremely dangerous; there is no science to show that immunity to this virus even exists for sure.

Its not that we will lose our loved ones – we will lose our own lives.

Paul T
Paul T
2 years ago
Reply to  kdelafield

Please confirm your expertise in this field. Are you a qualified epidemiologist for instance?

kdelafield
kdelafield
2 years ago
Reply to  Paul T

Given you don’t use your actual name I do not consider you qualified to ask me personal questions. However I offer you this quote from Margaret Harris, World Health Organisation whose professional qualifications are in the public domain…

‘We don’t know enough about the science of this virus, it hasn’t been in our population for long enough for us to know what it does in immunological terms. ‘Every virus functions differently in your body and stimulates a different immunological profile. ‘We can talk theories, but at the moment we are really facing a situation where we have got to look at action.’

Paul T
Paul T
2 years ago
Reply to  kdelafield

That’ll be a ‘no’ then Kazza

Paul T
Paul T
2 years ago
Reply to  kdelafield

OK, well your posts would have more credibility if you were to cite the sources you relied on, rather than making out that you are an expert.

R Fogh
R Fogh
2 years ago

– The UK strategy is different from the choices made in Denmark, Norway, Austria, Poland, Singapore, … . Yet the science is the same. Just what is different in the UK that caused the government to choose a different road?

– As the article says, herd immunity will work only if enough people stay immune after they have been sick. Is that the case for Coronavirus? This is a new disease, so *we just do not know*! Does not sound like a very solid base for a strategy.

wolffeeney
wolffeeney
2 years ago

I, like thousands of others have been wrecking my brain trying to think of any sane reason why the UK seem to be doing NOTHING to stop the virus. I keep hearing “Its only a flu”, “Why is everyone worried”, “Masks are no good”, Death rate is only 2%” What the hell is going on folks,

This is NOT the flu, it is (at R0 2%) 200 times deadlier than any flu
Everyone is worried because some of their family is going to die (or them)
Wearing a mask WONT stop you getting it, but it WILL stop you spreading it !
2% of 67 Million people is ONLY 1,340,000 people dead.

“Herd immunity is an untested, unverified way of going about this. This is the reason why out of the 130+ countries that have been infected, NONE of them have tried it ! And the fact that this has been the UK Gov plan from the very start is showing a monumental mistake by the so called UK “Experts”, and the government that are pulling their strings. None of them should EVER be forgiven for what they are about to put this country through… Wolf Feeney