Where’s our herd immunity?

Where’s our herd immunity?

In the early stages of the COVID-19 pandemic, there was a lot of discussion about herd immunity. Once a certain number of people had become immune through being infected, the epidemic would grind to a halt. This level is known as the Herd Immunity Threshold (HIT). There was (and still is) uncertainty as to exactly where this would kick in. Let’s call it 70%.

How many people have been infected? If we just consider those diagnosed, that ignores the asymptomatic cases. If we consider those who have tested positive, that understates the position because the tests weren’t available to start with, and there may still be asymptomatic cases not detected because they have no reason to take  a test. We can start with the number of deaths (as recorded on death certificates – currently about 150k). If the case fatality rate (CFR) is 1%, that translates to 15 million cases. (The CFR is higher in elderly and otherwise highly susceptible, and lower in younger people, but let’s focus on the concept rather than the accuracy of the data.) That’s about 20% of the population.

Of course, those earlier calculations were just based on becoming immune through being infected – but we now have the vaccines to consider. The headline figures that are generally quoted (e.g., BBC TV News) are that 90% of the adult population have had one dose, and about 66% have had both doses. You might think that we could add that to the above 20% who have been infected, but it’s not that simple – the two overlap (i.e., some of the vaccinated people will have had COVID already). Nevertheless, it is clear that the percentages are at or above the HIT. That is supported by surveys carried out by ONS which show that about 90% of UK adults have antibodies to SARS-CoV-2.

The snag here is the word ‘adults’ in the previous paragraph (i.e., those 18 or over). The ONS antibody survey shows that it is effectively 100% in those 50 or over, 95% for 35-49. dropping to 80% for 25-34 years, and only 60% for those in the 16-24 group. They don’t give figures for anyone under 16. (These figures are for England only; the details are different for the other nations, but the trends are the same).

Does this matter? It seems likely that if we consider the impact of vaccination and natural immunity combined, we should be near the HIT for the whole population. To show that it does matter, we have to consider the assumptions behind the concept of herd immunity. The most basic forms of epidemic modelling make two important assumptions: i) that the population is homogeneous, and ii) that it is randomly mixed. In more simple terms, this means that whoever you are, and wherever you are, if you have COVID, the number of people you are likely to infect is the same. This is obviously not true. If you live in a remote part of Scotland, you are less likely to infect others than if you live in central London. (I should add that modern epidemic modelling is much more sophisticated than the simple model, but the basic concept of herd immunity is still affected by these factors).

The most relevant, but somewhat less obvious, limitation here is the assumption of random mixing. Is the population in a specific area randomly mixed? If you have an infected 16 year old, are they more likely to pass it on to another 16 year old or to someone at 76? (I can’t remember the last time I came into close contact with a 16 year old!). So with the younger age groups not fully protected, it is quite possible to have rampant spread of infection there, while the older age groups are well protected.

To some extent, this is borne out by looking at the age group distribution of test positivity rates. Over the last month, these have risen from a very low level to 2-3% (daily) in school year 7 to age 24, and up to 1.4% in age 2 to school year 6, and those between 25 and 34. While there were increases in older age groups, these were much less. (A pinch of salt here – if more tests are done, you might get an apparent increase in the number of positive tests without there being an actual increase in the number of cases – you are just detecting them better).

So the current increase in the number of cases is being driven, to a large extent, by infections in younger age groups, who have not been vaccinated. This highlights the decision that was made in the vaccination campaign – to start with the older age groups and work downwards. Why was this decision taken? If the focus had been on simply controlling the number of cases, it might have been better to go all out for vaccinating everyone – especially as the younger age groups tend to mix more than the elderly do. The reason for starting with the most vulnerable comes down to the effect on the NHS. Older people (and others who are more vulnerable) are more likely to require hospital treatment, and especially intensive care, while those who are younger will often have only relatively mild  symptoms.

If we are to finally get on top of this disease, it is essential that vaccine uptake is increased in the younger age groups (and also in other groups that are at present under-vaccinated: e.g., some towns, some minority ethnic groups). It is disappointing therefore that the rate of vaccination has slowed markedly. In April/May, the total number of vaccinations (first and second doses) was as high as 600,000, Now it is down to not much over 200,000. Of these only 71,000 are first doses – so when they become eligible for a second dose, the number will be correspondingly low.

However, I am optimistic. The number of deaths is still very low compared to previous waves, either because the vaccination is reducing the effects of infection, or the proportion of cases amongst younger people is higher (or both). And if younger people are being infected without becoming seriously ill, that is doing the same job as vaccinating them – increasing the level of immunity. Let’s keep hoping!

Jeremy Dale

16 July 2021

Herd Immunity Revisited – an update

Herd Immunity Revisited – an update

There have been a number of responses to my previous post, most of which have missed the point. Perhaps I didn’t put it clearly enough. So let’s try to clarify it.

The main point is that I am definitely not advocating withdrawal of any attempt to control the epidemic and relying solely on the development of natural herd immunity. It is highly probable that this would lead to hospitals being stretched beyond breaking point (remembering that the policy of this and previous Tory governments in systematically reducing the ability of the NHS to cope with unforeseen crises has played a major part). I say ‘highly probable’, because nothing is certain when it comes to COVID – I suspect the virus has more tricks up its sleeve yet.

I should also say that I am in no way associating myself with the so-called ‘Great Barrington declaration’. At the same time. it does not make sense to automatically reject everything someone says simply because that person has, or is associated with, some unsavoury political views. If Mussolini made the trains run on time (whether that’s true or not), that doesn’t make it a bad idea. Nor does it mean that anyone advocating railway punctuality must be a fascist.

My central point is that lockdown and test-and-trace procedures will not by themselves permanently eradicate the virus. Textbook models of epidemics say that if you get R below 1 and keep it there, the disease will disappear. But one of the central assumptions of that model is that you are dealing with an isolated community. In the absence of herd immunity, the population is still largely susceptible, and any movement of people risks re-introducing the infection.

New Zealand can be considered as an ‘isolated community’, and they seem to have achieved that position – but at the cost of rigorous quarantining of all entrants to the country. A few other islands have done so too. The Isle of Man is making a valiant attempt to do it. Of course Britain is an island as well. But can we consider quarantine for all who come to Britain? Not just asking them to self-isolate, but putting them all into dedicated quarantine hostels for 1-2 weeks? Can you imagine that? It would have to include those coming from Northern Ireland too, which would cause a political storm.

In my view, the real purpose of the current restrictions is, or should be, to slow down the epidemic so that the NHS can continue to cope. And it buys time in the hope that an effective vaccine will become available. In the absence of a vaccine, where will it end? If the restrictions succeed in bringing the 2nd wave under control, my guess is that we would ultimately get a 3rd wave, a 4th wave and so on, until eventually enough people have been infected and we reach the unmentionable herd immunity. In that situation, we would end up with having had the same number of cases, and (unless we get more effective treatment) the same number of deaths. Those taking decisions about the nature and extent of the lockdowns need to balance those objectives against the economic, social, and psychological costs of the lockdowns.

Jeremy Dale


COVID-19 – a bit of perspective: update

An update

According to ONS, in the week up to Sept 11th, there were 9,215 deaths from all causes in England. Of these, COVID-19 accounted for 97 deaths – that is about 1% of the total. Of course, we would expect that figure to rise as the number of cases increases. For the whole period of the epidemic (up to Sept 11), COVID deaths (52,482) account for about 12% of the total number of deaths (434,618) in England and Wales..

If we assume a case fatality rate (the number of deaths as a percentage of the number of cases) of 1%, this means that just under 9% of the population have been infected.  But we don’t know the case fatality rate for sure; I have seen estimates that put it as low as 0.3%. If that is true, then the number who have been infected (England and Wales) would be as high as 17.5 million, or 29% of the population.  That would mean, given the highly uneven spread of cases, that there would be some areas that would, at some point during the ‘second wave’, approach the level required for herd immunity to kick in (usually taken as about 60%).

Jeremy Dale 25/9/20

COVID-19 – a bit of perspective

Every death is a tragedy for those affected. Especially so for COVID, where someone may die in isolation, unable to see, or be seen by,  their loved ones. In no sense do I want to diminish that – and if you have been affected by the virus you may not want to read on. But in the midst of all the publicity about COVID, I feel it is necessary to try to put it into perspective.

First of all, let’s look at the data for COVID deaths. This is very confusing. The Government dashboard (1 August) puts it at 46,193 in the UK. This is the number who have died having had a positive test result. This has been much criticised, as it would seem to mean that even if you get run over by the proverbial bus, having been tested positive months ago, you are still counted as a COVID-related death. On the other hand, if you die from COVID but have never been tested (or the test didn’t work), you wouldn’t be counted.

If you look at the data from the Office of National Statistics (ONS), you get a different figure,  50.800, in England and Wales only. This comes from death certificates, and is the number of times COVID was mentioned (even if other causes such as pneumonia were also mentioned). I’ll stick with the ONS figure, mainly because I want to compare it with other data from ONS.

So, 50,000 deaths. That’s a lot of tragedies. But death is a part of life. During the period of the epidemic, 245,000 people have died from all causes – so that’s getting on for 200,000 people have died from something other than COVID. And during each of the last five weeks, more people have died from what is recorded as Influenza/pneumonia than from COVID.

Another way of looking at the impact of COVID is to consider the excess deaths – that is the number of people who have died from any cause, compared to the average number who died in the same period over the last five years. This shows that since the epidemic started there have been over 53,000 excess deaths. That measure includes the possible indirect effects of COVID, e.g., people who didn’t get appropriate treatment in time. If we look at the weekly breakdown of excess deaths, we see that during the last five weeks it has been negative – that is, fewer people are dying than expected. The likely reason for this is that one effect of COVID has been to cause the death of some people who would otherwise have died soon anyway.

Historical comparisons

The current crisis has highlighted the fact that we are no longer used to people dying in large numbers from infective diseases. Medical advances, including antibiotics and vaccines, coupled with improvements in nutrition, housing, public health and other environmental issues, have in general made such diseases of historic interest only, at least in countries like the UK. (This is not of course true for most of the world, where diseases such as malaria and tuberculosis are causing death and suffering on a large scale – in low income countries, communicable diseases represent 5 of the top 10 causes of death).

A look at the death statistics (from ONS) for 100 years ago (1915) illustrates the point. In that year, there were 66k deaths from pneumonia/bronchitis and 39k deaths from tuberculosis. We can add others – 13k deaths from measles. 5k each from diphtheria and flu (not an epidemic year), 4k from whooping cough and nearly 2k from scarlet fever.

In the more distant past, there are numerous examples of devastating infections. The Black Death (1381) is thought to have killed a third of the population. In the nineteenth century, there were repeated epidemics of cholera, with tens of thousands of deaths, and tuberculosis was rampant (at its height, causing a third of all deaths).

In more recent times, the best comparison is with pandemics of influenza. (Technical note: Influenza viruses are classified by their H and A antigens, the most common type being H1N1. Various H1N1 strains are similar but not identical in both, so you get a degree of cross-immunity, while another type say H2N2 differs in both and there is no cross-immunity between them. Major pandemics usually occur with a virus that has ‘shifted’ to different H and N types)

In 1957-58 there was a pandemic of so-called ‘Asian flu’ (H2N2), which caused some 20-30k deaths in the UK. Then in 1968-69, we had an H3N2 strain (labelled ‘Hong Kong’ flu) for which the estimates of the number of UK deaths go up to 80k. In neither case was the official response anything like what we are currently seeing with COVID-19. And the media managed to find plenty of other news to cover.

More recently, there was some concern about ‘swine flu’ (2009). The incidence rose to about 110k cases per week in July, before dropping off, and then re-emerging in the autumn to about 84k cases per week in October. However, mortality was low (<1,000 deaths in UK), probably because this was an H1N1 strain, and older people had already encountered H1N1 strains and so had significant immunity to it.

Why all the fuss?

Why is it that fifty years ago we could face a disease that caused up to 80k deaths, not exactly with equanimity but at least without the massive sacrifices that we are currently making for a disease of (apparently) similar magnitude? Of course, we have to recognise that without the control measures it might have been much worse. Based on what was known about the disease, the initial assessment was that, if left unchecked, the disease would spread until Herd Immunity was achieved, and that would happen when about 60% of the population had been infected. Assuming a case fatality rate of 1%, that implied something like 350,000 deaths, which was deemed unacceptable. Of course we will never know if that would have happened, but a comparison with other countries is interesting. We hear a lot about the numbers of deaths in the USA and Brazil, but if we look at the numbers of deaths per million population, we are still some way ahead of either of them (UK 680, USA 477, Brazil 440) – although I am well aware of the dangers of reading too much into such comparisons, given the different methods and reliability of reporting deaths. But superficially, it could mean that our lockdown didn’t have much effect, and the original estimate of 350,000 deaths was over the top.

I’m not saying that we should all ignore the advice, and go out and party. But let’s keep a sense of perspective. At the individual level, unless you are in an extremely vulnerable category, there are plenty of other ways of dying that we don’t bother too much about. But collectively, we still have a duty to try to limit transmission so as to protect those who are more vulnerable. Above all, don’t panic!

Jeremy Dale

2 August 2020

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