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!
16 July 2021