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The Office for National Statistics (ONS) have released the latest report from the COVID-19 infection survey, looking at antibody and vaccination data for the UK.
Dr Peter English, Retired Consultant in Communicable Disease Control, Former Editor of Vaccines in Practice, Immediate past Chair of the BMA Public Health Medicine Committee, said:
“We do not yet know for sure what the ‘correlates of protection’ for Covid-19 are. It is likely that cellular immunity is part of the picture. Cellular immunity is not easy to measure, and is not measured by measuring antibody levels.
“It is likely that the majority of people with detectable antibodies will have protection against severe disease, and probably against infection. Some people without detectable antibodies may also be immune, protected by cellular immunity and the ability of a primed immune system to very rapidly produce high levels of antibodies in response to exposure to the virus.
“To achieve herd immunity a sufficient number of the population has to have sufficient immunity that each case can, on average infect fewer than one other person. This means that herd immunity relates to a germ’s infectiousness. This is usually described as the basic reproduction number – R0, for short – which refers to the number of people that, on average, each infected person will pass the infection on to, if there are no behavioural changes to reduce infectiousness. (It is an oversimplification, but is still a useful concept.) For measles, for example, R0 is estimated to be 15-18. Let’s call it 15 for the sake of argument. Vaccination can prevent people being infected and passing the disease on to others. In order to prevent measles from causing big outbreaks, we have to ensure that each person with measles can infect at most 1 other person (and on average fewer than one). This means that at least 14 of the people – all but one of the people they would have infected – must be immune, and unable to pass the infection on. So at least 14/15 (that’s over 93%) of the population must be immune; and (since the vaccine isn’t 100% effective) at least 95% of the population or thereabouts must be fully vaccinated. Thankfully, in the UK we are able to achieve coverage close to the 95% target with the MMR vaccine, which is a massive testimony to the NHS’ ability to provide vaccination.
“With Covid-19, the original strain was thought to have an R0 of approximately 3.1 (Perhaps 3.5 or so, but let’s keep the arithmetic simple!) This would require >2/3 (all but one; at least (R0–1)/R0 if you like equations) of the population to be immune, to provide herd immunity. (>2.5/3.5, or >71% if you use the 3.5 figure.)
“The so-called ‘Kent’, B.1.1.7 variant was thought to be approximately 1.5 times as infectious, so if the original R0 was 3.5, the B.1.1.7’s variant’s R0 would be 5.25; and the B.1.617.2 (‘Indian’) variant is thought to possibly be up to 1.5 times higher again, if we assume the higher end of the estimate then that would give it an R0 of nearly 8, meaning that at least 7 in 8 of the population (88% of the population) would need to be immune.
“These figures all have to be viewed with caution as yet; but they do give an idea of the proportion of the population that may have to be immune if we are to achieve herd immunity.
“It is, of course, more complicated. It’s obviously more important to ensure that the people most likely to spread the virus are immune and unable to spread it – particularly given that the number of people each individual infects can be extremely variable for Covid-19 – some people or events are ‘superspreaders’.
“Given that we may need to achieve immunity levels close to 90%, it is likely that we will, in due course, need to vaccinate children if we are to do so, particularly given that children generally have a lot of contacts that they could spread the virus to.
“In the meantime, however, we can be reassured that, by vaccinating everybody over 50, the number of people who become seriously ill and require hospital or ICU admission, or who will die, will be greatly reduced. But until and unless we achieve herd immunity, people who are vulnerable (perhaps they cannot be vaccinated, or for some reason the vaccine doesn’t work for them) will continue to be exposed and become ill. Some will be seriously ill, and some will die.
“And we should also remember that, while hospital admissions and deaths are vitally important, the long term consequences of ‘Long Covid’ are also crucial, even in people under the age of 50; so we should still aspire to achieving herd immunity, ideally globally, through vaccination, when we can.” 2 3
References
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“The estimates of the numbers who would test positive for antibodies are based on a survey, and any estimates based on a survey are subject to some statistical uncertainty. But the people tested for the survey form a reasonably representative sample of the population, and adjustments are made to allow for aspects that aren’t entirely representative, so I wouldn’t expect any major biases. The statistical margin of error means that the true percentage who would test positive for antibodies might be a bit less than 75%, or a bit more, but not too much (For England, the central estimate is about 76%, but the margin of error goes from about 73% to about 80%.) This is generally good quality data.
“I can’t say whether this is enough for herd immunity, because that would need more detailed modelling (and that would be beyond my own expertise). But I’ll make a few points. First, statements made early in the pandemic about the level of immunity in the population required for herd immunity may no longer still be valid, because the predominant variant here (the so-called Kent variant) is more transmissible than the original variants, and that would increase the percentage required for herd immunity. Second, we’ve got to remember that the 75% figure from these data is only for the population aged 16 and over, and about a fifth of the population of the UK is aged under 16. People under 16 are very unlikely to get seriously ill if they are infected, but they can be infected and they can pass the infection on to others, even if that is rather less likely than for older people. Some of the under 16s will have antibodies because they have previously been infected, but the figure in under 16s will be considerably less than 75%, so the national figure for the whole population must still be considerably less than 75%. Third, what’s required for herd immunity is a large enough proportion of the population being immune from infection, not a large enough proportion having antibodies. As the ONS bulletin rightly points out, antibody tests relate to only one aspect of immunity – they cannot measure others such as T cell responses. So antibody positivity is an imprecise measure of immunity – and also we still don’t have good data on exactly how long any immunity from antibodies might last.
“Finally, it’s worth pointing out that herd immunity wouldn’t mean no Covid-19, even if the immunity is very long-lasting. Some people will still be susceptible to infection, and there can still be outbreaks involving those people. Those outbreaks will be likely to peter out pretty quickly of their own accord, it’s true, but not without putting at risk people who are not immune.”
All our previous output on this subject can be seen at this weblink:
www.sciencemediacentre.org/tag/covid-19
Declared interests
Dr Peter English: “No conflicts of interest to declare.”
Prof Kevin McConway: “I am a Trustee of the SMC and a member of its Advisory Committee. I am also a member of the Public Data Advisory Group, which provides expert advice to the Cabinet Office on aspects of public understanding of data during the pandemic. My quote above is in my capacity as an independent professional statistician.”