A Full Fact investigation has revealed that NHS England released an incorrect statement claiming that 3.4 million children in England are “unprotected” against measles.
Measles is a highly contagious and potentially serious disease, but this figure includes many children recorded as having had one dose of the MMR (Measles, Mumps and Rubella) vaccine who were considered “not fully protected”. It is also likely to include some children who hadn’t missed a dose but whose records weren’t up to date.
Other data suggests the true number of unprotected children is substantially lower. And further details about the NHS estimates which Full Fact has obtained under the Freedom of Information (FOI) Act suggest that at least a million of those counted were thought to have had one MMR dose.
The 3.4 million figure was widely quoted —across the media, in Parliament and beyond—after an NHS England statement first published on 22 January said: “NHS figures show more than 3.4 million children under the age of 16 years are unprotected [against measles, mumps and rubella].”
However, after we asked a series of questions, NHS England has since admitted that the figure is actually an upper estimate of the number of children aged 1-15 who might have missed at least one MMR vaccine dose.
A response to our FOI request subsequently said: “The 3.4 million estimate is not an official statistical position on the number of children unvaccinated or partially vaccinated. It represents the number of children we may contact directly to encourage a check of their immunisation status, update of their vaccination record and if clinically appropriate vaccination.”
In other words, it seems NHS England actually calculated the number of children who might have missed at least one vaccine dose.
NHS England amended its press statement on 29 January after we got in touch, to say that the figure referred to children who are either “unprotected or not fully protected”. However the amended version still does not say that the figure is an upper estimate. Nor does it make clear that it is likely to have included children who hadn't missed a dose but whose records were out of date.
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Why this matters
Trustworthy information is vitally important in a public health campaign.
Measles is a highly contagious viral disease, which can cause serious complications. The MMR vaccine offers safe and effective protection against it. Two doses are required to reach full protection of about 99%.
We don’t know exactly how many children in England are susceptible to measles, or how many have missed doses of the MMR vaccine. However other publicly available data suggests that both numbers are much lower than the NHS figure. Neither we nor any of the experts we asked to look at the figures have been able to replicate an estimate close to 3.4 million.
While NHS England has made an initial correction to its press release, the amended version still fails to explain that the figure may include children with out-of-date records. The original claim that 3.4 million children are “unprotected” continues to appear on various different NHS websites and remains uncorrected by most media outlets. (After we got in touch, NHS England North West added a footnote, and a reference to 3.4m children being unprotected also seems to have been deleted from a North Cumbria NHS Trust statement.)
A different version of the claim relating to “unvaccinated” children has also circulated widely. As recently as 26 February, the Department of Health and Social Care (DHSC) posted on X (formerly Twitter): “More than 3 million unvaccinated children are at risk of catching measles in England”. This is almost certainly not correct. We have asked the DHSC about this, but it has not replied.
An NHS England spokesperson told us in relation to its original 3.4 million claim: “The figure used was an estimate at that point in time to inform who would be invited as part of the NHS MMR catch up campaign.”
Health authorities must give the public accurate information, and properly explain what it means. When a mistake happens, they must correct it promptly. Failure to correct mistakes can damage trust in public bodies and officials.
We will be writing to NHS England to ensure that all uses of the figure have been fully corrected, and to ask why it released incorrect information, as well as what it will be doing differently to reduce the possibility of this happening again. We will pursue the matter with the public statistics authorities if necessary.
Who are the “unprotected”?
Following news of a measles outbreak in the West Midlands, NHS England announced the launch of its MMR catch-up campaign on 22 January, and claimed that “more than 3.4 million children” in England were “unprotected” against measles.
In an apparent reference to the same figure, the NHS UK account on X (formerly Twitter) also said on 24 January and 26 January: “More than 3 million unvaccinated children are at risk of catching measles in England.” This post was in turn shared by the DHSC.
From the beginning, however, it wasn’t clear what the 3.4 million number really meant. It first appeared on 21 January on the front page of the Sunday Times, which said in its print edition: “The figures include those under one, who are too young to be vaccinated, and those who have only had one jab.”
It’s true that two MMR doses are required for full protection, and they are very effective at providing it. But children who receive one dose—including those between one and three years and four months, who are not yet eligible for a second—are as a group at least somewhat protected, according to the government’s Green Book, which is the official source of vaccine information for health professionals. And children under one generally inherit some immunity from their mothers, which wanes over time.
Professor Andrew Pollard, director of the Oxford Vaccine Group and chair of the Joint Committee on Vaccination and Immunisation, told Full Fact: “It is true that one dose can provide protection but it is much more reliable to give two doses. We don’t know who is unprotected with one dose and so the correct messaging is that two doses are needed for protection.
“The bottom line is that everyone should make sure that they have completed two doses of MMR vaccine to protect them from the measles virus in this dangerous outbreak. The virus will kill if the outbreak is not contained.”
Dr David Elliman, a community paediatrician at Great Ormond Street Hospital, also told us via the Science Media Centre of the 3.4 million figure: “They may be unprotected, but it is not strictly accurate to say they are unprotected.” He added: “About 95% of children who have had one dose of vaccine will be protected, but this will go up to 99% after the second dose. There is no simple way of identifying those children not protected after the first dose and so all are offered a second dose.”
For these reasons, it is fair to say that children who have missed doses are less protected than they could be. However, a significant proportion of people who have received one dose will be at least partly protected against measles.
After we asked NHS England about the 3.4 million figure, it gave us more detail and amended its press release on 29 January to say that 3.4 million children are either “unprotected or not fully protected”. It also added a footnote saying that the 3.4 million figure came from “NHS management data” and was “the number of children aged 12 months to 16 years in the groups the NHS is targeting with its catch up campaign, who are eligible for either one or two doses of the MMR vaccine but have not yet received it”. (This contradicted the Sunday Times report which claimed the figure included children under one. The Sunday Times declined to comment.)
Finally, following more questions, we learned that the 3.4 million figure was calculated to work out who should receive an invite for the MMR catch-up campaign, and that it was a theoretical maximum to ensure a catch-all approach rather than miss people out. Crucially, it was also subject to the limitations of the available data. For example, the quality of vaccination data can degrade over time as GP computer systems are upgraded and the way vaccinations are recorded changes.
In other words, the 3.4 million figure was never an estimate for the number of children definitely known to be unprotected against measles.
This was confirmed by the response to our FOI request, which also included a recalculation of the estimate using the data as it stood on 15 February. (The data changes on a daily basis, so this did not produce the same results that NHS England reached before.) In this new recalculation, 1,174,554 children were recorded as possibly having received one dose only, amounting to more than a third of the total which came to 3,135,053. The remaining 1,960,499 were suspected to be “unvaccinated”.
Whatever the true figure, this shows that a large number of the children in the NHS estimate are neither unvaccinated nor entirely “unprotected”.
While it’s clear the original NHS claim was incorrect, it’s impossible to reliably say how many children are “unprotected”, or have missed MMR doses. However, public data does suggest that the true figure may be much lower than 3.4 million.
Working out how many children are “unprotected” is particularly difficult given the ambiguity of that phrase. Many babies are protected, for example, because they often inherit some short-lived immunity from their mothers, but NHS England is now clear that children under one were not included in its estimate.
However, in simple terms, population estimates from the Office for National Statistics (ONS) suggest there were about 10 million children aged 1-15 inclusive in England in 2022. We know the NHS actually used data from the NHS England Master Patient Index, which rightly or wrongly has a higher population. But if the ONS estimate is roughly accurate then the 3.4 million figure would amount to about a third of all these children.
Yet a third is much higher than we would expect from the vaccine uptake data. For example, in July 2023, the UK Health Security Agency (UKHSA) published an analysis “based on coverage rates of routine MMR vaccination, catch up vaccination campaigns and the effectiveness of the MMR vaccine”, which suggested, with assumptions about unrecorded doses, that up to about 9% of children in younger age groups were susceptible to measles, and up to about 5% of children in older age groups.
This data doesn’t cover children born after 2016, so it’s possible that susceptibility rates might be higher with a more up-to-date calculation, and these figures only considered children old enough to have had at least one MMR dose. Even so, the proportion of children who are susceptible in the age groups it does cover is a long way short of the third that NHS England claimed.
Equally, if we just consider how many children have missed at least one MMR dose, NHS vaccine uptake figures suggest that this is likely to be much less than a third of all children.
Official data on two-year-olds and five-year-olds shows that MMR uptake in England has not fallen below 82.7% (for two doses) or 88.2% (for one dose) at any point since 2009. And it would need to be more like 66% across all children if about a third had missed at least one dose.
Neither we nor the experts we asked to look at the figures have been able to replicate an estimate close to 3.4 million with publically available data. In fact, when we attempted a more detailed analysis, matching age groups to their uptake in the relevant year, making moderate assumptions about when children received their doses, and making the unlikely assumption that no child received a catch-up dose after the age of five, it resulted in a figure of about two million. This is not a reliable estimate of the correct figure, but it does increase our confidence that the true number of children who have missed an MMR dose is substantially lower than 3.4 million.
Commenting on that analysis before we received the FOI data, Dr Hugo Pedder, a research fellow at Bristol Medical School and a statistical ambassador with the Royal Statistical Society, said: "I think what you've done in terms of the numbers is a perfectly reasonable approximation given the data that's been made available. The difference is huge."
We also spoke to Sebastian Funk, Professor of Infectious Disease Dynamics at the London School of Hygiene & Tropical Medicine, via the Science Media Centre. He agreed with our analysis, and stressed that the NHS approach revealed in our FOI does not mean that 3.4 million children in England are unprotected against measles. However, he said that the number unprotected may still be worryingly high.
Kevin McConway, Emeritus Professor of Applied Statistics at the Open University, also told Full Fact via the Science Media Centre, before seeing the FOI data, that the 3.4 million figure appeared too high when considering other data. He said “The key thing is that it's a third of the age group, and I know vaccination rates have been falling, but that is simply too high to be believed by me.”
How widely did this spread?
The 3.4 million figure has been used very widely since it was first published by the NHS. It was quoted by health minister Maria Caulfield in the House of Commons and a health minister in the Lords, Lord Markham, both of whom said it was the number of children in England under 16 who are “unprotected”.
It also appeared in different forms in much of the media including BBC News, ITV News, the Evening Standard, Times, MailOnline, Financial Times, Guardian, Telegraph, Mirror, Express, Independent, Spectator, Metro, LBC [00:47:05], Jeremy Vine on 5 and several times on Radio 4’s Today [3:35].
A story in the Sun which originally said 3.4 million children are “unprotected” has since been updated to reflect the correction to the NHS press release. A report from Sky News that originally said "more than 3.4 million children under 16 have not been vaccinated" has since been updated to say that these are children who are "eligible for either one or two doses of the MMR vaccine but have not yet received it."
We have approached all the politicians and publications mentioned in this article for comment.
People can receive the MMR vaccine at any age. Anyone who has missed any doses is advised by the NHS to contact their GP to arrange a catch-up appointment. (Your GP surgery should be able to tell you whether you are up to date, or you may be able to check for yourself in your own health records.)
Image courtesy of Dave Haygarth