NHS England measles figure causes confusion over encephalitis risk

14 May 2024
What was claimed

Around 10,000 new cases of encephalitis could emerge from the 3.4 million children in England who are unprotected or not fully vaccinated against measles.

Our verdict

This figure is likely too high, because it’s based on an incorrect NHS estimate. The 3.4 million figure was actually NHS England’s upper estimate for the number of children who might have missed at least one dose of the MMR vaccine. Other data suggests the true number who are unprotected against measles may be much lower.

Among the 3.4 million children in the UK who were unprotected or not fully vaccinated against measles, around 10,000 new cases of encephalitis could emerge.

An article in the Sun has quoted a claim from the charity Encephalitis International that 10,000 children could develop encephalitis—a potentially dangerous swelling of the brain—as a result of catching measles.

This claim, which was also quoted by the Standard and the i earlier in the year, as well as in an earlier Sun article, is based on an incorrect figure from NHS England that we wrote about in March.

The total number of children in England who are susceptible to measles is probably much lower than 3.4 million, so the number who are at risk of encephalitis as a complication of it is probably much lower too.

Measles is a highly contagious and potentially serious disease, and encephalitis is a serious condition which can be fatal. But it is important that charities, the media and the NHS especially take care to use correct figures when describing public health so that people get an accurate picture of the risks in the real world and have confidence in public health messaging

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Where did ‘10,000 children’ come from?

Full Fact asked Encephalitis International (EI) how its figure of 10,000 children was calculated, and it told us that the figure was based on the assumption that 3.4 million children in England were unvaccinated against measles, after NHS England said that number were “unprotected” in January.

EI has previously said that a child’s risk of encephalitis with a measles infection is between one and three in 1,000. It also said that the risk is about one in 1,000 in the month after infection. The National Institute for Health and Care Excellence says the overall rate is about one in 1,000.

Extrapolating from these numbers, the charity calculated that about 10,000 children were therefore at risk, because a rate of three in 1,000 (or 0.3%) applied to 3.4 million children produces a figure of 10,200.

What’s the problem with it?

Encephalitis is a known potential complication of measles. If it were true that 3.4 million children in England were susceptible to measles, and that 0.3% would develop encephalitis if they caught it, then the estimate of about 10,000 being at risk of encephalitis would also be correct.

However, as we explained in our earlier article, NHS data did not really show that 3.4 million children were “unprotected” against measles.

The figure was actually an upper estimate for the number of children who might have missed at least one vaccine dose, and therefore needed to be reminded to check their vaccination status. This overstated the number of susceptible children for two reasons.

Firstly, although two doses of the MMR vaccine are required to give maximum protection against measles, one dose does give protection to many people. So someone who is overdue for a second dose isn’t necessarily susceptible.

Secondly, in a series of emails and in a response to a Freedom of Information request, NHS England told us the figure was also subject to the limitations of the available data. In particular, the quality of vaccination data can degrade over time as GP computer systems are upgraded and the way vaccinations are recorded changes.

The FOI response 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.”

Although we don’t know how many children in England really were susceptible at the time of the NHS announcement, as we wrote in our investigation past vaccination coverage data suggests the true number was much lower.

After Full Fact queried the figure earlier this year, NHS England amended its initial press release to say that the figure referred to children who are either “unprotected or not fully protected”. But 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.

We have contacted NHS England, the Office for Statistics Regulation and health ministers about the claims, and continue to be in correspondence about this. NHS England has published a subsequent explainer document, but we have asked it to do more to correct its use of the figure to avoid any further confusion.

When we asked EI about its 10,000 figure, it told us it was calculated on the assumption that 3.4 million children were unvaccinated. It also said it would bear the problems with the NHS figure in mind in its future work.

Vaccination is important 

Measles is a highly contagious viral disease, which can cause several serious complications besides encephalitis. The MMR vaccine offers safe and effective protection against it. Two doses are required to reach full protection of about 99%.

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.)

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