Measles outbreak feared in London and South East
Doctors are concerned they are witnessing the start of a measles outbreak in London and the South East.
Public Health England has detected 20 cases in the region since February, compared with 91 in all of England in the whole of last year.
Most cases were in young adults and needed hospital treatment.
Doctors are advising people to ensure they are vaccinated to prevent a repeat of the 2013 Swansea outbreak in which 1,219 people were infected.
Dr Kevin Brown, from Public Health England, told the BBC News website: "The concern is this may be the beginning of another outbreak like we've seen in the past."
So far, there have been 12 cases in London, three in Cambridge, three in Hertfordshire and two in Essex.
They have mostly been in people in their 20s and 30s. The disease is usually more serious in adults than in children.
The patients presented to hospital with high fever and a rash.
"Measles is not pleasant in the older age group and I think people tend to forget that... They are unwell enough to need to be in hospital," Dr Brown added.
All the cases have been caused by the same strain of the virus suggesting the infections are linked, although there is no evidence that the virus has changed to spread more easily.
Dr Brown argued: "Measles is the most infectious infection that we know and it really is very good at seeking out those few members of the community that have not been vaccinated.
"I don't think it will be of the numbers we had for the Swansea outbreak, which was predominantly affecting school-age children.
"But there is still the potential for us to have an increasing number of cases, especially in young adults and they are the ones that tend to be hospitalised and don't do as well."
He said unvaccinated older people thought they had "dodged the bullet" with measles, but in fact needed to be vaccinated.
MMR (measles, mumps and rubella) immunisation rates are now at record levels in children.
So far, there has been little spread of the infection to school-age children, which suggests there is a high level of protection in those age groups.
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