More than 60 cases of measles have been reported in London in a “fast-spreading” outbreak.
It is said that for every one infected person, 18 unvaccinated people can contract the disease.
The UK Health Security Agency (UKHSA) said a large measles outbreak in the city could infect between 40,000 and 160,000 people.
Large numbers of children would be hospitalised – and some would die.
Last July, a child died from measles in Liverpool after a small outbreak.

Measles rash spread downwards, appearing three to five days after a fever and cold-like symptoms (file photo)
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She said the capital had “one of the lowest, if not the lowest, vaccination uptake rates” in the country.
In Enfield, more than a fifth of children are unvaccinated against measles, mumps and rubella by age five.
Ms Sher-Arami said average vaccination figures obscure “pockets” which create prime areas for measles to spread.

The outbreak is attributed to low rates of MMR vaccination in London (file photo)
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She told The Times: “We know that measles has got some very nasty complications. It can cause deafness.
“It can cause brain damage and one in five children can need hospital treatment.”
Temporary vaccination clinics are now being established in schools throughout Enfield and families are receiving letters from Ms Sher-Arami.
Since the beginning of the year there have been 96 confirmed cases of measles in England, with the majority of cases in children under 10.
Enfield has the highest numbers followed by Birmingham, with the UKHSA warning the figures were likely to be an underestimate.
Vaccination against childhood diseases has slumped to a 10-year low, with some parents feeling hesitant to vaccinate their children after the Covid pandemic.
A Government campaign promoting childhood vaccination is set to start this week, with adverts on social media, YouTube, and radio.
A pilot programme in Enfield has been established with the local council and NHS working to hold clinics in school and community centres to promote vaccination.
NHS nurses speaking Bengali, Turkish, and Somali will be available to encourage families to vaccinate their children.
