Cases of measles are rare in the UK but there are several current outbreaks in Europe, particularly in Romania, Italy and Germany.
In July, the World Health Organisation (WHO) said there had been thousands of cases and 35 deaths in the preceding 12 months.
Dr Mary Ramsay, head of immunisation at PHE, says in a statement: "Although the UK recently achieved WHO measles elimination status, due to ongoing measles outbreaks within Europe, we will continue to see imported measles cases in the UK in unimmunised individuals and limited onward spread can occur in communities with low MMR coverage and in age groups with very close mixing."
Measles is an extremely infectious viral illness that starts with cold-like symptoms. These are typically:
These symptoms are followed later by a distinctive rash.
The disease usually gets better by itself but in some cases, it can cause pneumonia and inflammation of the brain.
The MMR jab
Parents are being urged to ensure their children receive the MMR vaccine when it is offered at 1 year of age and as a pre-school booster at 3 years and 4 months.
Figures published in September by NHS Digital showed that the proportion of children receiving the first dose of the MMR vaccine by their 5th birthday had increased year on year since 2006-07. The latest coverage rate is 95%, which is generally considered sufficient to protect the community at large.
However, the rate of children being given the first jab by their 2nd birthday fell in 2016-17 for the third year in a row to 91.6%.
Confidence in the MMR vaccine has taken time to recover after a now-discredited study in a medical journal in 1998 linked the jab with autism and bowel disease. This may partly account for why cases in the current outbreaks in England all involve children and young adults.
"If children and young adults have missed these vaccinations in the past, it's important to take up the vaccine now from GPs, particularly in light of the recent cases in Liverpool and Leeds," Dr Ramsay says.
'Stay at home if you have measles symptoms'
Anyone with measles symptoms is being urged to stay at home and phone their GP or NHS 111 for advice. The advice from PHE is to stay away from surgeries and accident and emergency (A&E) departments to avoid the risk of spreading the illness to others.
It is understood that the outbreak in Liverpool is linked to travel to Romania.
Travellers are being urged to ensure they are vaccinated before travelling to affected areas of Europe.
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