An NHS trust has apologised after more than 500 children were wrongly given an additional immunisation.
Parents have received a letter of apology from NHS Worcestershire after the mistake which was made during routine vaccination appointments.
It is thought the three and four year olds were given the extra immunisation at 25 surgeries in the county.
Dr Ash Banerjee from the trust said there was no health risk.
"Instead of having the vaccination which contains protection against four diseases, they got the one that gives these four, plus an additional disease," he said.
"There are a number of reasons why this happened and what we have done is to look into it and try to make sure it doesn't happen again.
"But, what happened is they had an extra dose which won't do any harm and they had all the vaccine and the protection that they need.
"I'm very clear there is no risk to children."