These youngsters would have been eligible for a jab when the vaccination rates began to fall in the late 1990s and early 2000s.
Take up of the MMR jab started to drop following a 1998 report by Andrew Wakefield which falsely linked the jab with autism, according to a UK Health Security Agency (UKHSA) 2023 measles briefing document.
Now vaccine coverage is the lowest it has been for more than a decade, with just 85% of youngsters having both doses of the jab before they start school aged five.
The West Midlands initially reported the largest measles outbreak outside of London, however cases have now been identified in the North West, London, East Midlands and Yorkshire and the Humber.
Appointment invitation letters are being sent to those living in the West Midlands, London and Greater Manchester.