Two measles cases have been confirmed in the Merthyr Tydfil area, and the health board says "we don't want to see measles spreading any further."
Clinics will be held at the schools where immunisation levels are lowest:
- Cyfartha High School - Friday 19 April
- Bishop Hedley High School - Monday 22 April
- Porth County Comprehensive - Tuesday 23 April
- Mountain Ash Comprehensive School - Wednesday 24 April
The health board says more schools may be targeted in future.
Cwm Taf says all of Wales' health boards "are developing plans to vaccinate unprotected children and young people and to provide rapid implementation of school based immunisation in response to cases and outbreaks."
2013 is expected to be the worst year for measles in the UK for the past two decades. Read more about the illness and how it is treated.
If the Government's top adviser on immunisation thinks the risk of an emergency in England is a possibility, I believe him.