Though 47 U.S. states allow vaccine exemptions, it is becoming increasingly difficult to obtain such an exemption for religious or personal reasons. Many states will only grant an exemption if a child is allergic or has some other health reason – a compromised immune system due to chemotherapy, for example. The decision not to vaccinate can have far-reaching implications for a family in terms of access to schooling and other issues.

The three vaccines which are touted as being the most important, since they are for the most common and supposedly most easily preventable diseases, are: The MMR vaccine (mumps, measles and rubella); the DTaP vaccine (diphtheria, tetanus and whooping cough); and the varicella vaccine (chickenpox).

Four of the states have really pushed for all three vaccinations, namely Maine, Rhode Island, Massachusetts and Nebraska.

According to the American Academy of Pediatrics, other states which have thrown their full weight behind immunization programs are:

MMR: North Dakota, New Mexico, Ohio, Mississippi, Tennessee and Maryland.

DTaP: California, New Mexico, South Dakota, Iowa, Illinois and New Hampshire.

Varicella: Kansas, North Carolina, Georgia, Maryland, Connecticut and New Hampshire.