IPA: /ˌʌnɪˈmjunʌɪzd/
KK: /ʌnˈɪmjunˌaɪzd/
Not having received a vaccine or immunization to protect against disease.
The clinic is offering free vaccinations for unimmunized children.
Unimmunized → It is formed from "un-" (meaning not) and "immunized" (from Latin "immunis", meaning exempt or free from). The word "unimmunized" means not exempt from disease or not having received immunity.
Think of 'not' ('un-') being added to 'immunized', which means someone who is not protected against disease.