Timeliness of MMR vaccination--influence on vaccination coverage.

Over the last seven years, and especially in 2001, a declining coverage for MMR vaccination in 2-year-olds has been noted in Sweden. By recording actual date of vaccination in a cohort of almost 4,000 children in a county in central Sweden, we found that parents' decision to postpone vaccination by up to 1.5 years beyond the stipulated age of 18 months accounted for about half the reported drop in 2001. Even if coverage thus improves with time, postponed vaccination adds to the pool of unprotected children in the population. The design of the current national surveillance system overestimates coverage at 2 years and fails to record delayed vaccination. To avoid future outbreaks that can appear around imported cases of measles it is crucial to attain high coverage levels by timely vaccination.