Why are there measles outbreaks in the United States?
Despite an aggressive vaccination program, there are still Americans who are vulnerable to measles infection. Infants under 1 year of age have not yet been vaccinated and are susceptible to infection. A minority of youngsters are not vaccinated because of religious beliefs or from fear of an adverse reaction to the vaccine. Finally, those who have received only one MMR vaccine may have lost immunity to measles and need a booster shot.
