In Washington, what ingredient must not be included in vaccines for children under 3 years?

The correct answer identifies mercury, specifically in the form of thimerosal, which is a preservative that has been phased out or eliminated in most childhood vaccines in the United States due to concerns about its potential effects on developing brains. The decision to restrict its use in vaccines for children under 3 years reflects a commitment to minimizing any risks associated with vaccine ingredients in this vulnerable population.

While aluminum is used as an adjuvant to enhance the immune response and is considered safe at the levels present in vaccines, and live vaccines are used under specific guidelines that take into account the child's health status, thimerosal has been the focus of significant scrutiny and is no longer recommended in vaccines for young children. Understanding this distinction helps ensure both safety and efficacy in immunization practices for infants and toddlers.