The number of children in private health plans getting properly immunized declined by as much as 3.5 percent last year, according to a recent report by the National Committee on Quality Assurance (NCQA), a nonprofit organization that tracks health care quality.

In many cases, a routine vaccination is missed due to ignorance or inadequate health care. Vaccination rates for children on Medicaid hover below those of private health plans by several percentage points, according to the NCQA.

But some parents are purposely avoiding vaccinations, opting for a “philosophical exemption” to excuse their child from school immunization requirements, said Jeffrey Dimond of the Centers for Disease Control and Prevention (CDC). They point to rumors littering the Internet that suggest shots can make children autistic or otherwise chronically ill.

Here are five myths about vaccines, and the truth behind them.