Washington and Oregon, which are experiencing a measles outbreak, lets parents opt out of vaccines for their children if they have personal objections against vaccination, CNN reports.
"I've been saying now for the last couple of years [that] it's only a matter of time before we see a horrific measles outbreak in the Pacific Northwest," Peter Hotez, MD, PhD, co-director of the Texas Children's Hospital center for vaccine development in Houston, told CNN.
All states require immunizations for children to attend school, and 47 states let parents skip vaccines if they have religious beliefs against immunizations, according to the National Conference of State Legislatures.
Among those 47 states, 18 states also allow parents to opt out of vaccines if they have personal, moral or philosophical beliefs against immunizations.
Oregon and Washington are among the states that allow personal belief exemptions.
These states have especially high levels of unvaccinated children and are vulnerable to measles outbreaks, according to a study Dr. Hotez and his colleagues published last year.
The Pacific Northwest is a hot spot for unvaccinated children, including Seattle, Spokane (Wash.) and Portland.
Washington Gov. Jay Inslee declared a state of emergency Jan. 25 after 35 confirmed cases of measles and 11 suspected cases were reported.
Two more cases in Washington and one case in Oregon have been confirmed since then. Hawaii also reported two cases in visitors from Washington who were infected before traveling.