Two measles patients this week, a child from New Mexico and an adult from Minnesota, contracted the virus on domestic flights.
Of the 2,066 counties, 1,614 (78%) showed a decline in coverage of the measles, mumps, and rubella vaccine.
In other developments, states with links to the large West Texas outbreak reported a few more cases today.
Other states report more cases, including Minnesota, with two, one of which involves a child whose infection suggests community spread.
Some of the latest cases are tied to a large outbreak centered in West Texas, while others are linked to instances of community transmission or travel to other states or countries.
Iowa's patient is an unvaccinated adult, and Nebraska's is a vaccinated child who had not traveled outside the state.
Texas and New Mexico report more outbreak cases as other states raise alerts about exposures linked to new illnesses.
Georgia reported its fourth case, a resident of the Atlanta area who had traveled internationally.
Fifty-seven percent of those polled said they were unsure of complications if a pregnant woman contracts measles.
Texas remains the country's hot spot, with 718 cases.
