The measles vaccine uses a weakened virus that's never been shown to spread to others. Samples from the outbreak in Texas also show that a wild-type virus is responsible. Yet, social media posts have falsely claimed that the outbreak is due to a vaccine strain.

Texas' outbreak, which has largely spread in undervaccinated Mennonite communities, could last a year, based on studies.

State health officials worry that declining vaccination rates have left many communities vulnerable nationwide.