The number of measles cases associated with an outbreak in western Texas has grown to 259, with 36 cases reported over the last three days, according to new data released Friday. Almost all of the cases are in unvaccinated individuals or in individuals whose vaccination status is unknown. Two cases have occurred in persons vaccinated with two doses, according to the Texas Department of State Health Services. Breakthrough infections, when a vaccinated person is infected are rare, as the measles vaccine provides up to 97% protection after 2 doses.