As measles cases in West Texas are still on the rise two months after the outbreak began, local public health officials say they expect the virus to keep spreading for at least several more months and that the official case number is likely an undercount.

Vaccination efforts have faltered, and many residents have turned to alternative treatments endorsed by Robert F. Kennedy Jr., the health secretary.

The Texas measles outbreak has expanded to 309 cases, affecting four new counties, with most cases in children 17 and younger.