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 measles outbreak that began in Texas before spreading to at least two other states swelled to 355 confirmed cases on Friday — and officials say there is no sign it’s slowing.