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.

Measles cases in West Texas are still rising two months after the outbreak began. Local public health officials say they expect the outbreak to last for at least several more months and the 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.