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.

Three hundred and fifty-five cases have been reported in the ongoing measles outbreak in Texas, New Mexico and Oklahoma, but local officials say that it’s an undercount and that the virus could continue to spread spread for months.

A measles outbreak in Western Texas has now climbed to 309 cases, surpassing the total number of infections from the virus in the entire United States last year.