A measles outbreak in Arizona and Utah shows no sign of slowing, putting the United States dangerously close to losing its elimination status for the vaccine-preventable disease.

With holiday travel and gatherings approaching, doctors worry that transmission could escalate. If measles continues to spread through Jan. 20, that will mark a year of sustained transmission in the United States. At that point, the disease would no longer be considered eliminated and it would instead revert to being endemic, or constantly present.

The backward step seems highly likely.