Nearly 150 people have been infected with measles in Texas and New Mexico, and last week, Texas health officials confirmed the first death in the ongoing outbreak: an unvaccinated child. While almost all of the infected are believed to be unvaccinated, five of them told the Texas health department they have received at least one measles shot.

The United States declared measles eradicated in 2000 because the disease was no longer spreading locally (most cases in the following years were linked to travel to countries where the virus was still prevalent) But in recent years, outbreaks of the measles have been growing as more people across the country opt not to vaccinate. The recent reports of cases even among vaccinated people have added another layer of anxiety to the crisis. More than 90 percent of Americans got their measles shot as a kid. Exactly how reliable is that protection, given this news?

First of all, there is no need to panic. It is not clear whether the five people who contracted measles were vaccinated as children. The Texas health department told me they were investigating, but they “wouldn’t classify” these as breakthrough infections. Lara Anton, a department spokesperson, told me unvaccinated people who have been exposed to measles are given a vaccine within 72 hours if possible because it could reduce the severity of their symptoms.

“We are looking into how many of these cases received a dose of MMR after they were exposed to the virus,” Anton said.

To be clear, breakthrough cases can happen. But the risk today remains very low — only about

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