What causes breakthrough measles infections even if someone is vaccinated?

A small number of people who have been fully vaccinated against measles may get sick if exposed to the virus, research finds. Why do these infections happen?

More than 800 people in the U.S. have had measles since the beginning of the year, according to NBC News data. The majority of cases are in West Texas, where an outbreak that shows no signs of abating began in January.

Nearly all of the cases are among people who haven’t been vaccinated, but 3% of the identified cases are so-called breakthrough infections. People got sick despite being either partially or fully vaccinated with the measles, mumps and rubella (MMR) shot.

Infectious diseases doctors are at pains to emphasize that the MMR vaccine is one of the most effective vaccines on the market, but as Rodney Rohde, a professor at Texas State University explains, a small number of people who are fully vaccinated may get sick during a large outbreak.

One dose of the MMR vaccine is 93% effective at preventing measles, with the second dose increasing that to 97%.

“The vaccine is highly effective,” said Rohde. “But it means that after two doses, while 97 out of 100 people will develop strong immunity and be protected if exposed to measles, the remaining three out of 100 could still be vulnerable.”