Did I read that right? A measles outbreak in New York City has been traced back to a fully vaccinated patient ? Yes, even two shots of the measles vaccine does not guarantee that you won’t get it -- or that you won’t give it to someone else, maybe even someone who has also been vaccinated.

The viral infection had been almost eliminated in the U.S., where most people are vaccinated shortly after their first birthday . A second dose of the measles-mumps-rubella vaccine as a booster a few years later is recommended, and some universities require proof that both of these shots were done during the right time intervals before you’re allowed to start classes.

But in 2011, a 22-year-old theater employee living in New York developed measles despite having received both doses, and became unwittingly contagious. The startling case of “Measles Mary” begins with “vaccine failure,” followed by neither hospitalization nor quarantine, and ends with at least four people who, as far as anyone knew, should have been immune. It’s the first time scientists have documented transmission from a twice-vaccinated individual

They found that, of the 88 people Measles Mary contacted while ill, there were four “secondary cases” of transmission. Two of contacts were confirmed to have previously received the two-dose vaccine. The other two have past positive measles IgG antibodies, which means their immune systems have responded to measles in the past -- this usually confers immunity. There were no additional cases among the 231 contacts of the secondary cases.

Measles Mary’s blood work showed the immune response to measles that’s typical in a person who’s never had it or never been vaccinated. The blood contained an arsenal of IgG antibodies, except none of them were actually capable neutralizing the virus. It was as if her immunity had simply faded away.

The case highlights the jarring reality that “the actual duration [of immunity] following infection or vaccination is unclear,” Rosen tells Science . And though she doesn’t believe a change in vaccination strategy should be based on a single case, the work does underscore the need for thorough investigation of suspected cases regardless of vaccination status. This warning of waning immunity is particularly important given recent outbreaks in other urban areas, including Orange County and Quebec . Just last month, another 16 confirmed cases were announced in New York City.

A vaccine failure rate of just 3 percent could devastate a dense high school, according to Robert Jacobson with the Mayo Clinic . Still, he tells Science , “the most important ‘vaccine failure’ with measles happens when people refuse the vaccine in the first place.”

The latest version of the findings were reported in Clinical Infectious Diseases this week.

Image: Centers for Disease Control and Prevention