An 11-year-old boy has died of the flu, but he was vaccinated. What happened?

Luca Calanni just turned 11 years old in January. Earlier this month, he came down with the flu despite having already received this year’s vaccination a month or two before. Calanni was an otherwise healthy young boy, but not even a week later, he died.

This year's flu is proving to be deadlier than in previous years. Already 9.7 million cases have been reported with 39 pediatric deaths, according to the CDC. That's more children dying of the flu than in the past 16 years, and it's happening sooner in the season than normal.

But Calanni was vaccinated. Vaccines are supposed to be protective against the flu. How could he have died?

According to Doctor Richard J. Webby, a researcher at the Department of Infectious Disease at St. Jude Children’s Research Hospital, vaccines are no guarantees against the flu.

“The flu vaccine is a good vaccine, but unfortunately it’s not a great vaccine,” Webby says. “So getting the vaccine is not a guarantee, unfortunately, that you won’t get the flu.”

At issue is the mutagenic capacity of the influenza virus. The virus itself comes in many strains and this year's vaccine is well-matched against certain strains than others.

RELATED: This Year's Flu Shot Isn't Fighting Against The Virus That's Causing Complications

Calanni is believed to have contracted a strain of influenza B, which normally doesn't arrive until later in the season. Influenza B usually hits harder as it's a less common strain and so the body has fewer antibodies to fight it off. This is especially true in young children who might have never been exposed to influenza B.

Speaking with People, Webby says that this year's vaccine is "not ideally matched" against influenza B, and so provides less protection. Less is not zero, however.

“It does reduce your chances of catching it, and if you do get sick, of getting very severely affected by the virus,” he says, but that’s still not a guarantee. “Even when there’s an almost perfect match between the vaccine and virus, there’s still a chance of dying from the disease.”

Source: WKBW, CDC, People

NEXT: Teen Suffering A Seizure Saved By Online Gamer Friend Who Called For Help