The government is investigating five reports of teenagers who came down with a serious neurological disorder after receiving a new vaccine against meningitis (search).

Doctors don't yet know whether the cases of Guillain Barre syndrome (search) are related to the shot, called Menactra (search), or are coincidence, the Food and Drug Administration emphasized Friday.

The government recommends the vaccine for adolescents and college freshmen living in dormitories, and FDA said there was no reason to change that advice — but it alerted the public as a precaution.

Menactra protects against rare but devastating cases of bacterial meningitis (search). The shot is particularly targeted to college students, because close contact is the biggest risk factor for its spread. While there are fewer than 3,000 cases of invasive meningococcal meningitis (search) each year, and 300 deaths, people ages 15 to 24 have the highest mortality rates, and survivors can suffer mental disabilities, hearing loss and paralysis.

Menactra is a new vaccine approved last January; an older meningitis vaccine is available, too, but doesn't provide as much protection.

Guillain Barre syndrome, or GBS, causes increasing weakness in the legs and arms, sometimes severe enough to cause paralysis. It can strike suddenly, either spontaneously or after certain infections.

The five cases reported two to four weeks after Menactra vaccination occurred in 17- or 18-year-olds in New York, Ohio, Pennsylvania and New Jersey, FDA said. All are reported to be recovering.

Studies of more than 7,000 Menactra recipients by manufacturer Sanofi-Pasteur showed no GBS cases, and more than 2.5 million doses have been administered since it hit the market, FDA said. The rate of GBS is similar to what is expected without vaccination, but the timing of the cases is of concern and warrants further investigations, FDA said.