UPDATE 3-US probing seizures reported after Sanofi flu shot

* Rise in fever-related seizures reported after Fluzone

* FDA: Data are preliminary, investigation ongoing

* No children had long-term effects

* Sanofi shares up slightly on NYSE (Adds details of cases from FDA paragraphs 3, 6-8; updates shares)

By Lisa Richwine

WASHINGTON, Jan 20 (Reuters) - U.S. health officials are investigating a rise in reports of a type of seizure following vaccination with a Sanofi-Aventis SA (SASY.PA) (SNY.N) flu vaccine, the Food and Drug Administration said on Thursday.

The febrile seizures, or seizures related to a fever, have primarily been reported in children younger than 2, the FDA said in a statement on its website.

The FDA said 42 cases of febrile seizures had been reported as of Dec. 13 and it and the Centers for Disease Control and Prevention were investigating if they could have been caused by the vaccine, called Fluzone, or if other factors were involved, the FDA said.

Having a fever can cause a seizure in some children. About 4 percent of young children will have at least one febrile seizure in their lifetimes, the FDA said.

"In the cases reported, all children recovered and no lasting effects have been seen," the FDA said.

An FDA spokeswoman said 36 of the cases reported to its vaccine adverse event reporting system were in babies and toddlers aged 6 months to 2 years, with 10 serious cases. She said 38 of the febrile seizures occurred within a day of getting Fluzone.

Experts stress that just because something happens soon after receiving a vaccine, it does not necessarily mean the vaccine was the cause.

The FDA spokeswoman said the vaccine surveillance system did not detect extra reports of febrile seizures after the 2009-10 seasonal flu vaccine or the 2009 H1N1 swine flu vaccine.

The CDC recommends everyone older than 6 months get a flu vaccine annually. That recommendation has not changed, the FDA said.

The Sanofi-Aventis flu shot is the only one recommended for infants and children ages 6 months to 23 months during the current flu season.

The company said it was working with the FDA to investigate the cases.

"At this point no correlation between influenza vaccine and febrile seizures has been established. Adverse events after vaccination may be causally related to vaccine or may be coincidental," Sanofi-Aventis said in a written statement.

The risk of severe illness from influenza is higher among young children, especially under age 2. About 9 out of 10,000 children ages 6 months to 23 months land in the hospital each season for flu-related problems, the FDA said.

Shares of the French drugmaker were up 2 cents at $34.12 in afternoon trading on the New York Stock Exchange.

The FDA posted the notice at link.reuters.com/cep47r. (Reporting by Lisa Richwine; editing by Andre Grenon, Dave Zimmerman)