By Keith Mulvihill

NEW YORK (Reuters Health) - Parents who wish to provide their young child with a thimerosal-free flu vaccine next flu season are being advised to let their pediatrician know now so it can be ordered in advance.

Thimerosal has been used for over 60 years to prevent microbial contamination in vaccines, but health officials have begun phasing it out because it contains mercury.

While the bulk of influenza shots given in the U.S. still contain at least some thimerosal, the U.S. Food and Drug Administration has approved preservative-free flu vaccines. Previously, the pharmaceutical company Evans Vaccines was granted approval for a thimerosal-free flu vaccine for use in children over the age of 3.

Last September Aventis Pasteur got approval for its Fluzone preservative-free vaccine, which can be used in children 6 months and older. It became available to children in late November 2002.

Still, parents may need to ask their health care provider in advance about thimerosal-free flu vaccines to ensure that they have it on hand come next flu season, explained Dr. Walter Orenstein, director of the Centers for Disease Control and Prevention's National Immunization Program.

Most health care providers order their flu vaccine supplies in the spring before the fall and winter flu season.

"Parents have to ask their health care professional for a thimerosal-free flu vaccine," said Orenstein. "There is a limited supply since it is more difficult to produce, as I understand."

Nonetheless, Orenstein stressed the fact that thimerosal-containing flu vaccines pose no danger to youngsters.

"There has been controversy about thimerosal and there has been some concern that some parents might not want to give their child the flu vaccine unless they were offered a thimerosal-free version, explained Orenstein during an interview with Reuters Health.

"This was offered for those who wanted it as a potential alternative," he added.

Currently, Orenstein noted, the CDC's advisory panel says the preservative-free vaccine is equivalent in efficacy and safety to the one that contains thimerosal.

As such, the CDC has seen no reason to promote the options, Orenstein noted.

Orenstein also stressed that parents need to be aware of the risk influenza poses to very young children.

"Influenza disproportionately affects young children," he said. "The hospitalization rate for young children -- and the younger the child the higher the risk -- approaches that of adults over the age of 50 who are hospitalized," said Orenstein.

In 2001, the vaccine advisory committee of the CDC "encouraged" parents to have children 6 to 23 months old vaccinated for influenza starting in the 2002-2003 flu season.

Children in this age group are at "substantially increased risk for influenza-related hospitalizations," the CDC's Advisory Committee on Immunization Practices noted at the time.

The CDC is expected to make a full recommendation sometime in the near future that these children receive the flu vaccine annually but are still looking into the logistics of recommending the annual vaccination.

So, for the time being, flu vaccination for young children remains an "encouragement," Orenstein said.

According to the CDC, vaccination is most important for people who are at increased risk of complications from the flu. These include pregnant women, adults 65 and older and anyone over the age of 6 months with a chronic medical condition such as diabetes, asthma, or heart, lung or kidney disease. People are strongly advised to get flu shots at the start of flu season, generally in October and November.

Because of the late release of the thimerosal-free vaccine -- the flu season kicks off each October and parents are encouraged to have youngsters inoculated around that time -- demand for Fluzone was not very high last year, according to Aventis Pasteur spokesman Len Lavenda.

"Usage of Fluzone was low last flu season due to the timing of availability," Lavenda, who is based in Swiftwater, Pennsylvania, told Reuters Health.

Many pediatricians most likely did not have an opportunity to order the newer alternative.

"Our hope is that this year, with more advanced notice we will see interest in the vaccine pick up," said Lavenda.

"Our current challenge is for doctors to place orders as soon as possible to ensure timely delivery," he added.