As consent forms for free flu shots for students are being sent home this week, a group of concerned parents is warning that the vaccines may contain thimerosal — a preservative containing mercury.

But officials from the state Department of Health are reassuring parents that the thimerosal-containing vaccinations are safe.

The Coalition for Mercury Free Vaccines, a makeshift organization led by advocates for autistic children, is urging parents to choose FluMist, a nasal flu spray alternative to the flu shot.

Parents are being offered the option for their child to receive either the shot or the nasal mist in the $2.5 million program, which is open to public and private elementary and intermediate school students, according to the state Department of Health.

Kalma Wong, president of the Hawai'i chapter of Cure Autism/Autism Speaks, said the jury is still out on the possible adverse effects of thimerosal. She warns that some scientific studies say that the mercury-containing preservative is toxic and possibly causes autism.

"Even scientists say it's inconclusive. So we should err on the side of caution and not allow thimerosal in vaccines," she said. "Until you know for sure, don't put it in there."

However, state epidemiologist Dr. Paul Effler said numerous studies have shown that there is no connection between thimerosal and autism. Further, he said, although the preservative contains mercury, the quantity is insignificant.

"The fear is not scientifically valid," Effler said. "You get much more mercury from eating a can of tuna fish than getting a vaccine."

Effler said there are two types of mercury — methylmercury and ethylmercury. Methylmercury is found in fish and other animals and at high levels can be toxic to people, he said.

Thimerosal contains ethylmercury, which studies have found breaks down faster and doesn't tend to accumulate in the body and cause harm as its counterpart does.

"While the science doesn't support it, there may be some parents that are still concerned about thimerosal, so we've given them options in this program," Effler said. "Our goal is to get as many kids protected against influenza infection as possible."

In written materials being sent home, parents are being informed about the choice of flu shot or nasal mist, he said. Parents may designate which option they prefer.

However, Wong said she is concerned that parents may read the material and not be aware that thimerosal contains mercury. "We want this to be informed consent," she said.

Last year, there were 15 outbreaks of flu illness reported in Hawai'i public schools. The state said that in some flu seasons, more than 10 percent of schoolchildren have come down with the illness, which includes symptoms of high fever, muscle aches and nausea.

Vaccinations are open to public and private school students ages 5 to 13, with funding coming primarily from the Centers for Disease Control and Prevention.

Reach Loren Moreno at lmoreno@honoluluadvertiser.com.