The New York City Board of Health voted unanimously Wednesday to mandate flu shots for young children in day care and preschool.
Taking effect in 30 days, the measure will require kids between 6 months and 5 years old in those childcare programs to receive influenza vaccinations before Dec. 31 each year.
- PHOTOS: What's Brewing in Steamy Hallows, the Harry Potter-Inspired Cafe19 Pictures
- All of these celebrities have had their nudes leaked 36 Pictures
New Jersey and Connecticut are the only states that require flu shots for children in day care.
The Health Department believes the mandate will prevent 20,000 to 25,000 kids from getting sick annually. One third of children under 5 years old don't receive a yearly flu shot, according to the department.
"Young children often pass influenza to other children and family members, who then spread the infection to others in the community," the Health Department said in a statement. "This mandate will help protect the health of young children, while reducing the spread of influenza in New York City."
Children in day care are already required to be immunized against other diseases, including measles and chicken pox.
Still, some advocates strongly oppose the mandate, arguing that the shots could potentially do more harm than good.
John Gilmore, executive director of the Autism Action Network, said the group plans to file a lawsuit against the city opposing the ruling in the next few weeks.
"We don't think the City of New York has the authority to create its own vaccine schedule," Gilmore said. "We think they've done here the exact same thing with the big soda issue."
Mayor Michael Bloomberg's large soda ban was ultimately struck down by a state judges who ruled the measure, passed by the Board of Health instead of the City Council, was an overreach of executive power.
As for the flu shot mandate, Gilmore said Bloomberg has "dumped" the rule on Mayor-Elect Bill de Blasio.
Lis Smith, a spokeswoman for the mayor-elect, said de Blasio and his team were reviewing the measure, "but are supportive of proactive policies that promote public health."
Gilmore said that the shots are "ineffective" for children under five -- a notion Richard Kanowitz, president of Families Fighting Flu, strongly opposes.
His 4-year-old daughter died of the flu after her pediatrician followed the Centers for Disease Control's recommendation that only children between 6 months and 2 years old should be vaccinated.
"The flu kills healthy kids," Kanowitz said. "Had my daughter been vaccinated, her body's response to the flu virus would have been different."
Still, Kanowitz said that while the shots might not completely guard against nonfatal flu, they strengthen children's' immunity.
"I don't want any parent to be in the position I'm in, asking, 'What could I have done to save my child?'" he said, saying opposing the vaccination was akin to playing Russian roulette. He said his daughter died within 72 hours of contracting the flu, likely from a classmate.
But Gilmore said he feels it should be up to parents to decide if their children are vaccinated. "It's not like it's hard to get," he said.
Despite Gilmore's feelings about Bloomberg's new rule, he said he was grateful for the mayor's actions against cigarette smoking in public.
The flu shot will be required for about 150,000 children, though parents may opt out for religious and medical reasons.
Follow Anna Sanders on Twitter @AnnaESanders