New York (CNN) -- The New York City Board of Health unanimously approved a mandate Wednesday requiring all children under 5 enrolled in city-licensed preschools and day cares to receive the flu vaccine.
It is one of outgoing Mayor Michael Bloomberg's final health initiatives, which the New York City Department of Health proposed to the Board of Health in September.
"Young children have a high risk of developing severe complications from influenza," the Health Department said in a statement. "One-third of children under five in New York City do not receive an annual influenza vaccination, even though the vaccine safely and effectively protects them against influenza illness. Young children often pass influenza to other children and family members, who then spread the infection to others in the community."
The flu vaccine will be added to the seven other vaccines children under 5 are required to get, including measles, whooping cough and chicken pox.
"We know that influenza is a serious illness which affects an estimated 15% to 40% of children during annual flu season," said Dr. Jay Varma, who oversees infectious diseases as deputy commissioner of the New York City Health Department.
Richard Kanowitz, president of Families Fighting Flu, lost his daughter at age 4 to the virus.
"This is great news for children," he said. "Had the same law been in effect when my daughter was alive in 2004, she'd be alive today. She would not have gotten the flu."
"Only 60 percent of children under the age of 5 in New York City get vaccinated," Dr. Varma said. "In New Jersey and Connecticut, where they have this requirement, it's 80%."
The issue of vaccinating children is controversial. One consistent claim is that vaccines can cause autism.
"This was a very closed process," said John Gilmore, director of the Manhattan-based Autism Action Network. "We will most likely be filing a lawsuit fairly soon to overturn this. From a legal standpoint, we see this as similar to the soda ban," referring to the mayor's unsuccessful attempt to ban sugary drinks.
Varma said the myths that flu shots can cause the flu, or worse, are just that, myths. "It does not cause you to get the flu; it does not cause you to get autism," he said.
The flu vaccine is available in two forms -- shot or nasal spray -- and children are required to get vaccinated by December 31.
"To the extent that this saves lives, it's great news," Kranowitz said. "I wish my daughter had had the vaccination because, I, as well as other parents, were unaware how the flu killed healthy children. So the only way to protect them is to get them vaccinated."