In the United States, Connecticut, New Jersey and New York City are the three places that require a yearly flu shot to stay in daycare or preschool. The Connecticut law took effect in 2010.
"We found that of all the influenza hospitalizations in Connecticut, many fewer were in children 1 to 4 years old after the requirement than before the requirement," said lead researcher Dr. James Hadler, a clinical professor of epidemiology at the Yale School of Public Health.
The jump in flu vaccinations of young children -- to 84 percent in 2012-2013 from about 68 percent in 2009-2010 -- is thought to have caused the decline in hospitalizations, he noted.
"That difference, we feel, has resulted in children attending daycare being better protected against influenza and its severe complications," Hadler said.
Of 11 areas evaluated, Connecticut had the steepest decline in flu hospitalizations of children aged 4 and younger -- a drop of 12 percent between 2007-2008 and 2012-2013, according to the report from the U.S. Centers for Disease Control and Prevention. And the actual rate of hospitalizations for these young children compared to people of other ages was lower in Connecticut than for any of the other regions surveyed.
The report was published in the March 7 issue of the CDC's Morbidity and Mortality Weekly Report.
Young children are at particular risk for severe complications from flu, and they readily transmit the infection, Hadler noted.
"Where there are a lot of susceptible people in a small space, like daycare centers and preschools, you have the potential for easy spread of influenza," Hadler said.
"The kids get it, they give it to each other, they all take it home to their families, the families get it and spread it to other people," he added.
By vaccinating these children, you not only prevent children from getting the flu, but also prevent it from spreading in the community, Hadler explained.
And, he noted, flu shots are free for anyone who has health insurance, including Medicare and Medicaid.
For people who aren't insured, the state provides flu shots for free at community health centers, Hadler pointed out.
Dr. Jose Rosa-Olivares, director of the pediatric care center at Miami Children's Hospital in Florida, said efforts such as the one in Connecticut to standardize vaccination might have benefits.
"We know the proven benefit of the flu vaccine, and the more people that get vaccinated, the better it is for the community," he said.
The U.S. Centers for Disease Control and Prevention recommends that everyone 6 months and older get a flu shot every year.
Another expert thinks that because the flu vaccine has been shown to be effective, it is reasonable to mandate it around the nation before children are allowed to go into daycare or preschool.
"This is not saying that all kids have to have a flu shot. But if you're going into daycare you better have one because you are putting other kids at risk -- and if you don't like it don't go into daycare," said Dr. Marc Siegel, an associate professor of medicine at NYU Langone Medical Center in New York City.
For more information on flu, visit the U.S. Centers for Disease Control and Prevention.