Three years after Connecticut started requiring influenza vaccination for children in licensed child-care programs, vaccine coverage in young children was up substantially and their flu-related hospitalization rate was down about 12%, according to a study published today in Morbidity and Mortality Weekly Report (MMWR)

The findings don't prove a causal relationship, but they suggest that the vaccination requirement "might have helped to increase vaccination rates in Connecticut and reduced serious morbidity from influenza," says the report by researchers from the Yale School of Public Health, the Connecticut Department of Health, and the Centers for Disease Control and Prevention (CDC).

In September 2010, Connecticut enacted the rule that all children ages 6 to 59 months must receive at least one dose of flu vaccine each year to attend a licensed child-care program, the report says. New Jersey is the only other state that has such a mandate, though New York City adopted a similar rule in January of this year.

The researchers used several surveys to assess Connecticut and national flu vaccination coverage before and after the state enacted its requirement. The surveys included the National 2009 H1N1 Flu Survey, the Behavioral Risk Factor Surveillance System (BRFSS) for the 2009-10 and 2012-13 flu seasons, and the National Immunization Survey for 2012-13.

In 2009-10, the season before the Connecticut mandate took effect, vaccination coverage in the state's children between 6 and 59 months old was 67.8% (95% confidence interval [CI], 61.1% to 74.5%). For the 2012-13 season, coverage was 84.1% (95% CI, 78.2%-90.0%), up 16.3 percentage points.

In that same period the national flu immunization rate for that age-group rose from 57.9% to 69.8%, or 11.9 percentage points. Although the increase in Connecticut was greater, the difference wasn't statistically significant, the researchers said.

The authors used data from the Emerging Infections Program (EIP) to measure changes in flu-related hospitalization rates in Connecticut and other EIP sites. (The EIP is a CDC-led network of 10 state health departments, with various other collaborators, that conducts a wide range of surveillance and research.)

The team chose to compare the 2012-13 season with the 2007-08 season, because H3N2 was the predominant flu strain during both seasons. Eleven EIP sites, including two in New York, have been monitoring flu-related hospitalizations for all ages since the 2005-06 season.

In 2007-08, Connecticut's incidence of flu-related hospitalization in children up to the age of 4 was 58.6 per 100,000, the authors found. For 2012-13, the incidence was 51.5 per 100,000, a 12% drop, and the state was one of only two EIP sites to show a decline.

The researchers also found that the ratio of the flu-related hospitalization rate in children up to age 4 to the rate for children of all ages was 0.53, which was lower than at any other EIP site.

The authors say their study has several limitations, one of which is that factors other than the vaccination requirement might have contributed to the decrease in the hospitalization rate. Another is that the dynamics of flu circulation vary among the different EIP sites, and caution must be used in comparing changes over time at different sites.

"The Connecticut program might be helpful to public health agencies elsewhere considering requiring influenza vaccination of children in licensed child care programs and preschools," the report concludes.

CDC. Impact of requiring influenza vaccination for children in licensed child care or preschool programs—Connecticut, 2012-13 influenza season. MMWR 2014 Mar 7;63(9):181-5 [Full text]