Starting in the 2006/2007 influenza season, the US Advisory Committee on Immunization Practices expanded its recommendations for seasonal influenza vaccination to include healthy children aged 24–59 months. The parallel Canadian organization, the National Advisory Committee on Immunization, did not at that time issue a similar recommendation, thereby creating a natural experiment to evaluate the effect of the policy in the United States.
We examined data for 2000/2001 through 2008/2009 and estimated relative changes in visits to the emergency department for influenza-like illness at two pediatric hospitals, one in Boston, Massachusetts, and the other in Montréal, Quebec, following the US policy change. Models were adjusted for virologic factors, seasonal trends and all-cause utilization of the emergency department.
Of 1 043 989 visits to the emergency departments of the two hospitals for any reason during the study period, 114 657 visits were related to influenza-like illness. Adjusted models estimated a 34% decline in rates of influenza-like illness among children two to four years old in the US hospital relative to the Canadian hospital (rate ratio 0.66, 95% confidence interval 0.58–0.75) following the 2006 policy change of the Advisory Committee on Immunization Practices. This was accompanied by more modest declines of 11% to 18% for the other age groups studied.
The divergence in influenza rates among children in the US and Canadian sample populations after institution of the US policy to vaccinate children two to four years of age is evidence that the recommendation of the US Advisory Committee on Immunization Practices resulted in a reduction in influenza-related morbidity in the target group and may have indirectly affected other pediatric age groups. Provincial adoption of the 2010 recommendation of teh National Advisory Committee on Immunization in Canada to vaccinate childen two to four years of age might positively affect influenza morbidity in Canada.