Effect of expanded US recommendations for seasonal influenza vaccination: comparison of two pediatric emergency departments in the United States and Canada

Background:

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.

Methods:

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.

Results:

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.

Interpretation:

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.Seasonal influenza is an important cause of visits to the emergency department among children during winter months, and its control and prevention rely on annual vaccination.^1–4 In the United States, the Advisory Committee on Immunization Practices is responsible for guiding immunization practices, and it revises its recommendations annually. Until the 2004/2005 influenza season, vaccination was targeted to primarily older individuals and those with certain medical conditions.⁵ On the basis of evidence that young children are at elevated risk for admission to hospital because of influenza-related complications, the recommendations were expanded for the 2004/2005 season to include healthy children aged 6–23 months.⁶Starting in the 2006/2007 season, the recommendations of the Advisory Committee on Immunization Practices regarding influenza vaccination were expanded again to include healthy children aged 24–59 months, a shift that added 10.6 million children to the target group.⁷ For the 2008/2009 season, recommendations were further expanded to include all persons 6 months to 18 years old,⁸ and for the 2010/2011 season, the influenza vaccine was advised for all individuals over 6 months of age.⁹In Canada, the National Advisory Committee on Immunization is the federal organization responsible for guiding the use of vaccines. Until the 2006/2007 season, the recommendations of this committee for the use of seasonal influenza vaccines were harmonized with those of its US counterpart. However, it was not until the 2010/2011 season that the Canadian committee began recommending that children two to four years old be routinely vaccinated against influenza, and not all Canadian provinces have adopted this recommendation.We aimed to examine the effect of the 2006 recommendation of the Advisory Committee on Immunization Practices to expand influenza vaccination coverage to preschool-aged children. Interannual variation in severity of disease and effectiveness of the vaccine make it difficult to directly estimate the effect of an intervention on the incidence of influenza in any given community. Instead, we compared surveillance data from the emergency departments of two pediatric hospitals, one in Boston, Massachusetts, and the other in Montréal, Quebec, cities with similar epidemiologic dynamics for seasonal influenza.¹⁰ Our specific objective was to estimate the relative effect of the 2006 US recommendation on influenza-related emergency department visits to Children’s Hospital Boston, with the Montreal Children’s Hospital as an untreated control.