During the 2009 H1N1 influenza A pandemic, children and young adults aged 0-24 years were the most affected, comprising of almost 75% of all cases. Studying influenza vaccine effectiveness and health outcomes in this population is important so that we can find ways to improve care and prevent influenza-related deaths.

To measure vaccine effectiveness, we collected samples from patients admitted to the ICU during the flu seasons of 2010-2011 and 2011-2012. These patients, from across the United States, presented with symptoms of influenza-like illness. We collected respiratory samples and clinical data from all of the patients, as well as information on their vaccination history.  The children who did not have influenza were healthy comparisons to those who did have influenza, and we also recruited control subjects from the community. This study found that from 2010-2012, those who were vaccinated had almost a 75% lower risk of a life-threatening case of influenza in the ICU. We also discovered that the number of children being vaccinated is still low, even though the vaccine is the best way to prevent influenza infection.

Since that study was conducted, there is increasing evidence that the influenza vaccine is not as effective at preventing critical illness against the currently circulating strains of influenza virus. Therefore, we are repeating the influenza vaccine effectiveness study across 26 large pediatric centers with the aim of enrolling 100 children with severe influenza virus acute respiratory infection and approximately 300 children with infection from other pathogens during the 2019-2020 and 2020-2021 influenza seasons. 

We would like to thank the Centers for Disease Control and Prevention for providing the funding for these projects.