Although annual influenza vaccinations rates have steadily increased in recent years, more attention must be paid to matching available vaccination strands with the most prevalent disease strains during each season to mitigate the burden of pediatric influenza cases on hospitals and emergency departments, according to a study published in Pediatrics.
“Annual variability in influenza activity, strain virulence, and population immunity to circulating strains makes assessing the impact of [several recent] vaccination policy changes challenging,” wrote lead author Dr. Astride Jules of Vanderbilt University, Nashville, Tenn., and her coauthors (Pediatrics 2014 [doi: 10.1542/peds.2014-1168]). “However, with increasing uptake of influenza vaccines, influenza-related health care visits should decrease over time.”
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In a retrospective, population-based study, Dr. Jules and her associates estimated the proportion of laboratory-confirmed influenza-related hospitalizations and emergency department (ED) visits related to acute respiratory illness (ARI) and fever in children aged 6-59 months by using available data from the Centers for Disease Control and Prevention (CDC) and the Hospital Discharge Data System (HDDS). The investigators then took those proportions and multiplied them by the number of ARI/fever hospitalizations and ED visits for residents of Davidson County, Tenn., looking specifically for trends in vaccination coverage, and influenza-associated hospitalizations and ED visit rates.
For the purposes of this study, flu season was defined as starting in either the first week of November or the first week in which at least two patients were hospitalized with influenza-related illnesses, and covered all consecutive weeks in which influenza virus strains were presented in Davidson County. The only season that did not fit the typical outline was that of the H1N1 outbreak in 2009, when flu season lasted from May 3 to Dec. 5.
The data showed that the rate of fully vaccinated children between the ages of 6 months and 5 years increased from 6% during the 2000-2001 flu season, to 38% in 2010-2011 (P < .05). Over that span of time, influenza-related hospitalizations ranged from 1.9 to 16.0 per 10,000 children annually, with a median of 4.5 (P < .05). ED visits ranged from 89 to 620 per 10,000 children annually, with a median of 143 (P < .05).
However, for the years in which the A(H3N2) strain was prevalent – 2001-2002, 2003-2004, 2004-2005, 2005-2006, and 2007-2008 – median rates of both hospitalizations and ED visits were significantly higher, compared with the other years covered by the study: 8.2 vs. 3.2 for hospitalizations, respectively, and 307 vs. 143 for ED visits, respectively.
Investigators also noted that there were decreases in hospitalizations and increases in ED visits (P < .05 for both) over the course of the study, but these could not be definitively correlated to the increase in vaccination rates. Furthermore, despite more children receiving vaccinations in Davidson County, the rate still fell far below even 50%.
“Overall influenza vaccination coverage remained low (< 50%) in children [under] 5 years, and it is likely that substantially greater vaccination coverage levels and/or new vaccination strategies will be required before broad population level decreases in rates of influenza associated medical visits can be documented,” Dr. Jules and her associates wrote.
The study was funded by the CDC and a National Institutes of Health Clinical and Translational Science Award. Dr. Jules had no relevant financial disclosures, but some of her coauthors had ties to pharmaceutical companies.