SAN FRANCISCO – Influenza vaccination was associated with a substantial reduction in the risk of life-threatening influenza illness among children during the 2010-2011 and 2011-2012 influenza seasons, according to findings from a case-control study.
However, vaccine coverage was low in this study, even among children with comorbidities that increased their risk of severe influenza-related complications, Dr. Ed Belangia reported at an annual scientific meeting on infectious diseases.
Cases included 44 children with life-threatening confirmed influenza illness, 172 pediatric intensive care unit (PICU) control patients without influenza, and 93 community controls without influenza. Only 18% of cases and 31% of the PICU controls were fully vaccinated, compared with 50% of community controls; those who were fully vaccinated were 74% less likely to be admitted to a PICU for influenza-related illness, said Dr. Belangia of the Marshfield (Wis.) Clinic Research Foundation, who presented that data on behalf of study author Jill Ferdinands, Ph.D., of the Centers for Disease Control and Prevention.
Of the cases, 15 (34%) had conditions that put them at high risk of influenza-related complications, and of the PICU controls and community controls, 89 (51%) and 35 (37%), respectively, had such conditions; 20% of the PICU patients had three or more comorbidities, compared with 3% of community controls, said Dr. Belangia.
After the investigators adjusted for factors including age, sex, date of onset, medical conditions, and – among PICU patients – illness severity and days from illness onset to influenza testing, the vaccine efficacy rate was 77%.
Of note, the receipt of one vaccine dose by children in whom two doses were recommended did not appear to provide protection in this study.
Children in the study, who were aged 6 months to 17 years during the 2010-2011 and 2011-2012 influenza seasons, were recruited within 7 days of symptom onset from 21 U.S. PICUs in the Pediatric Acute Lung Injury and Sepsis Investigators Network. Cases included those with acute severe respiratory illness who tested positive for influenza by reverse-transcription polymerase chain reaction; controls were PICU patients who tested negative for influenza, and children from the community who were matched for comorbidities and geographic region and who had no recent history of an influenza-related hospitalization.
Vaccine status was verified by medical record review, except in the community controls, whose vaccine status was based on parent report.
The low vaccine coverage in this study population, even among those at increased risk for influenza-related complications, is troubling, he said.
Although numerous studies have looked at flu vaccine efficacy in children, this study is among the first to look at the effects on severe influenza-related illness in children, he said, noting that about 1-7/10,000 children are hospitalized with influenza every year, which translates to 8,000-54,000 children per year.
"So that’s substantial morbidity – and of those, about 4%-24% require ICU admission," he said.
The results highlight the value of increasing the use of influenza vaccine among children, he concluded.
Dr. Ferdinand and her coauthors reported having no relevant financial disclosures. Dr. Belangia was not involved in the study.