When vaccinating children against influenza, inactivated and live attenuated influenza vaccines show little significant difference in effectiveness against nearly all strains of the virus, according to a new study in Pediatrics.

However, the study – which examined the effectiveness of IIV and LAIV across four consecutive influenza seasons between 2010 and 2014 – cautions that the 2013-2014 season’s A/(H1N1)pdm09 showed an uncharacteristically large gap in effectiveness favoring IIV, a discrepancy likely due to a problem with a vaccine component in LAIV. (Pediatrics. 2016;137(2):e20153279)

“We found that lower LAIV effectiveness in 2013-2014 was specific to the A/(H1N1)pdm09 vaccine component and was consistent with a previously unexamined effect during the 2010-2011 influenza season,” Jessie R. Chung of the influenza division at the Centers for Disease Control and Prevention and associates wrote, adding that the impetus for the study was the lack of available data “from observational studies after the 2009 pandemic on relative effectiveness of LAIV and IIV in children and adolescents.”

Dr. Chung and coinvestigators enrolled children aged 2-17 years from clinics and hospitals in Michigan, New York, Pennsylvania, Tennessee, Texas, Washington, and Wisconsin during the 2010-2011, 2011-2012, 2012-2013, and 2013-2014 influenza seasons. Children brought in with symptoms of acute respiratory illness – cough, fever, or feverishness – had nasal and throat swabs collected to test for presence and type of influenza.

In total, 7,718 subjects were evaluated across the four influenza seasons, but after excluding subjects for various reasons – unknown vaccine type, indeterminate vaccine status, and inconclusive reverse transcription polymerase chain reaction results, among others – 6,819 subjects were included for vaccine effectiveness analysis, of which 2,703 were ultimately matched age appropriately and placed into IIV and LAIV cohorts for comparison. The IIV cohort consisted of 2,066 individuals (76.4%), while the LAIV cohort had 637 (23.6%).

During the 2010-2011 season, 66 of the 477 IIV subjects contracted influenza, versus 21 of 116 who received LAIV (14% vs. 18%, respectively). In the 2011-2012 season, 51 of the 499 IIV subjects (10%) contracted influenza, compared with 12 of the 152 LAIV subjects (8%). In the 2012-2013 season, 198 of the 622 IIV subjects (32%) contracted influenza, versus 61 of the 205 LAIV subjects (30%). But, in the 2013-2014 season, 36 of the 468 IIV subjects (8%) contracted influenza, versus 34 of the 164 LAIV subjects (21%).

After adjustment for age and season, the odds ratio for the 2013-2014 season was significantly higher than those of the other seasons across the entire age spectrum of 2-17 years: 2.88, compared with 1.49 (2010-2011), 0.67 (2011-2012), and 0.92 (2012-2013).

When comparing influenza type/subtype, adjusted odds ratio was 5.53 for those with A/(H1N1)pdm09 in the 2010-2011 season, compared with 2.65 for those with the same in the 2013-2014 season. Those with A/H3N2 did not show as significant a difference across seasons (2010-2013), nor did those with influenza type B (2010-2011, 2012-2013).

“We found no statistically significant difference in LAIV effectiveness compared with IIV against medically attended, laboratory-confirmed influenza illness due to A/H3N2 or B viruses,” Dr. Chung and colleagues concluded. “We found significantly higher odds of influenza A/(H1N1)pdm09 among participants vaccinated with LAIV, compared with IIV, [but] reasons for lower effectiveness of LAIV against the A/(H1N1)pdm09 virus, compared with IIV are not fully understood.”

The investigators added that “the finding appears to be specific to the A/(H1N1)pdm09 vaccine component; we did not detect any statistically significant differences in effectiveness for the other components.” Three previous randomized controlled trials indicated that trivalent LAIV was just as effective, if not more so, than IIV, making the findings of this study surprising and “unexpected,” the authors noted.

This study was supported by the CDC through cooperative agreements with a variety of universities and foundations, and funded by the National Institutes of Health. Dr. Chung and associates reported no relevant financial disclosures.


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