2014/15 Flu Season Live Vaccine Ineffective Against A/H3N2 Strains in Children

2014/15 Flu Season Live Vaccine Ineffective Against A/H3N2 Strains in Children
2014/15 Flu Season Live Vaccine Ineffective Against A/H3N2 Strains in Children

SAN DIEGO, CA—Quadrivalent live attenuated influenza vaccine (LAIV4) was effective against B strains vs. no vaccine or trivalent/quadrivalent inactivated influenza vaccines (IIV3/4), but not against A/H3N2 strains during the 2014–2015 influenza season in children, results of a multicenter study presented at IDWeek 2015 have found.

“Almost all circulating A/H3N2 strains were antigenically different from the vaccine strain,” reported Herve Caspard, MD, ScD, of MedImmune, Gaithersburg, MD, and colleagues.

The effectiveness of LAIV4 in children is monitored during each influenza season. For 2014–2015, the investigators enrolled children 2–17 years of age who sought outpatient care for febrile acute respiratory illness of less than 5 days' duration at 4 sites in North Carolina, Tennessee, Texas, and Wisconsin.

“Nasal swabs were tested for influenza by reverse transcription polymerase chain reaction (RT-PCR),” he noted. Medical records or immunization registries were used to document vaccination status; children considered vaccinated were those who had received 1 or more vaccine doses at least 14 days before onset of illness. Vaccine effectiveness was estimated as 100 × (1 – adjusted odds ratio) in influenza cases vs. test-negative controls.

The analysis included 1,671 children; 892 had not been vaccinated (202 cases and 614 controls), 244 had received LAIV4 (47 cases and 193 controls), and 535, IIV3/4 (77 cases and 424 controls). Mean age was 7.7 years (SD 4.1) and 532 (52%) were male.

Results showed that “RT-PCR was positive for influenza in 440 children: 326 tested positive for A/H3N2 and 114 for a B strain,” Dr. Caspard reported.

Effectiveness of the LAIV4 vs. no vaccine was 22% (95% confidence interval [CI]: -17%, 48%) among those with A/H3N2 influenza strain; 36% (95% CI: -467%, 93%) among those with vaccine-like strain; 86% (95% CI: 62%, 95%) against any B; 81% (95% CI: 45%, 93%) against B/Yamagata; 100% (95% CI: not evaluable to 100%) against B/Victoria; and 45% (95% CI: 21%, 61%) against any influenza strain.

Effectiveness of the LAIV4 vs. IIV3/4, adjusted for site and month of enrollment, age group, and number of outpatient visits in the previous 6 months, was -17% (95% CI: -83 to 25%) among those with A/H3N2 influenza strain; 58% (95% CI: -291% to 96%) among those with vaccine-like strain; 80% (95% CI: 41%, 93%) against any B; 73% (95% CI: 21%, 91%) against B/Yamagata; 100% (95% CI: not evaluable to 100%) against B/Victoria; and 16% (95% CI: -25%, 43%) against any influenza strain.

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