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An Optimized Hemagglutination Inhibition HI Assay to Quantify Influenza-specific Antibody Titers
Published on: December 1, 2017
Influenza Vaccine Effectiveness Against Antigenically Drifted Influenza Higher Than Expected in Hospitalized Adults:
Joshua G Petrie1, Suzanne E Ohmit1, Caroline K Cheng1
1Department of Epidemiology, University of Michigan, School of Public Health.
Background:
The 2014-2015 influenza season was severe, with circulating influenza A (H3N2) viruses that were antigenically drifted from the vaccine virus. Reported vaccine effectiveness (VE) estimates from ambulatory care settings were markedly decreased.
Methods:
Adults, hospitalized at 2 hospitals in southeast Michigan for acute respiratory illnesses, defined by admission diagnoses, of ≤10 days duration were prospectively enrolled. Throat and nasal swab specimens were collected, combined, and tested for influenza by real-time reverse transcription polymerase chain reaction. VE was estimated by comparing the vaccination status of those testing positive for influenza with those testing negative in logistic regression models adjusted for age, sex, hospital, calendar time, time from illness onset to specimen collection, frailty score, and Charlson comorbidity index (CCI).
Results:
Among 624 patients included in the analysis, 421 (68%) were vaccinated, 337 (54%) were female, 220 (35%) were age ≥65 years, and 92% had CCI > 0, indicating ≥1 comorbid conditions. Ninety-eight (16%) patients tested positive for influenza A (H3N2); among 60 (61%) A (H3N2) viruses tested by pyrosequencing, 53 (88%) belonged to the drifted 3C.2a genetic group. Adjusted VE was 43% (95% confidence interval [CI], 4-67) against influenza A (H3N2); 40% (95% CI, -13 to 68) for those <65 years, and 48% (95% CI, -33 to 80) for those ≥65 years. Sensitivity analyses largely supported these estimates.
Conclusions:
VE estimates appeared higher than reports from similar studies in ambulatory care settings, suggesting that the 2014-2015 vaccine may have been more effective in preventing severe illness requiring hospitalization.
Insights
The 2014-2015 influenza vaccine showed 43% effectiveness against severe A (H3N2) influenza, higher than outpatient settings. This suggests the vaccine protected against hospitalization during a season with drifted viruses.
Area of Science:
- Epidemiology
- Virology
- Public Health
Background:
- The 2014-2015 influenza season featured severe illness due to influenza A (H3N2) viruses antigenically drifted from the vaccine strain.
- Outpatient vaccine effectiveness (VE) estimates for this season were notably low.
Observation:
- This study prospectively enrolled hospitalized adults with acute respiratory illness in southeast Michigan.
- Influenza A (H3N2) was detected in 16% of 624 patients; 88% of tested viruses belonged to the drifted 3C.2a genetic group.
Findings:
- Adjusted VE against hospitalized influenza A (H3N2) was 43% (95% CI, 4-67).
- VE estimates were 40% for those under 65 and 48% for those 65 and older.
- Sensitivity analyses supported the primary VE estimates.
Implications:
- Influenza vaccine effectiveness may be higher for preventing severe, hospitalized illness compared to milder, outpatient cases.
- These findings highlight the importance of continued influenza vaccine development and monitoring, especially during seasons with drifted strains.
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