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Mismatched 2014-2015 flu vaccine more effective than initially reported
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Recent vaccine effectiveness data collected from Michigan hospitals suggested the antigenically mismatched 2014-2015 influenza vaccine performed better than was reported previously.
“[Influenza virus drift] was a particular problem in the 2014-2015 season when antigenically drifted influenza A(H3N2) viruses circulated widely resulting in high influenza illness rates and need for medical attention, including hospitalization,” Joshua G. Petrie, MPH, researcher at the University of Michigan School of Public Health, and colleagues wrote in Clinical Infectious Diseases. “Analyses from the U.S. Flu [Vaccine Effectiveness (VE)] Network conducted in ambulatory care sites indicated very poor VE against antigenically drifted strains. Specifically, VE against the predominant drifted A(H3N2) group 3C.2a viruses was only 1% compared with 44% against vaccine-like A (H3N2) group 3C.3b viruses.”
To better define VE during the 2014-2015 season, Petrie and colleagues examined adults hospitalized with acute respiratory infection (ARI) at two southeast Michigan health care facilities. Along with demographic characteristics, self-reported influenza vaccination status and assessments of frailty, they collected throat and nasal swab specimens for influenza identification via reverse transcription-PCR. The researchers then estimated VE by comparing vaccination status against positive influenza diagnoses.
Joshua G. Petrie
Within the cohort of 624 ARI patients, 54% were female, 92% had at least one comorbidity and 35% were aged at least 65 years. Two-thirds reported vaccination at least 14 days prior to illness onset, and 16% tested positive for influenza A(H3N2).
Influenza cases were less likely to be vaccinated, had shorter hospital stays and lower frailty scores than controls, but were otherwise similar. Overall adjusted VE against influenza A(H3N2) was 43% (95% CI, 4-67), but ranged from 67% (95% CI, 11-89) in adults aged younger than 50 years to 10% (95% CI, –140 to 65) in those aged 50 to 64 years. Eighty-eight percent of the 60 influenza A(H3N2) viruses tested by pyrosequencing belonged to the drifted 3C.2a group.
Although these findings were “dramatically different from those expected,” evidence from multiple sensitivity analyses detected no conclusive source of confounding bias, the researchers wrote.
“Continued consideration of the potential biological, including serum antibody, and methodological factors that could contribute to variation in estimates is critical,” they wrote. “In any event, we can be encouraged by the current results which suggest that inactivated vaccines may perform better in preventing hospitalization than mild disease in a year with antigenic drift.” – by Dave Muoio
Disclosure: Petrie reports no relevant financial disclosures. Please see the full study for a list of all other authors’ relevant financial disclosures.
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