The vaccine appears to offer the greatest protection, however, to patients who were not vaccinated within the previous 5 years, researchers wrote in Clinical Infectious Diseases.
In the observational study conducted across eight influenza seasons, researchers evaluated vaccine effectiveness in a cohort of 7,315 community residents within a 14-ZIP code area near Marshfield, Wis. The study was conducted between the 2004-2005 and the 2012-2013 influenza seasons. Participants in the 2004-2005 through 2006-2007 portion of the study period were limited to those for whom vaccination was recommended based on age or other high-risk status. From 2007-2008 onward, the study population was expanded to include all residents of the community aged 9 years and older.
Patients were recruited for each influenza season during clinical examinations for acute respiratory illness. They were interviewed about their symptoms, and nasal, nasopharyngeal or combined shallow nasal/oropharyngeal swabs were taken and tested for influenza. A real-time immunization registry was utilized to obtain vaccination history for participants; this data included vaccination status for the current, previous and past five influenza seasons. Logistic regression models with an interaction term for vaccination history were utilized to determine the vaccine effectiveness against influenza A(H392) and influenza B viruses.
The researchers found that during eight influenza seasons, 1,056 (14%) were positive for influenza A(H392) and 650 (9%) were positive for influenza B. Vaccine effectiveness for influenza A was comparable for patients vaccinated during the current season only, the previous season only, or both seasons (adjusted vaccine effectiveness range, 31%-36%) and protection also was comparable for influenza B regardless of coverage history over these two seasons (adjusted vaccine effectiveness range, 52%-66%).
The analysis of 5-year vaccination history revealed that patients with no prior vaccination history derived significantly higher current-season vaccine effectiveness against H3N2 (65%, 95% CI; 36%-80%) vs. vaccinated patients with a history of frequent vaccination (24%, 95% CI; 3%-41%). Vaccine effectiveness against influenza B was 75% (95% CI; 50%-87%) among patients with no previous vaccination history vs. 48% (95% CI; 29%-62%) of those who were frequently vaccinated. These associations persisted when the researchers confined the analysis to adults aged 18 to 49 years.
“Further observational studies that include simultaneous assessment of immune response, both to vaccine antigens and previously circulating viruses, and clinical protection would be helpful,” the researchers wrote. “In addition, we believe the results of this study and others support the need for another multiseason randomized clinical trial.”