Impact of repeated vaccination on vaccine effectiveness against influenza A(H3N2) and B during 8 seasons.
BACKGROUND
Recent studies suggest that influenza vaccination in the previous season may influence the effectiveness of current-season vaccination, but this has not been assessed in a single population over multiple years.


METHODS
Patients presenting with acute respiratory illness were prospectively enrolled during the 2004-2005 through 2012-2013 influenza seasons. Respiratory swabs were tested for influenza and vaccination dates obtained from a validated registry. Vaccination status was determined for the current, previous, and prior 5 seasons. Vaccine effectiveness (VE) was calculated for participants aged ≥9 years using logistic regression models with an interaction term for vaccination history.


RESULTS
There were 7315 enrollments during 8 seasons; 1056 (14%) and 650 (9%) were positive for influenza A(H3N2) and B, respectively. Vaccination during current only, previous only, or both seasons yielded similar protection against H3N2 (adjusted VE range, 31%-36%) and B (52%-66%). In the analysis using 5 years of historical vaccination data, current season VE against H3N2 was significantly higher among vaccinated individuals with no prior vaccination history (65%; 95% confidence interval [CI], 36%-80%) compared with vaccinated individuals with a frequent vaccination history (24%; 95% CI, 3%-41%; P = .01). VE against B was 75% (95% CI, 50%-87%) and 48% (95% CI, 29%-62%), respectively (P = .05). Similar findings were observed when analysis was restricted to adults 18-49 years.


CONCLUSIONS
Current- and previous-season vaccination generated similar levels of protection, and vaccine-induced protection was greatest for individuals not vaccinated during the prior 5 years. Additional studies are needed to understand the long-term effects of annual vaccination.
['McLean|Huong Q|HQ|', 'Thompson|Mark G|MG|', 'Sundaram|Maria E|ME|', 'Meece|Jennifer K|JK|', 'McClure|David L|DL|', 'Friedrich|Thomas C|TC|', 'Belongia|Edward A|EA|']
D000293:Adolescent D000328:Adult D000368:Aged D000369:Aged, 80 and over D002648:Child D015897:Comorbidity D005260:Female D006801:Humans D007117:Immunization, Secondary D053122:Influenza A Virus, H3N2 Subtype D009981:Influenza B virus D007252:Influenza Vaccines D007251:Influenza, Human D008297:Male D008875:Middle Aged D017063:Outcome Assessment, Health Care D012307:Risk Factors D012621:Seasons D014611:Vaccination D055815:Young Adult
vaccine effectiveness influenza