The effect of zanamivir treatment on influenza complications: a retrospective cohort study.
BACKGROUND
Complications of influenza are a major cause of morbidity and mortality during the influenza season. Clinical trials of zanamivir have reported a reduced incidence of influenza complications among high-risk patients.


OBJECTIVE
This retrospective study sought to determine whether the use of zanamivir lowers the risk of acute influenza complications in a broader population, based on an analysis of claims data from a large managed care organization.


METHODS
Medical and pharmacy health insurance claims data from October 1, 1999, through April 30, 2000, were compiled for UnitedHealthcare members in 19 states. All patients with a diagnosis of influenza (International Classification of Diseases, Ninth Revision, Clinical Modification diagnostic code 487.xx) associated with a physician visit were identified. From these, all patients were selected who had received zanamivir on the same day as the diagnosis of influenza. The propensity score matching technique was used to identify a comparison group with similar health service utilization and comorbidities who received a diagnosis of influenza but no antiviral therapy. Follow-up started the day after the influenza diagnosis and continued for 21 days.


RESULTS
From the 43,741 patients originally identified, 2341 were selected who received a simultaneous diagnosis of influenza and a prescription for zanamivir. The untreated comparator group numbered 2337. Fewer zanamivir patients than untreated patients were hospitalized for complications, and the absolute risks were low (0.6% and 1.0%, respectively; risk ratio [RR], 0.58; 95% CI, 0.30-1.12). Zanamivir-treated patients had an excess of outpatient visits (16.9% vs 14.5%; RR, 1.16; 95% CI, 1.02-1.33) and antibiotic use (16.3% vs 14.8%; RR, 1.10; 95% CI, 0.97-1.26), although the RRs were modest.


CONCLUSIONS
In the setting of a large managed care plan, patterns of influenza complications were similar in zanamivir-treated and untreated patients with a diagnosis of influenza. The results of this study are in contrast to those of published clinical trials reporting a reduction in the risk of influenza complications in zanamivir-treated patients.
Cole
J Alexander
JA
Epidemiology and Safety Research Division, Riverside Center, Ingenix, Inc., Auburndale, Massachusetts 02466, USA. acole@epidemiology.com
Loughlin
Jeanne E
JE
Ajene
Anuli N
AN
Rosenberg
Daniel M
DM
Cook
Suzanne E
SE
Walker
Alexander M
AM
D000293:Adolescent; D000328:Adult; D000368:Aged; D000998:Antiviral Agents / Q000627:therapeutic use*; D002318:Cardiovascular Diseases / Q000209:etiology / Q000517:prevention & control; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D005260:Female; D006146:Guanidines; D006801:Humans; D007251:Influenza, Human / Q000150:complications* / Q000188:drug therapy*; D007344:Insurance Claim Reporting; D008297:Male; D008875:Middle Aged; D010033:Otitis Media / Q000209:etiology / Q000517:prevention & control; D011714:Pyrans; D012140:Respiratory Tract Diseases / Q000209:etiology / Q000517:prevention & control; D012189:Retrospective Studies; D012306:Risk; D012621:Seasons; D012794:Sialic Acids / Q000627:therapeutic use*; D053243:Zanamivir
D000998:Antiviral Agents; D006146:Guanidines; D011714:Pyrans; D012794:Sialic Acids; D053243:Zanamivir