Effectiveness of an Adjuvanted Monovalent Vaccine Against the 2009 Pandemic Strain of Influenza A(H1N1)v in Stockholm County, Sweden

Background. Vaccination against the pandemic influenza A(H1N1)v was performed in many countries during 2009, but population-based data on vaccine effectiveness are lacking. Methods. We conducted a prospective cohort study involving all inhabitants in Stockholm County (n = 2,019,183) who were offered a monovalent AS03-adjuvanted influenza A(H1N1)v vaccine (Pandemrix, GSK), between 12 October and 31 December 2009. Overall vaccine coverage was 52%. A Web-based register with data on all vaccinated was linked by unique personal identification number to mandatory reports of influenza A(H1N1)v diagnoses. Vaccine failure was defined as a diagnosis or admission to hospital because of influenza > 14 days after vaccination. Risk factors associated with vaccine failure were investigated by conditional stepwise logistic regression in a nested case-control study. The weekly incidence rate ratio for being diagnosed with influenza among vaccinated versus nonvaccinated persons was calculated. Results. Vaccine failure was seen in 25 patients, 11 children and 14 adults, of 2594 patients diagnosed with influenza A(H1N1)v. Compared with age-matched controls, patients with vaccine failure were more often immunocompromised (Hazard Ratio, 4.89; 95% confidence interval [CI], 2.19—10.89). During the 4 weeks with maximum influenza activity, the relative risk per week for an influenza A(H1N1)v diagnosis in the vaccinated population was .06 (95% CI .008—.41), .13 (95% CI .06—.27), .05 (95% CI .02—.12), and .07 (95% CI .03—.15), respectively, corresponding to a weekly vaccine effectiveness of 87—95%. Conclusions. The monovalent AS03-adjuvanted influenza vaccine was highly effective in prevention of the pandemic influenza in Stockholm County. A single dose seemed to be sufficient in most, both children and adults, except in immunocompromised hosts.