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.