[Impact of vaccination against influenza (H1N1) 2009 in Navarre: comparison of different scenarios].

Background: A specific vaccination campaign against influenza A (H1N1) was conducted in 2009. We evaluated its impact in Navarre.

Methods: In the cohort of non-institutionalised population with chronic diseases covered by the Navarre Health Service (n=131,333), assuming 100% effectiveness from day 8 after administration of the pandemic vaccine, we estimated its impact on the prevention of influenza A (H1N1) 2009 cases and hospitalisations between weeks 47/2009 and 3/2010.

Results: In the nine weeks of the study, 973 cases of influenza syndrome were diagnosed (7 per 1000); but only 28% were due to influenza A (H1N1) 2009. In addition, there were 14 hospitalisations with virological confirmation (11 per 100,000). With 19% coverage with the pandemic vaccine (versus 40% with the seasonal vaccine), 7.7% of cases and 10.5% of hospitalisations were prevented during the study period. For each case prevented, 1092 doses of pandemic vaccine were administered, and for each hospitalisation avoided 15,021 doses were administered. If coverage had been the same as for the seasonal vaccine, it would have been possible to prevent 16.2% of cases and 22.2% of hospitalisations. If coverage had been double than for the seasonal vaccine and vaccination campaign had taken place two weeks earlier, it would have been possible to prevent 70.7% of cases and 68.0% of hospitalizations, with 261 doses needed to prevent one case and 6206 doses to avoid one hospitalisation.

Conclusion: Despite the high effectiveness of the vaccine, its impact in Navarre has been minimal due to low coverage and late initiation of the vaccination campaign.