Usefulness of health registries when estimating vaccine effectiveness during the influenza A(H1N1)pdm09 pandemic in Norway

Bernardo Rafael Guzmán Herrador,Preben Aavitsland,Marianne A Riise Bergsaker,Berit Feiring,Katrine Borgen

doi:10.1186/1471-2334-12-63

Abstract

BackgroundDuring the 2009-2010 pandemic in Norway, 12 513 laboratory-confirmed cases of pandemic influenza A(H1N1)pdm09, were reported to the Norwegian Surveillance System for Communicable Diseases (MSIS). 2.2 million persons (45% of the population) were vaccinated with an AS03-adjuvanted monovalent vaccine during the pandemic. Most of them were registered in the Norwegian Immunisation Registry (SYSVAK). Based on these registries, we aimed at estimating the vaccine effectiveness (VE) and describing vaccine failures during the pandemic in Norway, in order to evaluate the role of the vaccine as a preventive measure during the pandemic.MethodsWe conducted a population-based retrospective cohort study, linking MSIS and SYSVAK with pandemic influenza vaccination as exposure and laboratory-confirmed pandemic influenza as outcome. We measured VE by week and defined two thresholds for immunity; eight and 15 days after vaccination.ResultsThe weekly VE ranged from 77% to 96% when considering 15 days or more after vaccination as the threshold of immunity and from 73% to 94% when considering eight days or more. Overall, 157 individuals contracted pandemic influenza eight or more days after vaccination (8.4/100,000 vaccinated), of these 58 had onset 15 days or more after vaccination (3.0/100,000 vaccinated). Most of the vaccine failures occurred during the first weeks of the vaccination campaign. More than 30% of the vaccine failures were found in people below 10 years of age.ConclusionsHaving available health registries with data regarding cases of specific disease and vaccination makes it feasible to estimate VE in a simple and rapid way. VE was high regardless the immunity threshold chosen. We encourage public health authorities in other countries to set up such registries. It is also important to consider including information on underlying diseases in registries already existing, in order to make it feasible to conduct more complete VE estimations.

Highlights

During the 2009-2010 pandemic in Norway, 12 513 laboratory-confirmed cases of pandemic influenza A(H1N1)pdm09, were reported to the Norwegian Surveillance System for Communicable Diseases (MSIS). 2.2 million persons (45% of the population) were vaccinated with an AS03-adjuvanted monovalent vaccine during the pandemic
On 19 October 2009, just before the main wave of the pandemic, the vaccination campaign against influenza A (H1N1)pdm09 started in Norway with vaccine available from GlaxoSmithKline Biologicals S.A. (GSK) through an advance purchase agreement
Clinical trials containing A(H1N1)pdm09 indicated that one dose would provide sufficient immunogenicity in most formulations [2] and the recommendation was changed at the end of October 2009

Summary

Introduction

During the 2009-2010 pandemic in Norway, 12 513 laboratory-confirmed cases of pandemic influenza A(H1N1)pdm, were reported to the Norwegian Surveillance System for Communicable Diseases (MSIS). 2.2 million persons (45% of the population) were vaccinated with an AS03-adjuvanted monovalent vaccine during the pandemic. During the 2009-2010 pandemic in Norway, 12 513 laboratory-confirmed cases of pandemic influenza A(H1N1)pdm, were reported to the Norwegian Surveillance System for Communicable Diseases (MSIS). During the 2009-2010 influenza pandemic, surveillance for influenza in Norway was enhanced: laboratory-confirmed infection with the new A(H1N1)pdm virus was defined as a mandatory notifiable disease on a case based level. On 19 October 2009, just before the main wave of the pandemic, the vaccination campaign against influenza A (H1N1)pdm started in Norway with vaccine available from GlaxoSmithKline Biologicals S.A. Two doses were recommended by EMA based on data on the mock up vaccine, containing an A(H5N1) virus strain [4]. The recommendation was expanded to the whole population over six months of age on 23 October 2009 [5,6]

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