Comparison of observational studies evaluating the effectiveness of seasonal influenza vaccination to prevent 2009 pA/H1N1 morbidity in civilian populations.

All studies were conducted during the first wave of the 2009 pandemic (April–July 2009) and most include a majority of young adults.aNegative estimates of vaccine effectiveness indicate that vaccination may be a risk factor for 2009 pandemic illness, while positive estimates suggest a protective effect.bThree other study designs are considered in this publication but the Sentinel system is the most well-established.cAdjusted for age, comorbidities, province, interval between symptoms onset and sample collection.dAdjusted for age and comorbidities.eAdjusted for age.ILI, influenza like illness.