Initial studies on seasonal influenza vaccine effectiveness for 2013/2014

​Influenza vaccine effectiveness in Spain 2013/14: subtype-specific early estimates using the cycEVA study,  Jiménez-Jorge S, Pozo F, de Mateo S, Delgado-Sanz C, Casas I, García-Cenoz M et al. Euro Surveill. The authors used a test-negative case control design to estimate the 2013/14 influenza vaccine effectiveness (VE) in patients seen with influenza-like illness as outpatients in Spain during December 2013-January 2014.  A total of 61% of the cases in Spain during that time were due to influenza A(H1N1)pdm09 and 39% due to influenza A(H3N2).  They calculated the VE for all age groups and all viruses was 35% (95% confidence interval (CI) -9 to 62), to influenza A(H1N1)pdm09 was 33% (95% CI -33 to 67) and to influenza A(H3N2) was 28% (95% CI -33 to 61). The VE was slightly higher in populations targeted for vaccination.  The authors concluded that the sub-optimal protective effect of the vaccine should encourage healthcare workers to treat patients with suspected influenza with early antiviral treatment and to promote non-pharmacological prevention measures.

Interim estimates of 2013/14 influenza clinical severity and vaccine effectiveness in the prevention of laboratory-confirmed influenza-related hospitalisation, McNeil SA, Shinde V, Andrew M, Hatchette TF, Leblanc J, Ambrose A, et al. Euro Surveill.

The authors used a test-negative case control design to determine the vaccine effectiveness of the 2013/14 influenza vaccine to prevent influenza-related hospitalisations between November 2013 and February 2014 in Canada. Almost all of the cases had influenza A and, of those, the majority had influenza A (H1N1)pdm09.  The overall vaccine effectiveness for those >16 years of age, adjusting for medical co-morbidities and age, was determined to be 58.5% (90% CI 43.9-69.3) overall and 57.9% (90% CI 37.7-71.5) against influenza A(H1N1)pdm09.  Almost 90% of those with confirmed influenza had a co-morbidity associated with severe outcome but only 39% were vaccinated against influenza. The authors concluded that the vaccine provided moderate effectiveness against influenza hospitalization but that the public health impact is significant given the morbidity associated with hospitalization.  Furthermore, they conclude that the public health impact could be greater if more people in high-risk groups would be vaccinated.

ECDC comment

The results of the two vaccine effectiveness studies summarised above are similar to earlier estimates of the 2013/2014 influenza VE.  It is not clear why there are such different results between the studies from Canada and Spain. Jiménez-Jorge et al speculate there might have been different characteristics of the circulating A(H1N1)pdm09 in North America compared with Spain that might have affected the protective effect of vaccination. The use of different vaccine types, or that Canada had a higher proportion of its population vaccinated previously with the monovalent H1N1 vaccine than Spain, may also affect the comparability of the studies. Further investigations and on-going monitoring of effectiveness are needed to ensure timely improvement of vaccination programmes and to determine vaccine composition for the following season. Even with a poor effectiveness, influenza vaccine remains the most effective public health countermeasure against influenza.