abstract
The objective of this study was to estimate influenza vaccine effectiveness (VE) against medically attended, laboratory-confirmed influenza during the 2011-2012 season in Japan using a test-negative case-control study design. The effect of co-circulating non-influenza respiratory viruses (NIRVs) on VE estimates was also explored.

http://www.ncbi.nlm.nih.gov/pubmed/?term=24551167