How did the vaccine perform this year?

In mid-February, the Centers for Disease Control (CDC) released data regarding effectiveness of the 2017-2018 influenza vaccine. 81% of all influenza infections were caused by A strains, and 85% of influenza A strains were H3N2. This is an H3N2 predominant season which will typically see greater disease severity and lowered vaccine protection \cite{cdcc}. This is evident as the overall vaccine effectiveness during the 2017-2018 season is 36% whilst H3N2 specific protection is only about 25% effective. The greatest protection was for H1N1 strains (67%) and people under the age of 8 (59%). An average flu season will see overall vaccine effectiveness around 50%. Whilst this year is undoubtedly below average, it is similar to previous H3N2 predominant seasons the last of which was the 2014-2015 season when the average vaccine effectiveness was under 20% \cite{effectiveness,Flannery2018}.