5

Calculated by taking 100/average percent of population infected per year from 2008-2017

Average percent of population infected per year from 2008-2017 was calculated from the number of infections each year (Sources: 2008-2016:https://www.cdc.gov/vaccines/pubs/pinkbook/downloads/appendices/E/reported-cases.pdf 2017:https://wonder.cdc.gov/nndss/static/2017/annual/2017-table2j.html and the US population in each year (Sources: 2008-2010:https://fusiontables.google.com/DataSource?dsrcid=225439#rows:id=1 2011-2017:https://www.google.com/publicdata/explore?ds=kf7tgg1uo9ude_&met_y=population&hl=en&dl=en)