The following statistics are for the United States only.

2011: 2,125,573

2012: 2,180,201

2013: 2,145,086

2014: 2,150,287 (estimated average)

2015: 2,150,287 (estimated average)

 

Sources:

1. CDC – http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6053a1.htm

2. CDC – http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6153a1.htm

3. CDC – http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6253a1.htm