Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: (CNN) The United States is being hit with the most mumps cases in a decade, according to the Centers for Disease Control and Prevention. The latest tally as of November 5 is 2,879 cases in 45 states and the District of Columbia. Mumps is a contagious disease caused by a virus spread through saliva and mucus. The CDC recommends that children get two doses of the MMR vaccine, which protects against measles, mumps and rubella, but it is not 100% effective. To respond to this year's high numbers, the Advisory Committee on Immunization Practices -- a panel of medical and public health experts who meet three times a year to offer vaccination guidance for the United States -- is considering recommendation of a third dose of vaccine. It is unclear what age groups this would involve.The CDC reports 2,879 cases of the mumps in 45 states and DC, the most since 2006Arkansas and Iowa have been the hardest hit 
 Question:She said that studies show the current vaccine is effective against this strain and that the _ is not concerned about changing strains.

CDC