Refer to the graph below to respond to the following prompts.

Ten line graphs depicting the rate of polio cases versus vaccination coverage between 1980 to 2015. The y-axis is labeled Polio cases per million and starts at the bottom at 0 and stops around 300 at the top. The x-axis is labeled Pol 3 (percent of one-year-olds immunized) and starts at the left at 0 percent and goes past 80 percent. Five lines at the bottom start between 1 and 20 percent (x-axis) and between 25 and 90 (y-axis), and the end of all five lines end up at 0 (y-axis) and between 65 and 100 percent (x-axis). One line at the top starts to the left at 300 (y-axis) and 1 percent (y-axis), then ends to the right at 0 (y-axis) and 100 percent (x-axis). There are two lines that start to the left between 75 and 90 (y-axis) and approximately 5 percent (x-axis) then end all the way to the right at 0 (x-axis) and between 65 and 85 percent (x-axis). Two more lines start at the left between 140 and 160 (y-axis) and between 18 and 22 percent (x-axis), and the lines end at the right at 0 (x-axis) and 100 percent (y-axis).
© 2017 OurWorldInData
Creative Commons, CC BY

Part 1: Based on the information presented in the graph, describe the correlation between polio cases and vaccine coverage. Cite data from the graph to support your answer.
Part 2: A classmate is having trouble understanding how vaccines provide immunity. Using what you have learned in the lesson, write an explanation to share with your classmate about how vaccines work with your immune system to protect you against infection.