A study reported by Griffin et al. compared the rate of pneumonia between 1997 and 1999 before pneumonia vaccine (PCV7) was introduced and between 2007 and 2009 after pneumonia vaccine was introduced. Read the excerpts from the abstract, and answer the question that follows it. (Source: Griffin et al., “U.S. hospitalizations for pneumonia after a decade of pneumococcal vaccination,” New England Journal of Medicine, vol. 369 [July 11, 201]: 155–163) We estimated annual rates of hospitalization for pneumonia from any cause using the Nationwide Inpatient Sample database. . . . Average annual rates of pneumonia-related hospitalizations from 1997 through 1999 (before the introduction of PCV7) and from 2007 through 009 (well after its introduction) were used to estimate annual declines in hospitalizations due to pneumonia. The annual rate of hospitalization for pneumonia among children younger than 2 years of age declined by 551.1 per 100,000 children . . . which translates to 47,000 fewer hospitalizations annually than expected on the basis of the rates before PCV7 was introduced. Results for other age groups were similar. Does this show that pneumonia vaccine caused the decrease in pneumonia that occurred? Explain.