Randomly selected 100 children in the 5-year-old group for immunization prevention trial, followed up for 8 years, the results showed that 70% of immunized patients did not suffer from the disease of interest. In this study, which followingopinionis correct?
A、The vaccine is effective because 70% of children do not have the disease
B、The vaccine is ineffective because 30% of children have the disease
C、Can't conclude because there is no statistical test
D、Can't conclude because the observation time is not long enough
E、Can't conclude because there is no control group