Scientists at the National Institutes of Health are collaborating with the government of Thailand to test the effectiveness of a new vaccine in preventing HIV infection among prostitutes. Half the women receive the experimental vaccine and half receive an injection of salt water, and the women are then monitored for development of HIV infection. What kind of epidemiologic study is this? a) Cohort
b) Case-control
c) Intervention
d) Retrospective

[120 total points] (public health) in one of the largest human experiments ever conducted, in 1954 a randomized controlled trial was run to see whether a vaccine developed by a doctor named jonas salk was effective in preventing paralytic polio. a total of 401,974 children (ages 6–9), chosen to be representative of those who might be susceptible to the disease, were randomized to two groups: 200,745 children (the control group c) were injected with a harmless saline solution (a placebo) and the other 201,229 children (the treatment group t) were injected with salk’s vaccine.what was the point of giving saline solution to the children who didn’t get the vaccine? explain briefly.