Noroviruses Cause Roughly 21 Million Cases of Gastroenteritis ("Stomach Flu")

Noroviruses cause roughly 21 million cases of gastroenteritis ("stomach flu") annually in the United States. A study examined the effectiveness of a new norovirus vaccine. Healthy adult volunteers received, at random, either the vaccine or a placebo. They were then exposed to the norovirus. In all, 14 of the 38 vaccinated subjects suffered from gastroenteritis, compared with 27 of the 39 subjects who received a placebo. Which type of study is this?

A) Case-control observational study
B) Matched pairs or repeated measures experiment
C) Completely randomized experiment
D) Experiment with a block design