A company was testing a new drug it thought would help decrease the risk of transmission of a particular virus from mother to fetus. In an experiment to test the drug, an investigator gave 400 pregnant female rats a small dose of the experimental drug and inoculated each with the virus. At the same time, 400 other pregnant rats we're inoculated with the virus but not administered the drug. Of the rat pups born to the females that received both the virus and the drug, 203 showed no symptoms of infection; 205 rat pups born to the virus-only females showed symptoms. From these data, we can best conclude .