Many vaccines create immunity to viral diseases by introducing a certain portion of the disease-causing virus' s outer coating into the body. Exposure to that part of a virus is as effective as exposure to the whole virus in stimulating production of antibodies that will subsequently recognize and kill the whole virus. To create a successful vaccine of this type, doctors must first isolate in the disease-causing virus a portion that stimulates antibody production. Now that a suitable portion of the virus that causes hepatitis E has been isolated, doctors claim they can produce a vaccine that will produce permanent immunity to that disease. Which one of the following, if true, most strongly counters the doctors' claim? ['Most of the people who contract hepatitis E are young adults who were probably exposed to the virus in childhood also.', 'Some laboratory animals exposed to one strain of the hepatitis virus developed immunity to all strains of the virus.', 'Many children who are exposed to viruses that cause childhood diseases such as chicken pox never develop those diseases.', 'Researchers developed a successful vaccine for another strain of hepatitis, hepatitis B, after first isolating the virus that causes it.']