Vaccines contain antigenic materials; they stimulate the production by the body of antibodies that react specifically with the antigens. If the antibody is to be effective in preventing disease, it must match a particular antigenic component of the disease agent which is responsible for the symptoms. There must be enough of the antibody to do this, and it must be present in the right part of the body at the right time. Whether recovery from the natural disease leaves immunity or not is less important to the success of a vaccine than one would expect it to be. We could have good vaccines against diseases due to streptococci (sore throat, scarlet fever, and others) and pneumococci (lobar pneumonia), except that there are so many different varieties or types of each of these bacteria, each of which immunizes only against itself, that one can have streptococcal or pneumococcal disease repeatedly, and effective vaccines seem impractical. The same seems to be true of the common cold group of viruses: there are just too many of them. With influenza there is a related but distinct problem: the two main types of influenza virus are subject to mutation which alters them antigenically; a vaccine can be effective only if it matches the genetic type involved in an outbreak.

The most effective vaccines are those prepared against viruses or what are called "toxins" in a restricted sense; and they are most effective when the virus or toxin is present in the circulating blood at some time before the most serious symptoms of the disease appear. Viruses are the smallest infectious agents, being in effect single molecules, huge as such but very much smaller than bacteria. True toxins of which diphtheria and tetanus are the most important examples are also single molecules. As such, both viruses and toxins are neutralized by antibodies specific for them; the virus becomes noninfectious, the toxin nonpoisonous. The viruses of smallpox and polio, and the toxins of diphtheria and tetanus, do most of their damage after being distributed by the bloodstream. If enough antibody to neutralize the virus or the toxin is present in the blood before this happens, the serious symptoms do not develop. (see THE CHANCE FOR VD VACCINES PART III)