In healthy adults under the age of 65, flu vaccines work, too. A 2010 Cochrane review, also co-authored by Jefferson, estimated that during “good” vaccine years—when the vaccines match the circulating viral strain well, which Jefferson says happens about half the time—the vaccine reduces the relative risk that an adult under 65 will catch the flu by about 75 percent. In absolute terms, however, this means adults have about a four percent chance of catching the flu if they don’t get the vaccine and about a one percent chance if they do. Shay notes that while this estimate is reasonable, some flu seasons are worse than others, so the risk may be higher than 4 percent in some years (and some people) and lower than 4 percent in others. (And of course, the vaccine won’t protect against the nearly 200 viruses that cause flu-like symptoms but aren’t actually the flu.) Although scientists generally believe that the flu vaccine slows the spread of the virus through communities, there are no data showing that this is true, because “those studies are very difficult to do,” Shay explains.
(Again, one can do viral cultures and test this assertion, but it seems CDC does not wish to put it to the test, as this is one justification for vaccination that is not disputed by the data, since there are no data.–Nass)