Conventional wisdom suggests vaccinating elderly people first in flu season, because they are at greatest risk of dying if they contract the virus. This year's flu virus poses particular risk to elderly people and almost none at all to younger people, particularly children. Nevertheless, health professionals are recommending vaccinating children first against the virus rather than elderly people.
Which of the following, if true, provides the strongest reason for the health professionals' recommendation?
【选项】The vaccinations received last year will confer no immunity to this year's flu virus.