It will be interesting to read about what the virus is comprised of (seems to use the older influenza PR8 strain as a backbone, employing a reverse genetics approach to add in H7N9) and how the vaccine makers got around H7N9's predicted low immunogenicity issue, what the dosing regimen is and what was used as adjuvant (mentioned here, earlier). As Mike Coston notes on Avian Flu Diary, the announcements don't detail much of the preceding safety trials that should have been carried out for a vaccine to have reached this level of development.
Mike has an earlier post over on Avian Flu Diary that reminds us about the few that are sick enough to be obviously ill....and perhaps the many that do not seek medical attention because infection resulted in relatively mild disease. Largely, as Mike notes, any numbers assigned to infections that result in milder or even asymptomatic disease are guesstimates for now - at least until some actual testing is reported. History supports that mild infections are likely, but every zoonosis is its own beast.
More coming soon on the vaccine's development path and on testing to understand H7N9's reach.
Thanks to @makoto_au_japon for identifying the vaccine story through Twitter