Something weird about the game is the heart counter. In most games, you start with ¿5? hearts and you loose hearts, and if you have no heats the game ends, even if you have time.
It would be nice to have some objects that you should not shot, like white cells. But some moron will misinterpret that vaccines can kill white cells an avoid the vaccine and other moron will get angry at you because he misinterpret that you claim that vaccine kills white cells. Perhaps I'm too afraid of morons, but I guess it's better in this game to avoid things you should not kill.
Perhaps you can add some power-ups, like an additional heart (if you reverse the counting), or 5 more seconds, or more vaccines (if you make them finite). To avoid moron criticism, make them transparent to bullets. In most games you must move the syringe to pick them.
EDIT: Trash cans? What about some flying trash cans that you should not shot? How con morons misinterpret it?
The problem is that there are too many enemies and unlimited bullets, so the best strategy (in a keyboard) is to press:
up-space-up-space-up-space-up-space-up-space-...-down-space-down-space-down-space-down-space-down-space-down-space-down-space-...-up-space-up-space-up-space-up-space-up-space-...-down-space-down-space-down-space-down-space-down-space-down-space-down-space-...-up-space-up-space-up-space-up-space-up-space-...-down-space-down-space-down-space-down-space-down-space-down-space-down-space-...-up-space-up-space-up-space-up-space-up-space-...-down-space-down-space-down-space-down-space-down-space-down-space-down-space-...-up-space-up-space-up-space-up-space-up-space-...-down-space-down-space-down-space-down-space-down-space-down-space-down-space-...
and form a wall of bullets, without aiming. You should reduce the number of simultaneous bullets, or discount a small amount of health for a miss.
For the theme of the game, I think it's better to reduce the amount of bullets, because vaccine availability is not infinite.
Anyway, it seam to be too short, only 30 seconds. I'd like at least 5 minutes.