If getting a flu shot is really the best thing for everyone then someone who ran around inoculating people with flu shots when they weren't paying attention would be a superhero! "Thank you Flu Man!" People would scream while rubbing their arm where he had stabbed them. Turns out you're not allowed to do that because it's "illegal". Sorry I didn't know, it was a mistake! There's no reason to take legal action, Lady on The Park Bench Who Was Screaming And Thought I Had Stabbed Her With Drugs (if you're reading this).
Telling people they're selfish for not getting a flu shot is just plain rude. And the only reason someone who's already had the flu shot would be telling other people to get one is if they were still scared about getting the flu. That doesn't make sense. If you're so worried about still getting the flu after your shot then your confidence in the shot must not be too high in the first place!
Just do what I do and never leave the house without one of those Michael Jackson Contagion masks. Yeah people stare, but if you put a hat on, wear a toy soldier type jacket, and have a sparkly glove, people will really think you're the King of Pop so that's a thrill. If you're worried, get a flu shot. If you want to guilt others into getting the shot afterward just remember, it's best to keep your mouth closed because that's how the germs get in.