The shot is very effective for bacterial pneumonia, but not effective on viral pneumonia. When you go to get the shot you will be given a pamphlet that covers all the info you may want or need. I don't know if there is any preservative (thimerosal or mercury) in the shot. Usually multi-dose vials have some kind of preservative in them, but I don't know if you would be getting a dose from a multi-dose vial or a single dose vial. Either way, the amount of preservative is so miniscule as to be negligible. If you are over 65 and have asthma or any underlying health conditions it is a good idea to be vaccinated. The vaccine is probably far safer for you than getting pneumonia. I have seen the vaccine recommended for younger folks if they have other health problems.
Hope that helps.