i work in an ICU unit at a hospital. we give the flu vaccine all the time. best thing to do is take your temp. if you have a temp higher than 99.5, then wait. anything lower than that and you should be gtg with the vaccine if you're wanting to get it. with a temp higher than 99.5, you can book it that your body is more worried about fixing itself (read lowered immune system) and with the flu vaccine, you COULD start feeling bad/get sick after getting it. the flu vaccines aren't necessarily inactivated forms of the virus that get people as sick as they used to, but with a compromised immune system, you could end up feeling bad for a few days after.
hope this helps....