I have had similar swine flue worries since I just weaned my 18 month old fully this month. However, the fact is YOU have to have teh Swine Flu yourself in order to provide immunity. At least that's my understanding. The odds of you getting it are probably pretty small and thus the odds of offering the immunity are low.
Also, you and your child will qualify to get the vaccine first, so as long as you can maintain your health until the vaccine is available, you should be in good shape. In the meantime, you both should wash hands frequently, eat probiotics, take vitamins, etc.
As I mentioned I just weaned my 18 month old , so I haven't had an opportunity to observe changes in her immunity - although she has only ever had 1 cold and 1 stomach bug. Yea! But I weaned my older daughter, a day care baby, at 13 months. Unfortunately, she did get more infections as a toddler than as an infant. But tha tis also proabably diue to the nature of being a toddler - hands in mouth, touching other kids, etc.
I will say, BF doesn't have to be all or nothing. You might get down to one feeding at night or similar and still decide to keep it up for awhile. Jus tbe openminded. All I can say is good luck. I can't imagine nursing while pregnant, so good for you for getting this far. Enjoy this transition to a new phase with your oldest. I