Just like any vaccine, synagis helps baby build antibodies to the virus called RSV. It's important that you stay on the 28 day schedule for the shots through the season. Common sense things like hand washing are still important. It's not a 100% guarantee he won't get it, but it makes a big difference, and if he does get rsv it will likely be shorter and milder. Does he have ongoing lung issues? I'm guessing he does, considering his age. They don't frequently give a second season of synagis unless there are ongoing lung issues. 2/3 of kids get RSV by age one. RSV is a very common cold virus, but it can cause brochiolitis which is a more severe complication that can be devastating to preemies, those with chronic lung issues, certain heart defects, and the immune challenged. I'm glad he's getting the shots. Does he go to daycare?