I have used airborne with my son due to asthma if he is exposed to the flu virus. It has worked everytime. She may want to try that before getting on the plane. Many people get sick after flying. Those planes are germ incubators.
Handwashing is the best defense. Does she not wash regularly? Or perhaps she needs a little help on proper handwashing, i.e., she has to soap up and sing the ABC's or Yankee Doodle while she lathers. We buy disposable Kleenex drying towels for the bathroom any time we have a party/gathering so everybody isn't using the same gross hand towel since they mostly rinse and dry--ewwww. Just pull the hand towels out and they'll have to use the disposable towels--or put paper towel roll in there. I hate it when people touch my baby's hands because he's always putting them in his mouth. You're the gatekeeper of the germs so don't be afraid to speak up.
Maybe she shld come b4 U have the baby so that she can have time to adjust and if she gets sick she would be ova it when the baby comes. That's what I would do. Bcuz U really don't want baby sick right after birth. And try not to let her kiss baby n the mouth or face to reduce the risk of germs and use antibacterial hand gel b4 she handles the baby. That shld help. And explain these things to her so she won't feel like U don't want her around the baby. She shld b aware of UR concerns
There's this stuff they advertise for teachers and day care workers. I think it's called Emergen-C. I have no idea if it works but you can have her try it.