How to tactfully tell family my newborn son will not be passed around to be held due to pertussis?

My son is now 3 weeks old. He has only been vaccinated against Hep. B. I have to go to a large family function this weekend, and I am nervous that all of the aunts (who will be meeting him for the first time, and many of whom will go JUST because they want to meet him) will expect to hold him. How can I tactfully tell them I don't want to pass him around, because pertussis is very contagious and he has not been vaccinated? (Not to mention other illnesses.) I want to offer to bring him back next month about 3 or 4 weeks after he IS vaccinated. Will this fly?