Pat & I were on the 08 Discovery cruise and the only place that required yellow fever, at the start, was Brazil. At the last minute Richards Bay required it because we had been near an area with a suspected outbreak of yellow fever. Those who joined after Brazil and who didn't have the shot or their card were required to get one from the ship's doctor's office - no exceptions. Some who had a valid card but didn't bring it with them were able to get a fax copy to satisfy the requirement. Those who couldn't get a copy sent, including one couple who had a shot less than a year before, had to get another one. The only option was to debark in Kenya and rejoin the ship later at their own expense. We've never had a reaction to the shot and always carry our cards "just in case."
Jim