BruceC,
My wife and I had yellow fever vaccinations last year shortly after we turned 65. Other than a pain in our pocketbook, we did not have any adverse reactions to the vaccine.
Here's some information from the CDC that might be of interest to you:
http://wwwnc.cdc.gov/travel/notices/outbreak-notice/yellow-fever-brazil.htm and
http://wwwnc.cdc.gov/travel/yellowb...ine-requirements-and-recommendations.htm .
If you decide to get a vaccination waiver from your physician, be sure he or she fills out the appropriate section on the International Certificate of Vaccination or Prophylaxis in addition to giving you a letter of waiver.
Also, our doctor strongly recommended vaccination against Hepatitis A in addition to the yellow fever vaccination.