July 13, 2001 -- Within the past five years, seven people who had been vaccinated against yellow fever ended up contracting the disease from the vaccine itself. Of those, six subsequently died, according to reports that raise new concerns about the safety of a 60-year-old, widely used vaccine required for travelers destined for parts of South America and sub-Saharan Africa.
Yet experts emphasize that these incidents remain extraordinarily rare when compared with the numbers of people who safely receive the vaccine. And they stress that the risks of getting yellow fever in high-risk areas of the world without immunization are far greater than the risk of any severe side effects from the vaccine.
"This is a safe vaccine," says Thomas Monath, MD. "All vaccines have some side effects and sometimes these are severe. It really shouldn't change the way we use the yellow fever vaccine."
Monath is vice president of research and medical affairs at Acambis, Inc., a Cambridge, Mass., biotechnology firm. He was an author of one of the reports that appear in the July 14 edition of the medical journal The Lancet.
Despite the assurances, the yellow fever cases have startled public health experts. And they have underscored yet again that there is no such thing as a perfectly safe vaccine.
Monath's report described a 5-year-old girl and a 22-year-old woman -- both in Brazil -- who developed symptoms of yellow fever within four days of being immunized. Those symptoms included headache, feeling unwell, and vomiting, symptoms that ultimately progressed to jaundice -- the yellowing of the skin characteristic of yellow fever -- and death.
A second report by researchers at the CDC in Atlanta described four similar cases in older Americans (over age 63) who received the vaccination prior to travel. Three of those cases resulted in death.
And a third "research letter" from physicians in Australia describes a 56-year-old Australian who developed symptoms of yellow fever within days of receiving the vaccination and died a week later.
So why did these particular individuals succumb -- and not the millions of others who have received the vaccine?
Monath suggests that some people may carry an extremely rare genetic susceptibility that renders them likely to contract the illness when vaccinated. Others have questioned whether differences in the viral strains used to make the vaccine, contamination, or other factors account for events
And why are the reports surfacing now, when the vaccine has been in use for decades?
Public health experts suggest that better monitoring and reporting of bad reactions to vaccines and drugs may have helped turn up cases that have been missed in the past.
Martin Cetron, MD, an author of the CDC report, says more than 250,000 doses of the vaccine are dispensed in the U.S. each year, and the likelihood of serious problems is only one in 400,000.
"We are talking about something which is so infrequent that one might not see a case but every year or so," says Cetron, deputy director of the division of global migration and quarantine at the CDC.
Monath urges travelers to receive the vaccine only if they are traveling to the areas where yellow fever is commonplace -- including tropical South America or sub-Saharan Africa. However, he notes that some other countries may require proof of vaccination to enter, especially if they are passing through areas where the disease is rampant.
At the same time, Monath says travelers headed to dangerous areas are being under-vaccinated. And both he and Cetron urge travelers to recognize the life-threatening nature of yellow fever.
"If you are traveling in an area of ongoing transmission, such as Western Africa or the Amazon, your risk of getting yellow fever unprotected by vaccine is many times higher than your risk of [having problems] from the vaccine," says Cetron.