LONDON — Scientists are for the first time questioning the safety of the 60-year-old yellow fever vaccine after six suspicious deaths.
The deaths of three Americans, two Brazilians and one Australian after vaccination against the mosquito-borne disease are described in The Lancet medical journal.
"These are the first deaths supposedly linked to the vaccine virus," said Dr. Ray Arthur, head of the yellow fever division at the World Health Organization.
Experts are calling for an urgent investigation, but strongly recommend that people in parts of South America and Africa, as well as travelers to those areas, continue to get the shots.
The WHO estimates that yellow fever strikes 200,000 people a year, killing 30,000 of them. It occurs both in jungles and in urban areas.
The vaccine, which has been given to about 400 million people, has not changed since its introduction and has been considered one of the safest available.
It is likely that other deaths have occurred over the years but have gone unnoticed, Arthur said. New technologies now allow scientists to more accurately connect a fatality with the vaccine, he said.
Arthur, who was not connected with the reports, said the WHO does not plan to change its recommendation that people get vaccinated.
"I don't think we should be terribly concerned," he said, adding that although it is unclear what proportion of people vaccinated might get a bad reaction, complications are still very rare.
Dr. Thomas Monath, a virologist at Cambridge, England-based Acambis Inc. who described the Brazilian cases in the journal, emphasized that yellow fever is an untreatable disease that causes 1,000 times more illness and death than the feared Ebola virus.
The vaccine is created using a live version of the virus. The virulence is dampened so that a shot gives people a harmless bout of the disease. When they are later exposed to the real virus, their immune systems immediately recognize it and attack.
It now appears that some people can get very sick from the vaccine. Scientists suggested that some people may be genetically more susceptible to a bad reaction.
Arthur said the vaccine may need changing, but that it is impossible to tell yet.
Pedro Vasconcelos from the Center for Arbovirus Reference and Research in Brazil, reported two deaths, one a 5-year-old white girl, the other a 22-year-old Afro-Caribbean woman.
The symptoms in both victims were typical of yellow fever — fever, vomiting, muscle pain, jaundice and kidney failure.
Vasconcelos concluded that although such complications are rare, the safety of the vaccine should be reviewed.
The Australian report involved the death of a man showing symptoms similar to those seen in Brazil.
In a third report, scientists from the U.S. Centers for Disease Control and Prevention described the illness of four elderly patients, and the death of three of them, shortly after vaccination.
Unlike the cases in Brazil and Australia, the symptoms were not typical of yellow fever and seemed to be a new condition.
Yellow fever, so named because some patients get jaundice, is found in tropical and subtropical areas of Africa and South America. In response to a re-emergence of epidemics over the last 20 years, vaccination campaigns have increased.
"Despite the severity of these reactions, overall the findings indicate that no change in practice regarding yellow fever vaccination is needed," scientists from the Pasteur Institute in Lyon, France, advised in a commentary published in The Lancet.
"Nevertheless," they said, "the intriguing adverse effects reported today should rekindle research" into how the yellow fever virus attacks the body, how the vaccine activates the immune system and what factors might make some people react badly to the vaccine.
On the Net: U.S. Centers for Disease Control and Prevention, yellow fever travel information, www.cdc.gov/travel/yelfever.htm
World Health Organization fact sheet on yellow fever, www.who.int/inf-fs/en/fact100.html