Single Yellow Fever Shot Offers Lifetime Protection, W.H.O. Says One yellow fever shot confers lifetime protection and the customary "booster shot" given at 10 years is no longer necessary, the World Health Organization announced Friday. Yellow fever vaccine is relatively safe but causes more dangerous side effects than many other vaccines and is therefore normally given only to healthy people who are at risk. The mosquito-borne disease, for which there is no cure, strikes about 200,000 people worldwide each year and kills about 30,000 of them. Cases are increasing as deforestation increases contact with forest mosquitoes and the monkeys that harbor the virus. Only nine cases have been found in the United States over the past 30 years, all in travelers returning from Africa or Latin America, but eight were fatal, according to the Centers for Disease Control and Prevention. Dr. Thomas P. Monath, a yellow fever expert at the Harvard School of Public Health, said that the W.H.O. announcement made sense for almost all vaccine recipients, but that booster shots might still be needed for laboratory technicians working with the virus and for a few travelers who had one shot many years ago but whose immunity had waned.