Yellow fever outbreak in Ipetu-Ijesa: response after mass vaccination.

An outbreak of yellow fever in Ipetu-Ijesa, Nigeria, was confirmed by serological test and virus isolation. Serologically testing of subjects 12 weeks after an emergency mass vaccination following the outbreak showed a high degree of cross reactivity between yellow fever virus and seven other group B flaviviruses. There was no significant difference in the prevalence of neutralizing antibodies to yellow fever vaccine and wild virus among the vaccinated subjects. These findings are attributed to the probable hyperactivity and an all time exposure to yellow fever virus in circulation in the studied area.