The alert explains that a large outbreak of the disease began in Nigeria in September 2017 and has now spread throughout the country. According to the CDC, by the end of November 2018: 14 states had reported more than 55 laboratory-confirmed cases of yellow fever, all 36 states and the Federal Capital Territory had reported more than 3,500 suspected cases and a number of deaths, and Edo State had reported a large number of suspected cases, several of which had been laboratory-confirmed as yellow fever.
Travellers to Nigeria are advised to receive vaccination against yellow fever at least 10 days before travel and to take steps to prevent mosquito bites while there. Precautions against mosquito bites that can be taken include: covering legs and arms with long sleeves and trousers, sleeping under a mosquito net, avoiding standing in water that may breed mosquitos, and using Environmental Protection Agency-registered insect repellents. The alert states that people who have never been vaccinated against yellow fever should avoid travel to Nigeria during the outbreak.