As many as 101 million Nigerians are at risk of a possible outbreak of yellow fever, if a mass vaccination campaign is not carried out, the National Primary Health Care Development Agency has warned.
Some 377 local government areas in 25 states have been marked out as high risk areas, indicate an assessment survey of the country.
"We are sitting on a tinderbox," warned Prof Oyewale Tomori, chairman of Nigeria's Expert Review Committee in charge of polio eradication and routine immunisation.
"If we have not done a mass vaccination campaign, it means we have a large number of people who are vulnerable."
The warning came amidst a recent outbreak of yellow fever in six districts of Cameroun bordering Cross River state, which places Nigeria at risk because it is the only country among 13 in West Africa yet to conduct mass vaccination.
Mathematical models used in assessing Nigeria's situation suggest an urban area like Lagos could see up to 4.5 million cases along with 128,000 deaths, requiring millions of vaccine doses, which take considerable time to produce.
"An outbreak of this scale will have dire consequences on our health system," said Dr Akin Oyemakinde, chief consultant epidemiologist at the federal Ministry of Health and Nigeria's Centre for Disease Control.
"And there will be potential of Nigeria exporting yellow fever to other countries."
Nigeria's last vaccination against yellow fever was in 1995 after an outbreak that lasted eight years resulted in 20,000 cases and killed more than 5,000 people.
Millions currently have no protection against yellow fever virus, leaving herd immunity low around the country.
The mass vaccination programme is planned to start by 2013 and will take up to five years to complete because of the country's size.
But experts meeting to plan Nigeria's mass vaccination programme are certain that combining routine immunisation with mass vaccination is a better option.
The World Health Organisation has called it "one of the biggest campaigns ever" for which Nigeria has to be ready.
"Even one laboratory-confirmed case of yellow fever is an outbreak by itself," warned Dr Goitom Weldegebriel of the WHO.