Two of the largest mass vaccination campaigns against yellow fever ever seen in the world have begun in Nigeria and Brazil. Both campaigns, which are supported by the World Health Organization, aim to prevent the spread of the disease.

Nigeria plans to vaccinate more than 25 million people throughout the coming year, making this the largest yellow fever campaign in the country’s history. In preparation, the World Health Organization has trained thousands of health care workers on how to administer the vaccine.

The WHO says nearly 3,000 vaccination teams are being deployed across the states of Kogi, Kwara, Zamfara and Borno. In the case of Borno State, it says the campaign will focus on camps for internally displaced people and host communities.

WHO spokesman Tarik Jasarevic says the goal of the campaign is to reduce yellow fever transmission by achieving 90 percent coverage in those states.

“It is a part of an initiative to eliminate yellow fever epidemics,” he said. “As you know, we cannot … eradicate the yellow fever virus because it is being transmitted by mosquitoes. But, with the effective vaccine that exists for a number of years now, it can be prevented. So, mass vaccination is the best way to prevent outbreaks of yellow fever.”

The WHO reports the mass immunization campaign launched in Brazil will deliver so-called fractional doses of yellow fever to nearly 24 million people in Rio de Janeiro and Sao Paulo. Fractional dosing is a way of extending vaccine supplies so more people are protected from the spread of the disease.

A full dose of vaccine provides life-long protection against yellow fever. One-fifth of the regular dose confers immunity against the disease for at least 12 months and possibly longer. That is considered an effective short-term strategy in places where the vaccine is in short supply.