Brazil's south-eastern state of Minas Gerais has declared a public health emergency following a deadly outbreak of yellow fever.
At least 15 people have died there since December. Many areas, including the state capital Belo Horizonte, have been affected.
A mass vaccination programme is in place in three southern states.
But queues have formed outside clinics in Rio and Sao Paulo amid concerns that vaccines could run out.
In neighbouring Argentina, there have also been long queues for the vaccine in Buenos Aires and other cities as thousands of prospective tourists prepare to travel to Brazil for carnival.
On Tuesday the WHO recommended that travellers to Sao Paulo state get a yellow fever vaccine before visiting.
Minas Gerais has been the hardest-hit Brazilian state. The health emergency will be in place for six months and will allow local authorities to commission special services and buy in emergency materials.
What is yellow fever?
- Caused by a virus that is transmitted to humans by mosquitoes
- Difficult to diagnose and often confused with other diseases or fevers
- Most people recover after the first phase of infection that usually involves fever, muscle and back pain, headache, shivers, loss of appetite, and nausea or vomiting
- About 15% of people face a second, more serious phase involving high fever, jaundice, bleeding and deteriorating kidney function
- Half of those who enter the "toxic" phase usually die within 10 to 14 days
The WHO's advice is for all travellers to Sao Paulo to get a vaccination at least 10 days before travelling and to take measures to avoid mosquito bites.
Brazilian Health Minister Antonio Nardi responded by saying that most people attending Brazil's carnival celebrations in February should be safe as they are held in large cities and not the rural and forested areas which have seen the biggest increase in yellow fever cases.
Mr Nardi said Sao Paulo state authorities would speed up their vaccination campaign with the aim of vaccinating half of the state's population by the end of February.
More than 45 million people live in Sao Paulo state.