RIO DE JANEIRO, Jan. 16 (Xinhua) -- The World Health Organization (WHO) issued a warning on Tuesday to foreign travelers headed to Sao Paulo state, the most populated in Brazil, cautioning the state was at risk for yellow fever.

The warning was issued after an unusually large outbreak of the disease, which is spread by the same mosquito that transmits other tropical diseases, including Zika, in much of Brazil last year.

According to the Brazilian Health Ministry, 470 suspected cases of yellow fever have been reported in Brazil since July 2017, with 35 of the cases confirmed, including 20 in Sao Paulo state.

Last week, the ministry reported 11 confirmed cases, a significant rise in only one week, but the ministry denied an outbreak.

The Brazilian government has said that it is acting with excess care in order to prevent an outbreak and is vaccinating people in affected areas. In southeastern states, residents spend hours in long lines to be vaccinated.

"We have enough vaccines to inoculate the entire population if necessary," said Antonio Carlos Nardi, executive secretary of the Health Ministry.

According to the ministry, so far no confirmed cases originated in urban areas; all cases occurred among residents of rural regions or tourists who had access to those areas.

Brazil eradicated urban transmission of yellow fever in the 1940s.