The case of a three-year-old girl was caused by vaccine-derived poliovirus type 2, a joint statement by the WHO and UNICEF has said. The virus has also been detected in sewage in Manila, the nation's capital, and in the province of Lanao del Sur.
The weakened virus in vaccines replicates in children's stomachs before it is excreted. In rare cases, the virus can strengthen in regions where sanitation and hygiene are poor, the WHO has said.
Public confidence in vaccines in the Philippines is already at a low point after a dengue fever vaccine was blamed for the deaths of at least three children in 2017. The vaccination programme was halted when Sanofi, the manufacturer, revealed the vaccine could increase the risk of severe dengue infection.
Despite these concerns, Philippine's health officials plan to ramp up the polio vaccination programme. They are launching a mass vaccination campaign in November for children under the age of five.