The World Health Organization said on Tuesday that circulating vaccine-derived poliovirus type 2 (cVDPV2) had been detected in stool specimens of two healthy children in Papua New Guinea on May 9.

The detection of wild poliovirus or vaccine-derived poliovirus, including from samples taken from healthy children, is considered a serious public health event, WHO said in a statement.

It added that the detection of circulating type 2 poliovirus was classified as a “polio outbreak.”

Wild polio is only endemic in Pakistan and Afghanistan, but vaccine-derived polio continues to cause outbreaks in a wider range of countries. For example, this year, countries including Nigeria and Ethiopia, among others, have reported tens of cases of paralysis caused by polio.

This is the first polio outbreak in Papua New Guinea since 2018, when an outbreak was reported in the same area as the new detections, Lae city in Morobe province.

Vaccination protects against all forms of polio, but coverage rates in Papua New Guinea are only around 44 percent for the third dose needed for protection, the WHO said. Efforts are now underway to detect further transmission and boost vaccination coverage in the affected area.

Poliomyelitis, which is spread mainly through the faecal-oral route, is a highly infectious virus that can invade the nervous system and cause paralysis and death in young children, with those under 2 years old most at risk. In nearly all cases it has no symptoms, making it hard to detect.