A mother and child in Port Morseby, Papua New Guinea. Infants are bearing the brunt of the current polio outbreak. Clyde Lovuru / EyeEm/Getty Images

Almost two decades after being declared free of the disease, Papua New Guinea (PNG) is battling an outbreak poliomyelitis.

The re-emergence of the disease – the first cases seen in the country since 1996 – has been traced to the strain of poliovirus used in the standard vaccine administered to protect against the illness.

But the real cause, reports a team of medicos writing for the Centres for Disease Control and Prevention (CDC), is not the vaccine itself, but the fact that not enough people have received it.

The country was officially certified free of polio by the World Health Organisation in 2000.

Researchers led by Mathias Bauri of PNG’s National Department of Health report 26 confirmed cases of polio occurring between April and October 2018.

The cases have been recorded in nine of the country’s 22 provinces.

The first, or index, case of the current outbreak is identified as a boy aged six from Lae in Morobe Province, who succumbed to paralysis on April 25.

Investigating doctors discovered he had received two doses of the standard Sabin oral poliovirus vaccine (OPV).

Genetic testing revealed the cause of his illness was a variant of vaccine-derived poliovirus type 1 (cVDPV1).{%recommended 6030%}

The virus differed from the vaccine type across 14 nucleotides, leading researchers to conclude that it had been circulating in the community for more than a year.

Of the subsequent cases, all have been young, with 19 of them under five years old. Two-thirds of the cases have arisen in areas containing mines or plantations, which have highly transient populations.

Bauri and colleagues say the outbreak is because vaccination rates across the country remain too low, meaning cVDPV1 can be extensively transmitted person-to-person in conditions that allow it to mutate and turn virulent.

Because of a multitude of factors including inadequate access to healthcare, infant vaccination against polio was as low as 44% in 2017, and never climbed about 70% during the preceding decade.