Polio came back to a country that had been free of it for 22 years

Papua New Guinea's last poliomyelitis cases were in 1996, and it was among
**37 countries and areas of the Western Pacific certified free of indigenous
wild poliovirus in 2000.**

On 22 June 2018 it confirmed a polio outbreak. The index patient was **a boy
aged 6 from Lae, Morobe Province, paralysed on 25 April**, who **had received
two doses of oral polio vaccine.**

The virus that paralysed him was **circulating vaccine-derived poliovirus type
1** — descended from the vaccine.

How a vaccine virus becomes a paralysing one

Oral polio vaccine contains live, weakened virus. A vaccinated child sheds it,
and it can pass to others — which is a feature, spreading immunity beyond those
directly vaccinated.

**cVDPVs can emerge in underimmunized populations when Sabin vaccine
poliovirus is extensively transmitted person-to-person and reverts to
neurovirulence.**

The weakening is not permanent. Passed through enough unprotected people, the
virus mutates back. **Genetic characterisation found 14 nucleotide differences
from the Sabin 1 strain**, indicating more than a year of circulation before
anyone was paralysed.

So this outbreak is not evidence against the vaccine. It is evidence of what
happens when too few people get it: the virus needs unvaccinated hosts to
travel through, and the reversion is the consequence of low coverage rather
than a property of the vaccine.

Why coverage was that low

Fewer than half of infants completed the course, for more than a decade, with
substantial variation between regions. A year of undetected circulation in that
population is unsurprising.

The response

By 4 February 2019: 26 confirmed cases across nine of 22 provinces — 19 in
children under 5, six aged 5–14, one aged 17. **The most recent onset was 18
October 2018.**

Five rounds of supplementary immunisation, escalating as cases appeared beyond
the first provinces and in older children:

Reported coverage was 93% and 97% for the two national rounds — from 44%
routine coverage to the nineties in a campaign, which says the problem was the
routine system rather than willingness.

And the sentence that describes what that took:

**All SIAs faced logistical challenges requiring access to remote communities
via helicopter, boat, and by foot.**

Source: Centers for Disease Control and Prevention, MMWR.