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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
| Third-dose OPV coverage in infancy, 2017 | 44% |
| Peak during 2006–2016 | never above 70% |
| Last national vaccination campaign before this | 2012 |
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:
| Round | Scope |
|---|---|
| 1–2 (from 16 July, 20 August) | Subnational, children under 5 |
| 3–4 (from 24 September, 29 October) | National, 3.26 million children under 15 |
| 5 (from 26 November) | Subnational, under 15 |
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.
Licenza: CC0 1.0 (pubblico dominio) · Tratto da www.cdc.gov
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