The vaccine used to stop the outbreak was causing the next one

**Circulating vaccine-derived polioviruses emerge where population immunity is
low**, when the weakened virus in oral polio vaccine passes person to person
long enough to revert to a form that paralyses.

Between July 2019 and February 2020 there were **31 ongoing and new type 2
outbreaks**, and nine spread internationally.

The trap

**New cVDPV2 outbreaks were often linked to poor coverage with monovalent
Sabin oral poliovirus vaccine type 2 during outbreak response campaigns.**

The response to a type 2 outbreak is a campaign with type 2 oral vaccine. A
campaign that reaches most children stops the outbreak. A campaign that reaches
too few seeds the next one — it introduces live vaccine virus into exactly
the under-immunised population where reversion happens.

So the intervention has a threshold, and below it the intervention is the
cause. Thirty-one simultaneous outbreaks is what that looks like once it starts
compounding.

The two ways out

A better vaccine. The Global Polio Eradication Initiative planned to
introduce a **genetically stabilised, novel oral polio vaccine type 2 for
outbreak response in mid-2020**, expanding in 2021 — engineered to be much less
likely to revert, which breaks the loop without giving up the advantages of an
oral vaccine.

Stopping altogether. **Cessation of all OPV use after certification of
polio eradication will eliminate the risk of VDPV emergence.**

The second is the real end point and it is strictly conditional: you can only
stop using the oral vaccine once wild poliovirus is certified eradicated, and
you need the oral vaccine to get there. The novel vaccine exists to make the
interval survivable.

Source: Centers for Disease Control and Prevention, MMWR.