7 ms·

I was under the impression that vaccine derived polio would eventually fizzle out after the entire population becomes vaccinated (i.e. "herd immunity")?

ars 5y ago
That was the idea - but it failed. Type 2 does not exist in the wild, it's all from vaccine derived. It got so bad they stopping giving type 2 vaccine, but then people have no immunity to the vaccine derived version - but if you give the vaccine, then the vaccine causes more cases (of type 2) than it's stopping. There's no solution here - either option makes things worse. So I hope the new version of the vaccine works, because otherwise this will fail. Type 3 appears basically extinct - both in the wild, and vaccine derived. Only type 1 still exists in the wild, and vaccine derived type 2.
- gus_massa 5y ago
  IIUC the idea is to mix the oral vaccine with attenuated virus with only the types 1 and 3, and the injectable vaccine with inactive virus with the types 1, 2 and 3. https://en.wikipedia.org/wiki/Polio_vaccine#Schedule https://en.wikipedia.org/wiki/Polio_vaccine#Schedule So you have coverage for the three types. (It's more complicated. For example country without recent cases and good vaccine coverage only use the injectable vaccine.)
  - makomk 5y ago
    The problem is that although the inactivated vaccine provides good protection against severe polio it doesn't seem to work too well at stopping people from spreading the virus, which is not ideal for an elimination program. Presumably this was less of an issue in the developed world because polio spreads mostly through the oral-fecal route and there were already a lot of hygene and infrastructure improvements in developed countries aimed at stopping diseases from spreading that way.
- yholio 5y ago
  > if you give the vaccine, then the vaccine causes more cases (of type 2) than it's stopping. > There's no solution here - either option makes things worse. That's false and misleading. The OPV2 vaccine will stop both vaccine-derived cvdpv2 virus from spreading and the wild type 2, which has been eradicated. Since OPV2 is a live attenuated virus, it will also continue to multiply and confer protection to any person drinking contaminated water, the vaccine "spreads". It is thus highly effective at stopping polio and in no way it can be said that it "causes more cases than it's stopping". The problem with OPV2 is that it has a relatively higher chance of reverting to an variant that causes polio. This is not a problem if the population has a high level of vaccination, since the live attenuated virus cannot propagate and mutate. It's only problematic in low coverage areas where it can multiply extensively in many hosts. Thus, the cvdpv2 epidemic is an expression of low vaccination rates, similar in every way to a wild poliovirus resurgence. The novel OPV2 vaccine will improve the genetic stability of the attenuated virus, allowing further "viral vaccinations" in low coverage areas - but we could eradicate cvdpv2 today with the existing OPV2 vaccine, if only we could get good vaccinations rates everywhere, as it has happened in most of the world.
  - ars 5y ago
    You are completely misinformed about this. I spent quite a long time reading about this. What you write is what they hoped would happen. However it did NOT happen. The reason is that the more you try to vaccinate the entire population, the more cases of cvdpv2 you cause. Until you hit that magic 100% you cause more harm than you prevent. And in the real world 100% is impossible. So yes opv2 "causes more cases than it's stopping" - there are zero cases of wild type 2, so it's stopping nothing except itself. And in fact that people running this campaign noticed this and stopped vaccinating with opv2! (Which perhaps you did not know.) The new version of it will hopefully help, and we can eradicate this. But right now Polio eradication is failing, not because of Taliban, but because of cvdpv2. I have high hopes for the new version, but it'll be years before we know.
    - yholio 5y ago
      > The reason is that the more you try to vaccinate the entire population, the more cases of cvdpv2 you cause. Until you hit that magic 100% you cause more harm than you prevent. This makes little sense if you understand that both wpv2 and cvdpv2 were eliminated in the poorest regions of the world using OPV2. If what you claim is true, elimination would have been impossible, you would simply replace wpv2 with cvdpv2, since any attempt at eradication would seed new cvdpv2 cases. Inactivated injectable vaccine, which does not boost herd immunity, is inefective in these countries with limited health systems. > And in fact that people running this campaign noticed this and stopped vaccinating with opv2! (Which perhaps you did not know.) Perhaps you refer to the global coordinated action to move from trivalent vaccine to bivalent (wpv1+ wpv3) in 2016 after the wild type 2 virus was certified as eradicated. But monovalent OPV2 was still being used recently to target specific areas where cvdpv2 is endemic. It makes little sense to use it elsewhere and seed cvdpv2.

ars 5y ago

That was the idea - but it failed. Type 2 does not exist in the wild, it's all from vaccine derived. It got so bad they stopping giving type 2 vaccine, but then people have no immunity to the vaccine derived version - but if you give the vaccine, then the vaccine causes more cases (of type 2) than it's stopping. There's no solution here - either option makes things worse. So I hope the new version of the vaccine works, because otherwise this will fail. Type 3 appears basically extinct - both in the wild, and vaccine derived. Only type 1 still exists in the wild, and vaccine derived type 2.

gus_massa 5y ago
IIUC the idea is to mix the oral vaccine with attenuated virus with only the types 1 and 3, and the injectable vaccine with inactive virus with the types 1, 2 and 3. https://en.wikipedia.org/wiki/Polio_vaccine#Schedule https://en.wikipedia.org/wiki/Polio_vaccine#Schedule So you have coverage for the three types. (It's more complicated. For example country without recent cases and good vaccine coverage only use the injectable vaccine.)
- makomk 5y ago
  The problem is that although the inactivated vaccine provides good protection against severe polio it doesn't seem to work too well at stopping people from spreading the virus, which is not ideal for an elimination program. Presumably this was less of an issue in the developed world because polio spreads mostly through the oral-fecal route and there were already a lot of hygene and infrastructure improvements in developed countries aimed at stopping diseases from spreading that way.
yholio 5y ago
> if you give the vaccine, then the vaccine causes more cases (of type 2) than it's stopping. > There's no solution here - either option makes things worse. That's false and misleading. The OPV2 vaccine will stop both vaccine-derived cvdpv2 virus from spreading and the wild type 2, which has been eradicated. Since OPV2 is a live attenuated virus, it will also continue to multiply and confer protection to any person drinking contaminated water, the vaccine "spreads". It is thus highly effective at stopping polio and in no way it can be said that it "causes more cases than it's stopping". The problem with OPV2 is that it has a relatively higher chance of reverting to an variant that causes polio. This is not a problem if the population has a high level of vaccination, since the live attenuated virus cannot propagate and mutate. It's only problematic in low coverage areas where it can multiply extensively in many hosts. Thus, the cvdpv2 epidemic is an expression of low vaccination rates, similar in every way to a wild poliovirus resurgence. The novel OPV2 vaccine will improve the genetic stability of the attenuated virus, allowing further "viral vaccinations" in low coverage areas - but we could eradicate cvdpv2 today with the existing OPV2 vaccine, if only we could get good vaccinations rates everywhere, as it has happened in most of the world.
- ars 5y ago
  You are completely misinformed about this. I spent quite a long time reading about this. What you write is what they hoped would happen. However it did NOT happen. The reason is that the more you try to vaccinate the entire population, the more cases of cvdpv2 you cause. Until you hit that magic 100% you cause more harm than you prevent. And in the real world 100% is impossible. So yes opv2 "causes more cases than it's stopping" - there are zero cases of wild type 2, so it's stopping nothing except itself. And in fact that people running this campaign noticed this and stopped vaccinating with opv2! (Which perhaps you did not know.) The new version of it will hopefully help, and we can eradicate this. But right now Polio eradication is failing, not because of Taliban, but because of cvdpv2. I have high hopes for the new version, but it'll be years before we know.
  - yholio 5y ago
    > The reason is that the more you try to vaccinate the entire population, the more cases of cvdpv2 you cause. Until you hit that magic 100% you cause more harm than you prevent. This makes little sense if you understand that both wpv2 and cvdpv2 were eliminated in the poorest regions of the world using OPV2. If what you claim is true, elimination would have been impossible, you would simply replace wpv2 with cvdpv2, since any attempt at eradication would seed new cvdpv2 cases. Inactivated injectable vaccine, which does not boost herd immunity, is inefective in these countries with limited health systems. > And in fact that people running this campaign noticed this and stopped vaccinating with opv2! (Which perhaps you did not know.) Perhaps you refer to the global coordinated action to move from trivalent vaccine to bivalent (wpv1+ wpv3) in 2016 after the wild type 2 virus was certified as eradicated. But monovalent OPV2 was still being used recently to target specific areas where cvdpv2 is endemic. It makes little sense to use it elsewhere and seed cvdpv2.