24

The wikipedia entry on polio vaccines has the 1st graph below, showing that the rate of vaccine derived polio has increased massively over the last few years, with the rate of 1089 cases in 2020 compared to 56 - 71 per year from 2010 to 2014. The WEF page on the vaccine has the second graph below, show a fairly modest increase in the rate of immunisation coverage over this time.

What is the reason for a roughly 20 fold increase in vaccine derived polio in such a short period of time, when vaccination rate has increased by only a fraction?

Number of cVDPV cases since 2000

Poliomyelitis reported cases and Pol3 coverage
(source: who.int)

3
  • 3
    I took me reading @timeskull below to realize that you weren't saying it was caused by the vaccine. I would suggest changing the title if you can without detracting from the intent.
    – dgo
    Mar 10, 2022 at 15:16
  • 2
    I have to admit that I thought they were caused by the vaccine, but as you say the answer clearly states where it came from. I shall give the title a go.
    – User65535
    Mar 10, 2022 at 16:05
  • 1
    It still reads as if the polio vaccine causes polio. Perhaps reading it 10 - 20 times will get the right meaning, but that is too much. Mar 12, 2022 at 11:22

1 Answer 1

38

The short version is that in 2016 the polio vaccine changed.

A more thorough explanation requires some background on the immunology of polio and its vaccines, which is not straightforward. Polio virus is usually harmless, it reproduces in the gut and spreads through a fecal-oral route. In ~99% of infections it only causes mild diarrhea. In the remaining 1% of cases, however, it gets into the bloodstream and from there enters nerves, causing paralysis and/or respiratory failure. There are two types of vaccine: inactivated polio vaccine (IPV) and oral polio vaccine (OPV). IPV is a shot of killed virus particles and provides systemic immunity; someone who gets IPV but not OPV can still be infected with and spread polio virus, but they are protected from serious disease. OPV is a pill of live virus that has been mutated so it can reproduce in the recipient's gut but cannot get into nerves to cause disease. Someone who gets a full course of OPV will have mucosal immunity to polio virus - they will not be able to spread it. In areas with unsanitary water supplies, OPV is necessary to eradicate wild polio. However, the vaccine strains reproduce in and are excreted by the treated individual for 4-6 weeks, and can end up in the water supply, where they can infect new individuals. This wouldn't matter, except that the mutations that prevent the vaccine-strain virus from entering nerves eventually get reversed by new mutations. Unvaccinated people can then get sick from the circulating vaccine-derived polio virus.

There are three types of wild polio virus, so three strains of vaccine are needed to provide protection. The version of OPV administered before 2016 was "trivalent" - it included 3 vaccine strains to protect against 3 types of polio, while the new version is "bivalent" - it includes 2 vaccine strains. The switch was made because the excluded type, type 2, has been successfully eradicated in the wild, and the live vaccine against that type is the one that was mutating to regain its ability to cause paralysis. This "reverted" type 2 live vaccine is still circulating and there is a large and increasing group of people who have no immunity to it, hence the boom.

The OPV switch was supposed to be accompanied by a stepped-up vaccination program to eradicate the live type 2 vaccine strain, but a shortage of the needed vaccines, wars, and other difficulties prevented this. Also, the ability of the vaccine-derived strains to remain circulating in communities after vaccination stopped was underestimated.

The reasons for the sharp increase in circulating vaccine-derived poliovirus type 2 (cVDPV2) outbreaks following the OPV switch are discussed in greater detail in this report in China CDC weekly. A new vaccine against type 2 polio that is more genetically stable (and therefore less likely to revert to disease-causing strain through mutation) became available March 2021. I was not able to find a good answer to how effective the response to all this has been, but adding up all the 2021 cVDPV cases for the countries listed at https://polioeradication.org/where-we-work/ gives a total of 635. Some cases are listed for 2022, so the 2021 count is likely close to complete. If so, cVDPV is no longer booming.

8
  • 10
    Am I understanding correctly from this that these are then cases of cVDPV from doses given possibly many years ago, that are since spreading primarily through community transmission (despite being originally vaccine-derived)?
    – Bryan Krause
    Mar 9, 2022 at 19:00
  • 19
    yes, that's right. The use of "vaccine-induced" in the title is incorrect, the people getting sick have not been vaccinated, or at least not enough to gain immunity
    – timeskull
    Mar 9, 2022 at 19:10
  • 3
    I put it in with the (imo) necessary background information
    – timeskull
    Mar 9, 2022 at 20:41
  • 4
    @SSight3 I don't know what timeskull thinks but I don't understand your comment at all.
    – Carey Gregory
    Mar 10, 2022 at 5:21
  • 9
    @SSight3 It's a vaccine for the people who get it. It can rarely become a transmitted illness to people that aren't vaccinated but are exposed to those who are. The people that get ill aren't the same as the people vaccinated. The solution is to vaccinate everyone, then there's no one unvaccinated that can get ill.
    – Bryan Krause
    Mar 10, 2022 at 6:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.