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It's in the news this week that a case of polio was encountered in Gaza, the first case there in 25 years. The conflict between Israel and Hamas is blamed for reduced vaccination, so children are more susceptible to the disease.

But even if people are susceptible, how does the disease resurface? According to WHO, type 1 poliovirus remains in two countries, Pakistan and Afghanistan, but the child was infected with type 2, which was eradicated in 1999.

BBC wrote

According to the UN, Gaza, now in its 11th month of war, has not registered a polio case for 25 years, although type 2 poliovirus was detected in samples collected from the territory’s wastewater in June.

Where could the virus in the wastewater have come from?

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  • $\begingroup$ One answer to the general question is "not so perma" frost. Smallpox appears to be a candidate to make a re-appearance. $\endgroup$
    – Peter - Reinstate Monica
    Commented Aug 30 at 1:48

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Type 2 polio was still included in vaccines up until 2016. Recipients of the polio vaccine become immune, but in rare cases it's possible for vaccinated people to transmit disease to unvaccinated people, and for the virus to spread from there. Wild type 2 polio is eradicated; but some vaccine-derived cases still circulate. See also:

https://medicalsciences.stackexchange.com/questions/30853/why-is-the-rate-of-circulating-vaccine-derived-poliovirus-disease-booming

There's a tough decision as far as when to stop distributing a particular strain via vaccine; on one hand, you keep open the potential of vaccine-derived spread. On the other hand, the protection against vaccine-derived spread is vaccination of everyone, since if everyone is vaccinated there cannot be spread.

Not every type of vaccine holds these risks, but the oral polio vaccine involves a live, attenuated virus, so that's one that causes a risk.

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    $\begingroup$ @Barmar Not necessarily someone who was directly vaccinated, but someone who got it from someone who was (or someone who got it from someone who got it from someone who was). $\endgroup$
    – Bryan Krause ♦
    Commented Aug 29 at 18:47
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    $\begingroup$ @Barmar Other unvaccinated people can be symptomatic and spread it as well to other people who aren't vaccinated. That's why we keep vaccinating people. Polio also isn't always a severe disease even in people who are unvaccinated, so unvaccinated people who spread it to others may have brief/mild/no symptoms. The initial spread from someone vaccinated is an extremely rare event, but also many many people have been vaccinated against polio. $\endgroup$
    – Bryan Krause ♦
    Commented Aug 29 at 19:09
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    $\begingroup$ Please see the link in my answer to the Medical Sciences Q&A and read the links in timeskull's answer like ncbi.nlm.nih.gov/pmc/articles/PMC8393165 $\endgroup$
    – Bryan Krause ♦
    Commented Aug 29 at 20:09
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    $\begingroup$ @Barmar The typical infection by poliovirus is not a severe disease, poliomyelitis is. Ordinarily polio replicates in the gut and causes at most a minor illness. Not something you'd even consider unusual. Occasionally, it gets into the blood and if so then it can possibly affect the CNS, where it causes chronic illness. The vast majority of people that are infected with poliovirus do not get poliomyelitis. $\endgroup$
    – Bryan Krause ♦
    Commented Aug 29 at 20:46
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    $\begingroup$ @Barmar Usually "polio" as a disease is short for poliomyelitis. We don't really have a short name for the normal, non-poliomyelitis version of the infection because, well, it's so minor. $\endgroup$
    – Bryan Krause ♦
    Commented Aug 31 at 0:22

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