Just in the last two months, cases of paralysis caused by mutant vaccine viruses have been reported in the Philippines, Zambia, Togo and Chad. Because paralysis occurs in only about one in every 200 cases of polio, experts assume many more children have been infected.

Stopping such outbreaks typically requires vaccinating hundreds of thousands of children with both the injectable vaccine, which contains killed virus that cannot mutate, and the oral vaccine. The latter contains weakened viruses that normally cannot cause disease but provide better protection than killed viruses.

The strain that the Global Certification Commission for the Eradication of Poliovirus declared eliminated this week is Type 3 wild polio virus, the last case of which was seen in Nigeria in 2012. Type 2 was declared eliminated in 2015; the last case was detected in India in 1999.

Type 1, the only wild strain left, circulates only in Pakistan and Afghanistan.

(In the 1950s, the three strains had more evocative names: Brunhilde, Lansing and Leon. The first was named after a lab chimpanzee, the second after the Michigan city where it was isolated, and the third after a Los Angeles boy who died of it. The nicknames later fell out of favor .)

Enormous, multiyear surveillance efforts are required before a viral strain can be declared extinct. Children can be paralyzed by several other viruses, by bacterial brain infections and by neck and spine injuries.