Polio eradication faces a catch-22 as oral vaccines both protect and rarely spread

Oral polio vaccines have helped drive wild poliovirus to near extinction but can rarely mutate into vaccine-derived strains; a newer, more stable oral vaccine reduces that risk though rare recombinant viruses have been detected.

Decades of vaccination have pushed wild poliovirus to the brink, with just 39 recorded wild cases worldwide in 2025, confined to Afghanistan and Pakistan. Oral polio vaccines have been central to that success because they induce gut immunity and are inexpensive to deliver in low‑resource settings. However, the live weakened virus in those oral drops can rarely mutate after shedding and cause circulating vaccine‑derived poliovirus (cVDPV) outbreaks. Scientists have developed a more genetically stable oral vaccine that appears to lower that risk, but environmental surveillance has found at least one recombinant variant. Key points: - Wild poliovirus is now largely limited to Afghanistan and Pakistan, with 39 wild cases reported in 2025. - Since 2016, roughly 4,000 paralysing cases have been attributed to vaccine‑derived poliovirus, mainly in areas with low vaccination coverage. - Early testing found the newer type‑2 oral vaccine is about 70–80% less likely to revert to infectious forms, and nearly two billion doses have been used since 2021; studies report it helped interrupt transmission in Uganda after nationwide campaigns. - A single recombinant virus combining vaccine strain material with another enterovirus was detected in sewage and showed increased potential to affect nervous tissue in lab tests, though there was no evidence this variant had spread. Summary: The need for live oral vaccines in places with limited sanitation and low routine coverage creates a trade‑off: they are effective at stopping transmission but can, in rare circumstances, seed vaccine‑derived outbreaks. A more stable oral vaccine reduces that risk, yet rare recombinant detections show some residual risk remains. Undetermined at this time.