Open Outbreak

Poliomyelitis

Also known as: polio, poliovirus, WPV1, cVDPV2

Poliomyelitis, usually called polio, is a viral infection that spreads mainly through contact with infected stool, and sometimes through contaminated food or water. The virus multiplies in the gut. Most infected people have no symptoms, which is why spread can be hard to spot. In a small minority, the virus reaches the nervous system and causes weakness or paralysis that can be permanent. WHO states that paralysis happens in about 1 in 200 infections, and death occurs in about 5% to 10% of people with paralytic polio. The incubation period is usually 7 to 10 days, with a reported range of 4 to 35 days. There is no cure that removes the virus once infection has happened, so care is mainly supportive. The main protection is vaccination. Current public-health concern includes both wild poliovirus type 1 and circulating vaccine-derived polioviruses, which can appear where population immunity is too low for too long.

Key facts

TransmissionPerson-to-person, mainly by the fecal-oral route; less often through contaminated food or water.
IncubationUsually 7–10 days; reported range 4–35 days.
Case fatalityAbout 5%–10% among paralytic cases; overall infection fatality is much lower because most infections are asymptomatic.
Reproduction numberunknown
VaccineYes. Polio is vaccine-preventable. WHO outbreak notices describe use of inactivated polio vaccine (IPV), oral polio vaccine (OPV), bivalent OPV, and novel OPV2 in response settings.
TreatmentNo cure for the infection itself; treatment is supportive.
ReservoirHumans
Endemic regionsWild poliovirus remains endemic in limited settings; recent WHO notices also show outbreaks of circulating vaccine-derived polioviruses in multiple countries in Africa, the Eastern Mediterranean, Asia, and occasional detections elsewhere.

Transmission

Polio spreads mainly from person to person through the fecal-oral route. In plain language, that means virus from an infected person’s stool gets into another person’s mouth, often through contaminated hands, surfaces, food, or water. WHO also notes that spread can happen less often through a common vehicle such as contaminated food or water. The virus multiplies in the intestine, which is why sanitation and hand hygiene matter, but vaccination is the key protection. A major challenge is that many infections are silent. WHO says up to 90% of infected people have no symptoms or only mild illness, so transmission can continue without obvious warning. This is also why sewage surveillance is useful: poliovirus can be found in wastewater before paralytic cases appear. WHO’s recent notices describe genetically linked virus found in sewage in places with no human cases yet, showing that absence of paralysis does not always mean absence of transmission.

Symptoms and severity

Most people infected with poliovirus do not become noticeably ill. WHO reports that up to 90% of infections are asymptomatic or cause only mild symptoms. When symptoms do occur, they can include fever, fatigue, headache, vomiting, stiffness of the neck, and pain in the limbs. In a small proportion of cases, the virus invades the nervous system and causes paralysis. WHO describes this as often permanent. The commonly quoted figure in the WHO outbreak notice is about 1 paralytic case for every 200 infections. Among people with paralytic polio, WHO reports that about 5% to 10% die. The incubation period is usually 7 to 10 days, but WHO says it can range from 4 to 35 days. Because mild infections are common and severe disease is uncommon but life-changing, surveillance depends not only on hospital cases but also on testing stool samples and wastewater to detect spread early.

Treatment and vaccines

There is no cure that eliminates poliovirus after infection. WHO states this plainly: polio can be prevented by immunization, but there is no cure. Care is therefore supportive. In severe illness, treatment focuses on breathing support if needed, hydration, pain control, rehabilitation, and long-term help for weakness or disability. Prevention is much stronger than treatment. WHO notices show that countries use several vaccine tools depending on the situation: inactivated polio vaccine (IPV) for routine protection, oral polio vaccine (OPV) in some programmes, bivalent OPV in outbreak response, and novel OPV2 in some cVDPV2 responses. Vaccine-derived poliovirus can emerge rarely when weakened OPV virus keeps circulating in under-immunized communities and changes genetically over time. That does not mean the vaccine is the main danger; WHO repeatedly frames the real risk as immunity gaps. Where vaccination coverage is high, poliovirus spread is much less likely and outbreaks are easier to stop.

Where it occurs

Polio is now much less common globally than in the past, but it has not disappeared. WHO outbreak notices in this session show both wild poliovirus type 1 and circulating vaccine-derived polioviruses in several regions. Wild poliovirus type 1 was reported in Malawi on 2022-03-03 and Mozambique on 2022-06-23, with WHO linking the viruses genetically to strains from Pakistan. More recently, WHO notices show cVDPV2 in Papua New Guinea, Indonesia, Kenya, Tanzania, Burundi, Sudan, and earlier in the United Kingdom and the United States through environmental detection and one paralytic case in New York. WHO also reported cVDPV1 environmental detections in Israel in 2025. This pattern means the disease is not confined to one continent. Wild poliovirus remains geographically limited, but vaccine-derived outbreaks can appear wherever vaccination coverage and surveillance are weak enough to let virus circulate. Humans are the reservoir, so the map of risk follows immunity gaps rather than animal habitats.

Recent history

On this site’s archive, polio has 87 recorded WHO notices or major-event records since 1996, which shows how persistent the problem has been even in the eradication era. The recent WHO pattern is mixed. In 2022, WHO reported wild poliovirus type 1 in Malawi and Mozambique, confirming cross-border spread in southeastern Africa. The same year, WHO also described genetically linked cVDPV2 detections in sewage in the United Kingdom and the United States, with one paralytic case in an unvaccinated person in New York. In 2023 and 2024, WHO posted cVDPV2 notices from Burundi, Kenya, Tanzania, Sudan, and Indonesia. In 2025, WHO reported cVDPV2 in Papua New Guinea and cVDPV1 in Israel, detected mainly through environmental surveillance without reported paralytic cases in Israel. WHO’s broader view remains that the international spread of poliovirus is a serious public-health event, and outbreak reports repeatedly stress vaccination gaps and surveillance quality as the main drivers of ongoing risk.

What to watch

The biggest warning signs are not mysterious. They are the signals WHO keeps highlighting in its outbreak notices. First, new detections in regions that have not seen poliovirus recently, especially if sewage samples stay positive over time or if paralytic cases appear. Second, evidence of sustained person-to-person transmission, including genetically linked viruses found months apart or across borders. Third, falling routine vaccination coverage or hard-to-reach communities with low uptake, because these create the conditions for both wild and vaccine-derived poliovirus spread. Fourth, surveillance gaps: if testing of acute flaccid paralysis cases or environmental samples is weak, outbreaks can grow unnoticed. Fifth, operational problems such as conflict, displacement, or interruptions in vaccine campaigns. Finally, vaccine supply or deployment constraints matter, especially for outbreak-response products such as IPV or novel OPV2. Any combination of wider geographic spread, ongoing silent circulation, and low immunity would make polio more concerning very quickly.

Related outbreaks

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