Israel declares a polio outbreak found only in sewage, with no paralysis cases

On 4 August, Israel notified the World Health Organization under the International Health Regulations that it had declared an outbreak of circulating vaccine-derived poliovirus type 1 [s1]. WHO published the details on 20 August [s1]. No case of paralytic polio has been reported in this event [s1].

An outbreak declared entirely on the strength of sewage samples is worth explaining, because it represents surveillance working the way it is supposed to.

What was found

Between February and July 2025, nine genetically related vaccine-derived poliovirus type 1 isolates were detected in environmental samples collected from seven sampling sites, four of which are geographically non-overlapping, in the Jerusalem district and the Central Region [s1].

Laboratory analysis and whole-genome sequencing showed those viruses were genetically linked to each other and to multiple Sabin-like viruses isolated from environmental samples since October 2024 [s1]. As of 8 August, the virus had been detected only in environmental samples, with no paralytic cases — but WHO states that the evidence supports classifying the event as an outbreak with sustained community transmission [s1].

The genetic story extends back further. A Sabin-like type 1 virus, related to strains found in environmental surveillance, caused an acute flaccid paralysis case in an unvaccinated 17-year-old male from Jerusalem reported on 23 December 2024, classified as vaccine-associated paralytic poliomyelitis [s1].

A report published in Eurosurveillance on 28 August sets out the sequencing work behind that link. Whole-genome sequencing revealed a strong genetic connection between the Sabin-like type 1 variant from the paralysis case and the pre-VDPV1 and VDPV1 viruses isolated from environmental samples collected between October 2024 and April 2025, mostly in Jerusalem [s2]. The authors attribute the early detection to Israel's environmental surveillance system and advanced sequencing technologies, which enabled a rapid public health response [s2].

Why a vaccine-derived virus circulates at all

The oral polio vaccine contains live, weakened poliovirus that replicates in the intestine for a limited period, building immunity [s1]. On rare occasions, while replicating in the gut, those strains can genetically change and spread in communities that are not fully vaccinated — particularly where hygiene, sanitation or crowding create opportunity [s1]. The lower a population's immunity, the longer the virus survives and the more genetic changes it accumulates [s1].

In very rare instances the changed virus can regain the ability to cause paralysis, as wild poliovirus does [s1]. Detection of such a virus in at least two different sources at least two months apart, genetically linked and showing community transmission, is what defines a circulating vaccine-derived poliovirus, or cVDPV [s1].

Israel discontinued routine use of the bivalent oral polio vaccine in March 2025, and continues to administer four doses of inactivated polio vaccine up to 12 months of age, with a fifth dose in the second year of primary school [s1]. Inactivated vaccine cannot itself seed vaccine-derived virus.

The coverage gap

National coverage figures conceal what matters here. The WHO/UNICEF estimate of national immunisation coverage for three doses of inactivated polio vaccine in Israel in 2024 was 98% [s1]. Vaccination coverage in Jerusalem, however, is notably lower and below WHO's recommended threshold for maintaining sufficient population immunity to prevent transmission [s1].

WHO's risk assessment is explicit that circulation is likely occurring in vaccine-sceptic, under-vaccinated communities with known close ties to similar communities in other countries — and it cites the 2022-2023 multi-country circulation of cVDPV2 across the United Kingdom, the United States, Canada and Israel as the precedent [s1].

This is not the first such event in Israel. A genomic epidemiology study of the 2022-2023 cVDPV2 outbreak, published in The Lancet Microbe, detected 256 genetically linked environmental samples along with one acute flaccid paralysis case and four positive contacts, with most affected locations showing a high-density population of Jewish Ultra-Orthodox communities [s3]. That study drew on 15 routine environmental surveillance sites plus 30 additional sites dedicated to monitoring the outbreak, covering approximately 70% of Israel's population, and its authors credited environmental surveillance with enabling a prompt response including enhanced surveillance and a catch-up campaign with inactivated vaccine [s3].

Response and risk

A multidisciplinary emergency response team established for the cVDPV3 outbreak in 2022 and the cVDPV2 outbreak in 2023 continues to operate for this event [s1]. Enhanced acute flaccid paralysis and environmental surveillance are ongoing, and targeted inactivated-vaccine immunisation campaigns focused on under-immunised communities, running since 2022, are being intensified alongside measles vaccination efforts [s1]. WHO notes these campaigns are supported by tailored, community-specific communication strategies developed to address vaccine hesitancy [s1].

WHO assesses the risk of international spread as low, citing high overall population immunity, robust surveillance and response capacity — while noting that the community ties described above create potential for spread [s1]. The risk of further spread within Israel is assessed as likely moderate, because immunisation gaps persist in known high-risk groups [s1].

For context on the wider picture, WHO records 11 cVDPV1 cases reported in 2024 — ten in the Democratic Republic of the Congo and one in Mozambique — and notes that despite no cVDPV1 case detection for the past ten months, low routine immunisation and inactivated-vaccine coverage in several countries indicates continued risk of emergence [s1].

What this shows about surveillance

The international spread of poliovirus was declared a Public Health Emergency of International Concern on 5 May 2014, and the declaration was most recently extended on 28 July 2025 [s1]. Israel's outbreaks fall under it [s1].

The substantive lesson is about detection. This outbreak was found in sewage before it produced a single case of paralysis, and sequencing connected it to a paralysis case eight months earlier [s1][s2]. That is a much earlier warning than clinical surveillance alone would have given — and it only works where the sampling network is dense enough and the sequencing capacity exists. Most countries have neither.

Sources

• Circulating vaccine-derived poliovirus type 1 - Israel — World Health Organization, Disease Outbreak News, 2025-08-20
• Early detection of a circulating pre-vaccine-derived poliovirus type 1 (pre-VDPV1) variant linked to an acute flaccid polio case prior to VDPV1 emergence, Israel, 2024 to 2025 — Eurosurveillance, 2025-08-28
• Environmental surveillance of a circulating vaccine-derived poliovirus type 2 outbreak in Israel between 2022 and 2023: a genomic epidemiology study — The Lancet Microbe, 2024-09-13