Stopping a Vaccine-Derived Polio Outbreak in War-Torn Syria, 2017–2018

This report describes an outbreak in 2017 and the response through mid-2018.

Since the Global Polio Eradication Initiative (GPEI) began in 1988, most progress against wild poliovirus has come from oral poliovirus vaccine (OPV), given in mass campaigns and routine immunization. Because OPV contains live, weakened virus, it carries a rare risk: where very few children are vaccinated, vaccine virus can circulate for a long time and regain the ability to paralyze. These vaccine-derived polioviruses (VDPVs) can cause outbreaks of paralytic polio.

Wild poliovirus type 2 was last detected in 1999 and declared eradicated in 2015, yet most VDPV outbreaks have been caused by type 2. In April 2016 the world switched from trivalent OPV (types 1, 2 and 3) to bivalent OPV (types 1 and 3), and since then GPEI has treated any emergence of VDPV type 2 as a public health emergency.

How Syria became vulnerable

The civil war that began in 2011 badly damaged Syria's health system. Before the war, from 2001 to 2010, national coverage with three doses of OPV by age 1 was consistently above 80%. By 2016 it had fallen from 83% in 2010 to 48%.

Mass vaccination campaigns during a 2013–2014 wild polio outbreak made up for weak routine services in parts of the country, but they became less frequent and less thorough once that outbreak ended. In Deir-ez-Zor governorate in 2016, six campaigns were held; two used trivalent OPV, with reported coverage of just 7% and 23%. Local authorities controlling the governorate also banned vaccination campaigns at times.

Children with paralysis not caused by polio serve as a rough gauge of vaccination coverage. Among those born during the war, the share who were unvaccinated rose nationally from 3% in 2015 to 6% by the end of 2016, and in Deir-ez-Zor from 0% to 10%. National surveillance met its targets, but gaps showed locally: in Deir-ez-Zor, the share of paralysis cases with adequate stool specimens fell from 84% in 2015 to 61% in 2016.

The outbreak

In May–June 2017, VDPV type 2 was found in stool from two children with acute flaccid paralysis in Deir-ez-Zor. The first isolate differed from the Sabin vaccine virus by 22 nucleotides in the 903-nucleotide VP1 region, and genetic analysis linked the two cases, confirming a circulating outbreak. The strain had been circulating for about a year before it was detected, which may help explain why it became one of the largest documented outbreaks of its kind.

As of June 17, 2018, there were 74 cases:

• Timing: the first known case, a 22-month-old girl in Mayadeen district, became paralyzed on March 3, 2017; the last, a 5-month-old in Boukamal district, on September 21, 2017.
• Who: 46 (62%) were girls. The median age was 15 months: 26 were under 12 months, 35 were 12–23 months, 11 were 24–59 months, and two were 5 or older.
• Vaccination: 30 (41%) had never received OPV, 32 (43%) had received one or two doses, and 12 (16%) had received three or more.
• Where: 71 (96%) were in Deir-ez-Zor governorate, 58 of them in Mayadeen district, the center of both this outbreak and the 2013–2014 wild polio outbreak, 12 in Boukamal and one in Deir-ez-Zor district. One case each came from Tell Abyad and Thawra districts in Raqqa governorate and Tadmour district in Homs governorate.

Late detection resulted from local gaps in surveillance and from delays in getting stool specimens to laboratories amid the humanitarian emergency.

The response

Surveillance. Health workers stepped up active searches for paralysis cases in affected and neighboring districts, investigated cases quickly, and tested stool from patients' close contacts (the Raqqa cases were confirmed this way), from healthy children in affected areas, and from children arriving elsewhere from outbreak areas. From December 2017, six environmental sites in five governorates sampled sewage; none found the virus.

Vaccination. Campaigns with monovalent OPV type 2 (mOPV2) targeted children under 5 in two phases:

The Raqqa campaigns started late because of long negotiations with the several local authorities controlling different parts of the governorate. The third round was added because the virus was still spreading in Deir-ez-Zor. Inactivated poliovirus vaccine (IPV) was also given to children aged 2–23 months during the second round in Deir-ez-Zor and most of Raqqa, to high-risk groups in Damascus, Aleppo and Hasakeh in February 2018, to displaced children across Syria in the second half of 2017, and to refugees and high-risk communities in Lebanon, Iraq and Turkey.

Containment and communication. Under GPEI rules for type 2 poliovirus, nearly every mOPV2 vial was recovered after each round; unused vials went back to national stockpiles and opened or empty ones were destroyed at designated sites. Communication efforts were tailored to build trust in vaccines and counter misconceptions.

Outcome

Despite immense operational and security obstacles, the Syrian polio eradication program's response appears to have worked. The evidence indicates that sewage sampling alongside intensified surveillance, together with mOPV2 campaigns, interrupted transmission of the virus in Syria. The outbreak shows the risk of vaccine-derived polio wherever OPV coverage falls, and that good surveillance and targeted mOPV2 vaccination can control it.

Sources

• Mbaeyi C, Wadood ZM, Moran T, et al. Strategic Response to an Outbreak of Circulating Vaccine-Derived Poliovirus Type 2 — Syria, 2017–2018. MMWR Morb Mortal Wkly Rep 67(24). https://www.cdc.gov/mmwr/volumes/67/wr/mm6724a5.htm
• GPEI, declaration of the eradication of wild poliovirus type 2: http://polioeradication.org/news-post/global-eradication-of-wild-poliovirus-type-2-declared/
• The report was written jointly by CDC, WHO and UNICEF; its figures are not reproduced here.