When the Polio Vaccine Virus Spreads: Outbreaks in Congo and Somalia, 2017–2018

Oral polio vaccine has driven polio close to eradication. But it contains live, weakened (Sabin) virus, and where too few children are immunized, that virus can pass from person to person long enough to regain the ability to paralyze. The result is circulating vaccine-derived poliovirus (cVDPV). As of February 26, 2019, outbreaks in the Democratic Republic of the Congo (DRC) had paralyzed 42 children, and outbreaks in Somalia 12.

How vaccine-derived poliovirus is identified

The virus accumulates changes at roughly 1.1% (10 nucleotides of VP1) a year, so the number of changes shows how long it has been circulating unseen. It is "circulating" when genetically linked viruses turn up in paralyzed patients, their contacts or sewage.

The type 2 component of trivalent oral vaccine caused more than 90% of cVDPV cases in 2006–2015. After wild type 2 poliovirus was declared eradicated in 2015, every country using oral vaccine switched by May 1, 2016, to a bivalent vaccine without type 2. A single dose of inactivated vaccine (IPV) covering all three types was added to routine schedules — it protects against paralysis but not against spreading the virus — and monovalent type 2 vaccine (mOPV2) was stockpiled to fight type 2 outbreaks.

Four outbreaks in the DRC

Responding took repeated vaccination campaigns (supplementary immunization activities, or SIAs) — up to 10 in the Haut Lomami area. Ituri's lone case was genetically linked to Haut Lomami with no known contact, and campaigns there skipped health zones with confirmed or suspected Ebola in 2018. Delays, low coverage and campaigns too small in scope meant far more rounds than usual were needed — and the Haut Katanga outbreak shows the danger: when response campaigns reach too few children, the type 2 vaccine itself can seed new outbreaks.

Somalia, Kenya and Ethiopia

In Somalia, sewage testing found the virus first. Genetically linked type 2 virus turned up in sewage in Banadir province in October 2017 and January 2018, and type 3 in April 2018. The genetics suggested type 2 had circulated unseen for more than three years (36–44 changes) and type 3 for more than a year (15–17). The first paralyzed patient — infected with both types — was from Hiran province, with onset on May 11, 2018. Twelve cases were confirmed, the most recent with onsets on September 2 (type 2) and September 7, 2018 (type 3).

• Twenty-one Banadir sewage samples were positive for type 2, the latest collected October 11, 2018, and 12 for type 3, the latest August 23, 2018.
• One sewage sample in Kamakunji district, Kenya, in March 2018 held type 2 virus linked to Somalia, but no cases occurred in Kenya; neither type was found in Ethiopia.
• Somalia ran six mOPV2 campaigns from December 2017 to November 2018, two synchronized with campaigns in Kenya and Ethiopia, and three bivalent campaigns for type 3, two synchronized with Kenya. Kenya also ran a focal campaign in Kamakunji in May 2018.

Vaccinators went to bus stations and markets to reach children from inaccessible areas — but more than 300,000 unimmunized children are estimated to live there.

Why it keeps happening

Both countries had type 2 outbreaks in 2005–2013, and because routine immunization was chronically low, they held yearly preventive campaigns before the 2016 switch. The new outbreaks show those campaigns missed children in the affected areas. After the switch, preventive campaigns with type 2 vaccine are no longer possible, and low routine coverage kept IPV from filling the gap. Type 2 outbreaks also occurred in 2017–2018 in Mozambique, Niger, Nigeria and Syria.

Transmission in the DRC may have stopped, but in both countries longer surveillance is needed to be sure. Response campaigns must be timely and high-quality, and aggressive tactics like vaccinating at transit points should continue. Better routine delivery of bivalent vaccine would prevent type 1 and 3 outbreaks, though it takes time; preventive bivalent campaigns can raise immunity faster.

Sources

Based on Chukwuma Mbaeyi, Mary M. Alleman, Derek Ehrhardt, et al., "Update on Vaccine-Derived Poliovirus Outbreaks — Democratic Republic of the Congo and Horn of Africa, 2017–2018," Morbidity and Mortality Weekly Report, volume 68, Centers for Disease Control and Prevention; a work of the United States government in the public domain.