Vaccine-Derived Polioviruses, January 2017–June 2018: New Outbreaks in Five Countries

Since the Global Polio Eradication Initiative began in 1988, polio cases worldwide have fallen by more than 99.99%, mainly thanks to the live, weakened oral poliovirus vaccine (OPV) given in routine schedules and mass campaigns. Of the three wild poliovirus types:

OPV's record comes with a rare drawback. The vaccine virus can drift genetically as it replicates, and a strain that has drifted far enough is a vaccine-derived poliovirus (VDPV). Where too few children are immunized, VDPVs can circulate and cause outbreaks of paralytic polio; in people with primary immunodeficiencies, they can replicate and be shed for years.

After cVDPV2 outbreaks kept appearing over the previous 15 years, every OPV-using country made a synchronized switch in April 2016, from trivalent OPV (tOPV) — Sabin types 1, 2 and 3 — to bivalent OPV (bOPV), types 1 and 3. To control type 2 outbreaks, about 100 million doses of monovalent type 2 OPV (mOPV2) have gone to 11 countries, and to keep protection against type 2 paralysis, 176 OPV-using countries have introduced at least one dose of injectable inactivated polio vaccine, starting in 2015.

How a VDPV is identified

Laboratories of the Global Polio Laboratory Network screen isolates by real-time reverse transcription–polymerase chain reaction and then sequence the VP1 region. An isolate counts as a VDPV when its VP1 sequence differs from the vaccine strain by more than 1% (types 1 and 3) or more than 0.6% (type 2). Since August 1, 2016, every type 2 isolate has been sequenced.

Circulating VDPVs

From January 2017 to June 2018, circulation was found in six countries, one more than in the previous report:

Vaccine-derived polioviruses detected worldwide, January 2017–June 2018. CDC

• Of the 119 cVDPV cases, 113 (95%) were type 2 — a serotype mix like that of earlier years.
• 62 cVDPVs came from sewage sampled at environmental surveillance sites in Kenya, Nigeria and Somalia.
• No further cases came from the type 2 outbreaks first reported in Nigeria and Pakistan in 2016.
• After June 2018, new VDPVs linked to these emergences turned up in every outbreak country except Syria.

Circulating vaccine-derived poliovirus cases detected, by serotype, worldwide, January 2000–June 2018 (917). Data through June 2018, available by September 18, 2018. CDC

The new emergences

Each emergence is named for where it was found and how many have arisen in that region.

DRC — three separate type 2 emergences, February 2017–June 2018:

• HLO-1 spread through Haut Katanga, Haut Lomami, Ituri and Tanganyika provinces;
• MAN-1 and MON-1 stayed limited, in Maniema and Mongala.

Several mOPV2 rounds followed in the outbreak provinces and nearby high-risk areas.

Kenya and Somalia — two emergences, October 2017–May 2018:

• BAN-1 (type 2) was found at three environmental sites in Mogadishu (Banaadir province) and one in Nairobi, with two linked cases in Gedo and Hiran provinces of Somalia.
• BAN-2 (type 3) turned up at the same three Mogadishu sites and, during February–May 2018, in three cases in Middle Shabelle and Hiran.
• A new sampling site in Mogadishu, opened in October 2017, picked up type 2 cVDPV at once, then two type 3 cVDPVs and one aVDPV2.
• Response: three mOPV2 rounds in Somalia (December 2017–May 2018) and one in Kenya (May 2018).

Nigeria — two type 2 emergences at the same time, January–June 2018:

• JIS-1, in Gombe, Jigawa and Yobe states: 18 isolates from environmental samples and 4 from people with AFP;
• SOS-3, in Sokoto: 17 isolates from sewage.

Two mOPV2 rounds followed in May 2018 in Bauchi, Gombe, Jigawa and Sokoto.

Papua New Guinea — during April–June 2018, type 1 cVDPVs came from three patients with AFP and two contacts in Eastern Highlands and Morobe provinces. Routine vaccine coverage there has been low, 50%, for years; bOPV campaigns are under way.

Immunodeficiency-associated VDPVs

14 iVDPVs were reported from nine countries: 7 type 3, 5 type 2 and 2 type 1. Across all years since OPV was introduced in 1961:

Ambiguous VDPVs

aVDPVs were found in seven countries, down from 11 in the previous report. Of 28, 23 were type 2 — mostly after mOPV2 responses to outbreaks — 4 type 3 and 1 type 1; 23 (82%) came from environmental samples. Finding aVDPVs where polio vaccination coverage is below 60% can signal a risk that a cVDPV will emerge and spread.

One stood out: a highly divergent type 2 aVDPV (8.4% VP1 divergence) in an environmental sample from metropolitan Melbourne, Australia, collected on November 21, 2017, whose genome looked like that of an iVDPV.

What it means

• More outbreaks, more cases. Both rose compared with January 2016–June 2017, and cases kept appearing after June 2018 in DRC, Nigeria, Papua New Guinea and Somalia.
• More sewage testing finds more. Sampling now covers 244 sites in 41 countries. In the countries with recent wild-virus transmission — Afghanistan, Nigeria and Pakistan — sites grew from 21 at the end of 2011 to 153 by April 2018. That finds more wild virus, more VDPVs, and more leftover type 2 vaccine virus shed after mOPV2 campaigns.
• More iVDPV excretors are known, partly from extra surveillance of patients with primary immunodeficiencies. The antiviral pocapavir has recently been used to treat them under compassionate-use protocols in three countries.
• Type 2 immunity gaps are widening in high-risk areas as time passes since the switch. Type 2 VDPVs in 2017 and 2018 show that large groups of children missed type 2 immunization before it. Some detections were in or near security-compromised areas — DRC, Somalia and Syria — where surveillance quality is uncertain.
• Types 1 and 3 matter too. The new type 1 and type 3 outbreaks show why high immunity against them, and sensitive AFP surveillance, must be kept up.
• VDPV cases outnumbered wild-virus cases in this period — but wild-virus eradication must still be documented before OPV can stop. Ending all OPV use after polio eradication is certified will end the risk of new VDPVs.

Sources

Based on Jorba J, Diop OM, Iber J, Henderson E, Zhao K, Sutter RW, Wassilak SGF, Burns CC, "Update on Vaccine-Derived Polioviruses — Worldwide, January 2017–June 2018," MMWR volume 67, number 42, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report's introduction describes the 14 people excreting iVDPVs as all newly identified, while its iVDPV section says six of the isolates were new since the last report, so no count of new ones is given here. The report names "Easter Highlands" province; it is Eastern Highlands.