CDC Reports Sharp Increase in Vaccine-Derived Poliovirus Outbreaks

The number of circulating vaccine-derived poliovirus (cVDPV) outbreaks worldwide more than tripled to 29 in 15 countries between January 2018 and June 2019, according to an update from the US Centers for Disease Control and Prevention. That compares with 9 outbreaks in 6 countries during the previous reporting period (January 2017—June 2018).

“Worldwide over the past 10 years, over 12 billion doses of OPV [oral poliovirus vaccine] have been administered to more than 2.5 billion children, and as a result more than 3.5 million polio cases were prevented,” first author Jaume Jorba, PhD, told Contagion^®. “During that time, 1085 cases of paralysis caused by cVDPV have been reported.”

The report found that 86% of cVDPV cases were caused by cVDPV type 2, occurring both inside and outside of areas where monovalent type 2 OPV was used to control the cVDPV2 vaccine strain. In 2016, world health officials switched from trivalent oral poliovirus vaccine (tOPV, types 1, 2 and 3) to bivalent oral poliovirus vaccine (bOPV types 1 and 3).

“cVDPV is extremely rare and occurs when routine or supplementary immunization activities (SIAs) are poorly conducted and a population is left susceptible to poliovirus, whether from vaccine-derived or wild poliovirus,” Jorba said. “Hence, the problem is not with the vaccine itself, but low vaccination coverage. If a population is fully immunized, they will be protected against both vaccine-derived and wild polioviruses.”

Most of the recent cVDPV emergences were in central and western Africa, where 18 (72%) outbreaks were reported. Transmission of cVDPV2 occurred in 12 African countries and China, primarily in children born after the switch to bOPV. Type 1 variants were found in Indonesia, Myanmar and Papua New Guinea. Type 3 cVDPVs were seen in Somalia.

“In an under-immunized population, there is a very small risk that the type 2 poliovirus shed into the environment after mOPV2 vaccination could circulate and revert into a form that could cause paralysis,” Jorba told Contagion^®. “In response to that risk, as soon as a cVDPV2 outbreak is detected, vaccination campaigns with mOPV2 are rolled out in the surrounding regions.”

According to the CDC, mOPV2 campaigns should be administered within 14 days of confirmation of cVDPV2 emergence, and about 300 million doses of the vaccine having been administered since 2016.

“Stopping current vaccine-derived poliovirus (VDPV) outbreaks and limiting the risk of VDPV emergence requires:

• increasing assistance to countries where cVDPV2 outbreaks occur,
• achieving/maintaining high vaccination coverage in all countries, and
• ending all OPV use after certification of polio eradication,” Jorba told Contagion^®.

Jorba said CDC polio response is addressing the outbreaks by deploying 75-100 additional technical experts to provide technical support in countries with active outbreaks and in neighboring countries that are at risk to help stop the spread of the virus.

Development of a novel oral polio vaccine type 2 is underway. The vaccine, which is in clinical trials and could be introduced as early as mid-2020, is stabilized to decrease the likelihood of reversion to neurovirulence.

New outbreaks continue to be reported, including in the Philippines, where a young girl from Lanao del Sur was confirmed to have vaccine-derived polio virus type 2 (VDPV2) earlier this year. The virus was classified as circulating after being confirmed in environmental samples from sewage and waterways in the area.

A CDC report released earlier this year detailed revised emergency action plans for polio vaccination in Afghanistan, Nigeria, and Pakistan, where the virus continues to be endemic. Vaccination rates in those countries were in 2017 were 60%, 40% and 75%, respectively. The number of wild poliovirus cases reported in Afghanistan and Pakistan rose from 22 WPV1 cases in 2017 to 33 cases in 2018.