Circulating vaccine-derived polio virus or cVDPV are outbreaks of polio that are actually caused by the polio vaccine.
Very rarely, the attenuated (weakened) virus in the oral polio vaccine can revert to a form that can cause the person who was vaccinated or their close contact to actually develop polio.
It should not be confused with VAPP or vaccine-associated paralytic polio. With VAPP, the original strain of attenuated vaccine virus reverts to a form that can cause polio, but it does spread from one person to another, so does not lead to outbreaks.
Fortunately, both VAPP and cVDPV are rare.
How rare? VAPP only occurs in about 1 in every 1.27 million children receiving their first dose of OPV.
And according to the WHO, there had only been about 24 outbreaks of cVDPV over the past 10 years. Tragically, this has resulted in at least 750 cases of paralytic polio in 21 countries.
A new outbreak of cVDPV in Syria adds to those numbers though.
After being polio free for 15 years, since 1999, Syria began having cases of wild type polio again in 2013 (35 cases) and 2014 (1 case). Those polio cases and the emergence of an outbreak of cVDPV2 (there are three strains of polio virus – this outbreak was caused by the type-2 strain) highlight the effects of years of poor immunization rates because of war.
While there are many challenges to getting kids vaccinated in Syria, up to 355 vaccination teams with 61 supervisors will be working out of five vaccine distribution centers to vaccinate 328,000 children to control the outbreak and get kids vaccinated.
Circulating Vaccine-Derived Polio Virus
Just like wild type polio, we can stop cVDPV by increasing vaccination rates and increasing access to improved sanitation facilities.
Although anti-vaccine folks routinely cry wolf about shedding, the oral polio vaccine really does shed – in the stool of people who have been recently vaccinated. You can then be exposed to the attenuated polio vaccine virus (which can help give immunity to others in the community by passive immunization) or a strain of cVDPV (which can, unfortunately, help give others, especially if they are not vaccinated, paralytic polio) if they are exposed to open sewage or can not practice proper hygiene, etc.
Can’t we just stop using the live, oral polio vaccine?
Although a serious side effect of the vaccine, the vaccine’s benefits clearly outweigh the risk of both VAPP and cVDPV while polio is endemic (lots of cases) in a region, after all, without the vaccine, hundreds of thousands of children would get polio and would be paralyzed.
In polio-free countries, the risks of VAPP and cVDPV becomes greater than the risk of polio though, and they move to the inactivated polio vaccine. That helps prevent a situation in which the polio vaccines actually causes more cases of polio than wild type polio viruses.
Eventually, all countries will move to the IPV vaccine as we move closer to polio eradication. We came one step closer to that point in April 2016 when all countries that were still using the oral polio vaccine switched from trivalent OPV (three strains) to bivalent OPV (two strains) for their routine immunization programs. This could eliminate up to 90% of cases of cVDPV (most are caused by the type-2 strain which is not in bOPV)!
What To Know About cVDPV
Circulating vaccine-derived polio virus outbreaks are a rare side effect of the oral polio vaccine.