There are reports that oral polio vaccines have been linked to multiple cases of vaccine-derived poliovirus type 2 (cVDPV2) in Burundi and the Democratic Republic of Congo (DRC).
This article will try to clear up what is happening in Burundi and the DRC and discuss why it can happen. Also, I’ll try to explain why it’s nearly impossible for this to happen in more developed countries.
Polio and the oral polio vaccine
Polio, or poliomyelitis, is an infectious disease caused by one of three poliovirus serotypes: types 1, 2, and 3. Polioviruses are in the group of viruses called enteroviruses which usually infect the digestive tract. They are usually transmitted through the fecal-oral or oral-oral routes.
In about 0.5% of symptomatic cases, the virus moves from the intestinal tract to affect the central nervous system, and muscle weakness develops resulting in flaccid paralysis.
This can develop over a few hours to a few days. The weakness most often involves the legs, but may less commonly involve the muscles of the head, neck, and diaphragm. Many people fully recover. In those with muscle weakness, about 2 – 5% of children and 15 – 30% of adults die.
There is no cure for polio; however, it is easily prevented with the polio vaccine, which is available in two forms. The first is the injectable inactivated virus vaccine which is based on the original vaccine developed by Jonas Salk. This version is usually known as IPV or injectable polio vaccine.
The second version uses a live attenuated (weakened) virus given orally which was originally developed by Albert Sabin. This is generally known as OPV or oral polio vaccine. OPV has not been authorized for use in the USA since 2000.
The IPV vaccine is used throughout the developed world and is very effective in preventing polio. It also has an excellent safety profile. On the other hand, the OPV version is easier to give (no syringes and needles) and easier to transport, so it is used in developing countries and areas that have warmer climates.
Polio has more or less been eradicated across the world but may still be endemic in impoverished countries that have lower vaccination rates and lack clean water. Most cases remain in Afghanistan and Pakistan, according to the Centers for Disease Control and Prevention.
Unfortunately, OPV has been linked to circulating vaccine-derived poliovirus (cVDPV), a very rare virus caused by the mutation or recombination of the attenuated viruses used in the oral polio vaccine. While cVDPVs are rare, cases have been increasing in recent years due to low immunization rates within certain communities. cVDPV type 2 (cVDPV2) is the most prevalent, with 959 cases occurring globally in 2020.
The cVDPV virus is the form that has been identified in Burundi and the DRC, although there has only been a handful of cases. There have also been some cases in the USA, all of which came from individuals who had received the OPV outside of the USA.
There are two important points that I need to make:
- The injectable polio vaccine cannot cause cVDPV, because the virus is dead. Only the oral polio vaccine is linked to cVDPV.
- Individuals who have been immunized against polio cannot contract cVDPV. In other words, mass immunization with OPV may lead to a few cases of cVDPV, but it generally cannot cause a mass outbreak as long as most individuals are already vaccinated against polio.
The situation in Burundi and DRC
The Global Polio Eradication Initiative (GPEI) announced that several cases of cVDPV have been linked to the novel oral polio vaccine type 2 (nOPV2) with multiple cases detected in Burundi and the Democratic Republic of Congo.
The World Health Organization (WHO) reported that cVDPV was found in stool samples from seven children with acute flaccid paralysis in the DRC and Burundi. Cases were confirmed in an unvaccinated 4-year-old boy living in western Burundi and two other children who were in contact with the boy, as well as in five wastewater samples collected in Burundi.
These are the first instances of cVDPV linked with the oral polio vaccine since the vaccine became available in March 2021. GPEI stated that “while detection of these outbreaks is a tragedy for the families and communities affected, it is not unexpected with wider use of the vaccine.”
Although it is a tragedy even if one child contracts cVDPV as a result of the vaccine, we just put these numbers into a broader context. Over 600 million doses of nOPV2 have been administered across 28 countries — the risk appears to be much less than 1 in a million doses.
Anti-vaxxers jump on board
Of course, anti-vaxxers are using this small outbreak as “evidence” that all polio vaccines are not safe. Nothing could be further from the truth.
As I wrote above, the injectable polio vaccine uses a dead virus and cannot cause cVDPV, no matter how you try to convince everyone that it does. In the USA, Europe, Australia, Canada, Japan, New Zealand, and other developed countries, OPV is generally not approved for use, since IPV is just safer to use.
And, as I wrote above, the risk of polio caused by the oral polio vaccine is so rare, that the benefits far outweigh the risks.
Finally, those vaccinated against polio are immune to cVDPV, so it will not spread widely.
Unfortunately, in much of the world, we have no choice but to use the oral version of the vaccine because of distribution issues — IPV needs to be kept colder and requires syringes for injection, something that makes broad use of the vaccine difficult in many countries.
This story is not an indictment of any vaccine, let alone the polio vaccine. But the anti-vaxxers will try.