Why are we seeing polio now? Hadn’t it been eradicated?
Dr. Furuya: Polio has not been completely eradicated. Globally, two of the three strains of wild poliovirus (types 2 and 3) have been eradicated, and the remaining strain of wild poliovirus (type 1) is circulating in only two countries, Pakistan and Afghanistan.
All that being said, outbreaks of polio continue to occur, usually due to a strain of poliovirus that is given as part of the oral polio vaccine. Since 2000, the U.S has only been vaccinating people against polio using the inactivated polio vaccine (IPV), which is an injectable vaccine. However, in many other countries, the vaccine that is used is the oral polio vaccine (OPV) which contains live, weakened polioviruses. When people receive OPV, the live, weakened polioviruses can lead to transmission to other people. Normally this doesn’t cause problems if everyone else is vaccinated as well, but if there are enough unvaccinated or undervaccinated people around, and the weakened poliovirus from the vaccine circulates in that community for long enough, it can actually revert to a form that can cause paralytic disease.
Dr. Acker: Although we had considered polio eradicated in the Americas in 1994, polio continues to cause infections globally due to undervaccination. OPV is used globally in an effort to eradicate polio and is very effective at preventing polio infections. Yet, if there are enough undervaccinated individuals in the community, and the vaccine-derived virus circulates long enough, it can result in paralytic disease, as was seen in the recent case in New York in July which was a case of poliomyelitis caused by a vaccine-derived virus in an unvaccinated individual.
Can you explain what polio is and how it’s spread?
Dr. Furuya: Polio, or poliomyelitis, is a very contagious disease caused by a virus that lives in people’s gut, which can spread through contact with the stool of an infected person, as well as sometimes from droplets from an infected person sneezing or coughing. If you get the stool or droplets on your hands and then touch your mouth, you can get infected in what we call “fecal-oral” transmission.
More than 70% of people who get infected with the poliovirus actually have no symptoms but can still shed and spread the virus to others. About 1 in 4 infected people can get flu-like symptoms such as fever, sore throat, nausea, vomiting, or abdominal pain; these symptoms usually only last for a few days and then go away on their own. More rarely however, people can get more severe symptoms that affect the brain and spinal cord, specifically meningitis (inflammation of the fluid around the brain and spinal cord) and weakness and paralysis of the arms and/or legs (called acute flaccid paralysis).
If you were vaccinated as a child, are you still protected as an adult? What about kids who aren’t yet fully immunized? Who is at risk?
Dr. Furuya: It’s thought that getting the full vaccine series as a child should provide lifelong immunity from paralytic polio, although we can’t be 100% certain. Kids who aren’t yet fully immunized may be at some risk, although it’s been shown that 90% of people who received IPV are immune after two doses and 99% are immune after three doses, so most children who received their vaccine series on schedule should be protected, even before they receive their fourth dose at age 4 to 6 years.
Dr. Acker: Individuals who were fully vaccinated as a child are protected against paralysis from polio. Those considered at higher risk include healthcare providers treating patients who could have had polio or have close contact with a person who could be infected with poliovirus, laboratory workers handling specimens that may contain poliovirus, and travelers to areas where poliomyelitis is endemic or epidemic.