Polio has reemerged in the United States after decades of absence. In late July 2022, Rockland County officials in New York State announced that a young unvaccinated man living in the county had become the first case of paralytic polio in the US in nearly a decade. Soon after, state officials announced they found poliovirus in wastewater samples from Orange County (also in New York State, neighboring Rockland County) and New York City, confirming that the virus was present and circulating in those areas. Most recently, the virus was detected in a wastewater sample from this past February.
The last time most Americans had to worry about wild polioviruses was 1979, when mass vaccination campaigns led to the elimination of such viruses in the United States. Safe and reliable vaccines have been produced since 1960, preventing nearly 30-million cases of paralysis worldwide.
But now, a key component of that success, the weakened forms of polioviruses present in some vaccines, is causing problems of its own, suggesting that decades of progress may be lost. Incomplete global eradication efforts, vaccine hesitancy, and pandemic-related declines in routine childhood immunization led to the biggest resurgence of poliovirus the U.S. has seen in decades.
The most obvious explanation for the resurgence of polio in Rockland County is that less than 60% of residents are immunized against it (circa statistics from Summer 2022). However, the origins of the current crisis may be found in incomplete efforts to eradicate polio virus infections with vaccines.
Types 1, 2, and 3 are the poliovirus strains that have spread among humans in the past. These viruses, which infect the intestinal mucosa, primarily spread through touch with someone else’s feces. The majority of people who are unvaccinated experience only minor symptoms, or none at all. Around 25% of those who catch the virus will develop mild flu-like symptoms. But for every 2,000 infections with the virus, depending on the strain, one to ten people will experience paralysis as it travels from their gut to their brain or spinal cord.
Polio cases reached their highest levels in the United States around the late 1940s and early 1950s, when more than 35,000 people were paralyzed each year. Globally, incidence peaked in the 1980s, when polioviruses afflicted hundreds of thousands of children every year. Development of two types of vaccines in the mid-20th century have drastically improved the situation, eliminating wild-type polio from the U.S. and bringing the polio virus to the brink of eradication. These vaccines work well, but there are differences between them, which help explain current global conditions.
The inactivated poliovirus vaccine, often known as IPV, was created first by Jonas Salk and is the only polio vaccination used in the United States. It protects against paralysis but not infection, completely. By injection, this vaccine contains poliovirus which then inspires the production of antibodies in the bloodstream. This action blocks polioviruses from infecting the brain or spinal cord and causing havoc. However, similar to how covid vaccines don’t fully prevent SARS-CoV-2 infection, IPV doesn’t completely stop polioviruses from getting into the gut. Even though they’re vaccinated with IPV, people can still transmit the poliovirus.
OPV, or oral poliovirus vaccine, is a less expensive and easier to administer option than IPV. OPV contains live but weakened versions of up to all three strains of the poliovirus. The virus replicates and stimulates immunity in both the gut and blood. This prevents paralysis and infection caused by the disease polio. During the early days of global eradication efforts, because just a small proportion of people might eventually immunize an entire population, this was quite effective. The virus in oral polio vaccination (OPV) can be excreted through feces and transmitted to others, especially in regions with poor sanitation.
Since 1988, a worldwide vaccination campaign costing billions of dollars and reliant on OPV has wiped out wild poliovirus Types 2 and 3 from the planet. Type 1 wild-type poliovirus is still endemic in Pakistan and Afghanistan.
However, the live, attenuated virus has a drawback: approximately one in every six million doses causes paralysis in children who receive OPV or unvaccinated close contacts. (Because of this risk, the United States stopped using OPV in 2000.) Additionally, by evolving and spreading throughout individuals who have not been vaccinated, the weakened vaccine virus can reclaim its ability to paralyze. When you have low vaccine rates, the strains can keep circulating and evolve to be like wild viruses, capable of causing disease. Occasionally, this happen within an immuno-compromised person.
In the last 15 years, vaccine-derived polioviruses have become a more prominent issue, leading to thousands of cases of polio primarily in developing nations. Most of these instances are caused by the type 2 strain, which was declared extinct in the wild in 2015. Thousands of cases of vaccine-derived polio have emerged throughout the developing world over the last 15 years. And because paralysis occurs so infrequently as a result of this, it indicates that there is likely to be a much larger epidemic of transmission.
In 2016, in order to try and address this problem, the Global Polio Eradication Initiative told countries to switch to a version of OPV without Type 2. This would ideally mean that cases of vaccine-derived polio would slowly die out as the most problematic strain was removed from circulation. To make the switch work, mass vaccination campaigns had to be conducted just before removing Type 2 to build up immunity. Also, all countries needed to switch at the same time and that did not go as planned.
Instead, in many countries, ineffective campaigns have left large swaths of vulnerability that have only worsened since more children were born without any immunity to the vaccine-derived type 2 virus. Over two-thousand cases of vaccine-derived paralytic polio have been reported in 36 nations since the change. Since 2016, type 2 cases and transmission have surged in these countries, eventually finding its way to the United States.
If you received the standard childhood vaccinations, there is a high chance that you are now immune to polioviruses. In the United States, over 90% of the population is vaccinated against polio.
However, community protection has decreased throughout the U.S. in recent years for two primary reasons: an anti-vaccine movement and areas where childhood vaccinations have been declining due to the pandemic. Many children missed vaccine appointments due to Covid.
These growing pockets of susceptibility have given imported vaccine-derived polioviruses a foothold in the U.S. The 20-year-old who caught polio in June and suffered paralysis lived in Rockland Country, a suburb of New York City that has immunization rates for polio that range between 37 percent and 62 percent depending on the area’s ZIP code. Measles vaccination rates were low among Hasidic Jewish communities living in the area, which contributed to a large outbreak of measles in 2018, an event that nearly cost the U.S. its measles elimination status. Vaccine hesitancy has grown considerably over recent decades due largely to disinformation circulated online.
Although paralytic polio is relatively rare, a single case likely indicates that hundreds or thousands have already been infected, meaning the virus has been circulating for months. Based on wastewater samples in and around New York City, the Centers for Disease Control and Prevention said the virus started circulating in the region sometime in April.
Unvaccinated people give the virus more opportunity to spread, but it’s also possible that IPV immunization recipients (everyone born after 2000) are also contributing. Those with IPV protection may transmit the disease more than those with OPV protection.
It’s tough to say what will happen next in America, but it largely depends on how many people get vaccinated. If vaccination rates do not improve, the most negative outcome is that this virus spreads quickly and easily among those who haven’t been vaccinated – mainly children – in various communities. People unknowingly infected with the virus could travel to other areas with lower vaccination rates, and become “vectors” by seeding new chains of transmission.
If you are vaccinated, you will be protected from developing symptoms if you come in contact with the poliovirus. However, if the virus continues to spread and people who are not vaccinated come into contact with it, approximately one out of every two thousand people infected could develop paralysis. Some people who can’t get vaccinated, like immuno-compromised individuals and infants under two, will be at risk, something Americans haven’t had to think about for a long time.
The IPV vaccine may not be strong enough to stop the spread of poliovirus if it continues to circulate, and countries might have to go back to using OPV. However, this comes with the risk of causing vaccine-associated paralytic polio. The World Health Organization granted an emergency-use listing of a new OPV vaccine in 2020, which uses a modified virus that is less likely to cause ongoing transmission. However, this vaccine is not authorized in many countries, including the United States.
No matter what tools the U.S. employs, the main challenge will be getting a sufficient number of people to actually use them. With public health issues becoming increasingly controversial, it is a more difficult task that may impede our ability to continue to control any virus, including Polio.
