Low polio vaccination rates plus the presence of the virus in wastewater in a New York county suggest that others are at risk following a case of paralysis from polio in a young adult this summer, the Centers for Disease Control and Prevention reported Tuesday.
An unvaccinated individual in Rockland County developed paralysis from polio in June — just the second instance of community spread of the virus in the United States in 43 years.
"We haven't thought about polio in decades, and aren't we lucky?" said Helen Jenkins, an infectious disease epidemiologist at Boston University School of Public Health who was not involved with the New York case. "But now we're thinking about it again, and if we could stop circulation everywhere, we could actually forget about it forever."
Three doses of the polio vaccination shot are more than 99% effective at preventing paralytic polio, but vaccination rates against the virus in Rockland County are far below the national rates.
According to the New York State Immunization Information System, the CDC said, 60.3% of children under age 2 living in Rockland County had received 3 doses of the polio vaccine as of August. In at least one of the county's 26 ZIP codes, rates were as low as 37.3%.
Nationwide, the polio vaccination rate is about 93%.
The specific polio strain in the New York patient has also been found in wastewater samples dating to May in Rockland County and neighboring Orange County, indicating "community transmission and ongoing risk for paralysis to unvaccinated persons," the CDC researchers wrote in the report.
Polio infection is asymptomatic in the majority of cases. However, 1 in 200 cases result in paralysis, according to the World Health Organization. That paralysis can be deadly if muscles used to control breathing are affected.
The chances of paralysis are even lower among people infected with the strain detected in Rockland County, at 1 in 1,900 cases, the CDC said. This suggests that there are many more asymptomatic cases that have gone undetected in the area.
"Even a single case of paralytic polio represents a public health emergency in the United States," the researchers wrote.
Tracing the New York case
The last case of polio that originated in the U.S. was in 1979, though the virus still spreads in other parts of the world.
According to the CDC report, the Rockland County patient had not recently traveled outside of the country. It is unknown where the patient was infected, however the person did attend a large gathering eight days before symptoms — fever, constipation and abdominal pain — began.
Within three days, the patient also developed a stiff neck and weakness in the legs. The case was first thought to be acute flaccid myelitis, a condition that can mimic polio, and is likely caused by a different virus.
Stool samples, however, revealed the presence of vaccine-derived poliovirus type 2 — a specific strain of the virus linked to a polio vaccine that has not been used in the U.S. since 2000, but is still used in other parts of the world.
That vaccine, which is given orally, uses a live but weakened form of the poliovirus to induce immunity. That virus can be released in a person's stool, and in rare cases, mutate back into a form that can cause paralysis if an unvaccinated person is exposed. The injectable version of the vaccine that's used in the U.S. doesn't use a live virus and cannot do the same.
The Rockland County individual was therefore likely infected after a person who received oral vaccine abroad returned to the U.S. and shed the weakened virus, the CDC wrote.
Polio has also been detected in wastewater samples in New York City, though it's unclear whether it's related to the Rockland and Orange county cases.
Polio spreads mainly through a fecal to oral route. This can often occur, for example, among young children still learning proper hand-washing skills.
Polio vaccines are considered durable and extremely effective. The CDC recommends that children receive four shots: at age 2 months, 4 months, 6 to 18 months, and then around age 5.
Three doses are enough to confer at least 99% protection, and the fourth dose is meant to make that protection last a lifetime. Some people may need an additional booster later in life if traveling to countries where polio is common, or if they did not receive all of the doses as a child.