(Inside Science) -- On October 26, 1977, a Somali cook, Ali Maow Maalin, was reported with the last human case of naturally infected smallpox. In 1980, the virus that caused the ghastly disease was declared extinct in nature, the first time that has happened in history.
Since then, public health workers have been trying to do the same with polio. They have come very close but still haven't succeeded. According to new research at the University of Michigan, epidemiologists need to consider that the theory of the spread of the disease may be wrong. When the last known case is reported, the researchers said, the world ought to wait at least three years before proclaiming victory. In a statement, the CDC indicated that three years is sufficient to certify that wild polio virus is not spreading in a region.
People born after 1955 do not understand the terror polio once spread every late summer and early fall in the U.S. as thousands of people, mostly children, came down with the infection. Some of them became paralyzed, and in cases when the paralysis spread to their chests, some spent the rest of their lives in respirators known as iron lungs. The disease affected all classes and races equally, said Micaela Martinez-Bakker, a doctoral candidate at Michigan's department of ecology and environmental biology, lead author in the paper published this week in the journal PLOS Biology.
It happened every summer: Swimming pools, preschools, nurseries and theaters were closed in hopes of halting the spread. Like all viral infections, polio could not be cured.
The most famous case was Franklin Delano Roosevelt, who lost the ability to walk when he was stricken at the age of 39. He wore braces and, when out of public view, used a wheelchair.
In 1952, Jonas Salk began testing a vaccine containing dead viruses. It was injected into millions of children -- one of the largest medical tests in history -- and when it was announced as safe and effective it made the front pages of almost every newspaper. Salk became an international hero. Ten years later, there were fewer than 1,000 cases of polio in the U.S. The last naturally occurring infection in the country was in 1979.
Another vaccine containing weakened but living viruses was developed by Albert Sabin. The Sabin vaccine is administered orally. It was the most-used vaccine until recently, when it was abandoned in the U.S. and elsewhere because of safety concerns.
Scientists noted that up until the 20th century, polio was hardly a major public health risk, and paralytic polio was "fairly rare," Martinez-Bakker said. From 1900 on, however, cases exploded. By 1950, there were 35,000 cases in the U.S. annually. The fact that the increase coincided with the industrialization of the western world and improvements in sanitation and hygiene led scientists to develop what is known as the hygiene hypothesis.
The polio virus has no animal reservoir. It must be transmitted from human to human, usually through contact with fecal material. As sanitation improved, the thought was that diseases like polio should be greatly reduced, but the opposite was happening.
Many scientists concluded that it was the sanitation that expanded infection. Before the sanitation improvement, wild poliovirus was everywhere, and most people caught the disease as young children. They became immune.
"The thing about getting the infection in infancy is we get maternal antibodies that protect against paralytic polio," Martinez-Bakker said. People might get sick, but they would not get the paralytic form of the disease.
Improvements in sanitation reduced transmission of the virus, so most people were no longer infected as children and were no longer immune when they caught the disease later in life, according to the theory. Only about 1 percent of polio infections lead to paralysis, so what happened to the mature Roosevelt was extremely bad luck.
The Michigan researchers, using huge amounts of health data from the 48 U.S. states that existed before the development of the Salk vaccine, concluded that the evidence doesn't support the theory. The expansion of the disease is nothing more than the result of a burgeoning population that grew particularly rapidly after World War II and the baby boom. More people was the problem, not more virus.
What the hygiene hypothesis ignored was that fact that unlike smallpox, where every infection leads to symptoms, more than 90 percent of people infected with the polio virus show no symptoms. The "silent infections" don't show up in the data.
Scientists knew about the silent infections, but Martinez-Bakker and her colleagues were the first to track the spread of the disease before the vaccine.
Poliovirus and the disease still exist naturally in a few countries, including Pakistan, Afghanistan and Nigeria. Attempts to eradicate it in those countries have proven difficult, partly because of resistance from groups of people to Western medicine and conspiracy theories.
The Michigan researchers have concluded that if the time comes when no further cases of polio are reported--when it looks like the virus is extinct--epidemiologists should wait at least three years before declaring the disease eradicated. Based on the U.S. data, only a timespan that long could account for the silent infections.
What makes the Michigan study so valuable is that the data were gathered before the Salk vaccine altered the natural infection flow of the disease, said Yvonne Maldonado, professor of pediatrics at the Stanford University School of Medicine, who specializes in infectious diseases. She also works with the Gates Foundation on polio.
This is further complicated by the fact that in one in every million doses of the live vaccine, the virus causes an infection, which is the main reason it is no longer used in the U.S. and many other countries. So, Maldonado said, it is possible to wipe out wild polio but still have "vaccine polio."
In the U.S., the live polio vaccine was used exclusively until 1997, and wild polio was eliminated. All other cases came from the vaccine. Most occurred in young babies without vaccine immunity or where immune-compromised.
"Three years is not a bad guess," Maldonado said.
Editors' note (6/19/2015): Shortly after publication, Inside Science added details about the Sabin vaccine and information provided by the CDC.