In order to completely eradicate polio, the researchers recommend intense environmental surveillance alongside vaccination campaigns.
In the study, published in PLOS Biology, researchers from the University of Michigan demonstrate that the polio virus can be spread for long periods without any cases being reported, suggesting that aggressive surveillance and vaccination programs will be required to guarantee its eradication.
"Using transmission models, we show that you can have sustained chains of silent transmission in populations for more than 3 years, without a single person ever showing up as a reported polio case," explains graduate research fellow Micaela Martinez-Bakker.
"Once we've eradicated polio - or think we've eradicated polio - we probably should intensify the environmental surveillance to make sure the virus is not just lurking under the hood at very low levels," she adds.
Polio is a contagious viral illness that mainly affects children under 5, with 1 in 200 infections leading to irreversible paralysis. Although the last case of naturally occurring polio in the US was reported in 1979, the World Health Organization (WHO) reported 416 cases worldwide in 2013.
Rates of polio prevalence have fallen significantly in the past few decades. In 1988, the virus was endemic in more than 125 countries, but now it is only prevalent in Afghanistan, Nigeria and Pakistan.
A lot of this reduction is down to the success of polio vaccination. The authors report, however, that due to this success, "critical features of polio virus transmission remain obscure."
In order to investigate, the team analyzed polio case reports from large epidemics in the US during the pre-vaccine era. These data included birth statistics and census numbers from every state, allowing the researchers to examine the ecology of polio infection in a world without human intervention.
True extent of polio prevalence in 1950s in the US revealed
Polio prevalence in the US was at its highest in 1952, with 57,000 reported cases. Mass inoculations with the new vaccine occurred just 3 years later, after the vaccine was declared safe and effective.
Incidence of the virus in the US increased greatly between the 1930s and the 1950s, and researchers initially believed that this was due to the "disease of development" hypothesis.
The hypothesis is that improved hygiene had "reduced transmission and pushed the burden of infection onto children more susceptible to paralytic polio," the authors write, therefore increasing the likelihood of clinical manifestation of the virus.
Following their investigation, the researchers now believe this hypothesis is wrong. Instead, they attribute the increase in cases in the late 1940s to rising birth rates following the end of the Second World War.
"If you have more kindling, you can have a much larger forest fire," Martinez-Bakker explains. "The baby boom provided more kindling for polio epidemics - young children and infants over 6 months of age - so much more explosive outbreaks were now possible."
The data obtained for the study also allowed the researchers to track the unobserved and symptomless polio infections that occurred around this time. While the reported number of polio cases for 1952 was 57,000, the team discovered that more than 3 million people were likely to have been infected that year.
According to the authors, "the regular identification of, and rapid response to, these silent chains of transmission is of the utmost importance." Their study also offers further insight into why these historical polio epidemics fluctuated, were seasonal and varied geographically.
"Reaching eradication and preventing re-emergence of polio requires intimate knowledge of how the virus persists," Martinez-Bakker concludes. "Historical epidemics that predate the use of vaccines can be used to disentangle the epidemiology of disease from vaccine effects. They allow us to establish a baseline by studying the system in the absence of intervention."
Last year, Medical News Today reported on a study that found an extra dose of polio vaccine for children under the age of 5 could help to speed up eradication of the virus.