Years without reports of acute flaccid paralysis – the clinical manifestation of polio – does not mean that the wild poliovirus is gone, according to a new study.

The study of the prevaccination cases of polio in the United States also has debunked the previously held belief that improvements in sanitation and hygiene fueled the spread of the disease after the mid-1940s. Instead, the new research found the surging birth rate of the post-war baby boom to have caused the steep rise in instances of polio.

The study involved the examination of monthly polio case reports that predated the introduction of the polio vaccine. The data came from the U.S. Public Health Service’s Morbidity and Mortality Weekly Reports on the 48 contiguous states and the District of Columbia, dating from January 1931 to December 1954. The number of births by state, beginning at least as early as 1933 from Vital Statistics, and state population sizes from the population distribution branch of the U.S. Census Bureau also were examined. For exploratory analyses, the researchers quantified the relationship between disease fade-outs, which it defined as having at least 3 months without a reported infection, and population size.

The researchers used the polio data to fit and simulate mechanistic transmission models to track poliovirus and reconstruct the millions of unobserved subclinical infections that spread the disease.

“We demonstrate 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,” Micaela Martinez-Bakker, a graduate research fellow at the University of Michigan, Ann Arbor, said in a statement.

The study showed that widespread polio transmission likely occurred during multiyear periods when no cases of the disease were reported. “We estimate that over 99% of [polio] infections were subclinical, with the reporting of total infections regularly below 1%,” said Ms. Martinez-Bakker and her colleagues. “Importantly, subclinical infections are likely more common today than in the period we studied.”

The researchers advocate for the completion of future studies using similar transmission models on how demographic and environmental factors interact with polio vaccine coverage in endemic countries.

Read the full study in PLOS Biology (2015 June 19 [doi:10.1371/journal.pbio.1002172]).


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