Task: Question Answering
Input (English): Passage: Salks vaccine was then used in a test called the Francis Field Trial, led by Thomas Francis, Jr., the largest medical experiment in history at that time. The test began with about 4,000 children at Franklin Sherman Elementary School in McLean, Virginia, and eventually involved 1.8 million children, in 44 states from Maine to California. By the conclusion of the study, roughly 440,000 received one or more injections of the vaccine, about 210,000 children received a placebo, consisting of harmless culture media, and 1.2 million children received no vaccination and served as a control group, who would then be observed to see if any contracted polio. The results of the field trial were announced 12 April 1955 (the tenth anniversary of the death of President Franklin D. Roosevelt, whose Franklin D. Roosevelts paralytic illness was generally believed to have been caused by polio). The Salk vaccine had been 60-70% effective against PV1 (poliovirus type 1), over 90% effective against PV2 and PV3, and 94% effective against the development of bulbar polio. Soon after Salks vaccine was licensed in 1955, childrens vaccination campaigns were launched. In the U.S, following a mass immunization campaign promoted by the March of Dimes, the annual number of polio cases fell from 35,000 in 1953 to 5,600 by 1957. By 1961 only 161 cases were recorded in the United States. 
Question: By how many polio cases did the prevalence decrease in the four years following 1953? 
Answer type: Number 
Output (English): 29400.