Which sentence from the passage best supports the idea that before Salk's team, medical researchers had been unable to combat polio effectively?
Salk's Polio Vaccine
In 1953, a medical researcher named Jonas Salk attempted to inoculate himself, his family, and other volunteers with an injection for the polio virus. Salk was testing his research in hopes that his discoveries could be used to prevent polio epidemics, or outbreaks, across the United States and the world. Building on the research of previous scientists and doctors, Salk developed a polio vaccine by growing large numbers of the virus in test tubes. Salk then killed the virus with formaldehyde and used this dead virus to create vaccines for immunization. Once injected in a person's body, the dead virus lacked the ability to infect but still triggered an immune response that produced polio antibodies and prevented future polio infections. Analyzing their test results, Salk and his team were thrilled to see that their vaccine caused the immune system to produce polio antibodies without making a person ill. After years of research, scientists finally felt they had the upper hand in the expensive and prolonged battle against polio.
Researchers believe polio has been around for thousands of years. However, scientists did not identify the virus until 1908. Since the early 1900s, increased polio outbreaks in the United States caused a growing hysteria among many Americans. In 1916, a polio outbreak in New York City killed 2,000 people. Future US President Franklin Delano Roosevelt became ill with polio in 1921, and the infection caused lifelong paralysis in his legs. In 1952, the United States dealt with 57,000 cases of polio—the most cases ever in a single year.
In 1954, the government approved a massive vaccination trial test using Salk's vaccine for over one million children in the United States. The next year, scientists announced that the vaccination trial had been a success: the vaccine prevented polio infections in 80% to 90% of people. Soon thereafter, the government approved the vaccine and began delivering the vaccine across the country. The success of Salk's polio vaccine helped eliminate the virus from the United States and other parts of the world.