Read the entry on Jonas Salk from Compton's Encyclopedia/The Britannica School Edition, and answer the question beneath the text. Compton's Encyclopedia

Jonas Salk (1914-1995). For the first half of the 20th century, poliomyelitis, also known as infantile paralysis, was one of the most dreaded diseases to attack young people. By 1955 American physician and researcher Jonas Salk had developed a vaccine that almost eradicated the disease in the United States and other developed countries.

Jonas Edward Salk was born in New York City on October 28, 1914. He graduated from the City College of New York in 1934 and earned his doctorate in medicine from New York University's College of Medicine in 1939. While in medical school he had worked with Thomas Francis Jr. on killed-virus immunology studies. In 1942 he joined a group at the University of Michigan working on a vaccine against influenza. The vaccine was perfected in 1953.

In 1947 Salk became associate professor of bacteriology and head of the Virus Research Laboratory at the University of Pittsburgh School of Medicine. There he began working on the polio vaccine. Three viruses caused polio, and a vaccine had to be effective against all of them. By 1952 Salk had developed such a vaccine and conducted field tests on children. His findings were published in 1953, and a mass field test was done the next year.

On April 12, 1955, the vaccine was released for use in the United States. In 1957 Salk was named professor of experimental medicine at Pittsburgh, and in 1963 he went to San Diego as head of the Institute (later named the Salk Institute) for Biological Studies. In 1977 Salk was awarded the Presidential Medal of Freedom. He died on June 23, 1995, in La Jolla, California.

Answer the following question based on the Compton's Encyclopedia reading:

What was one of the challenges Dr. Salk faced in his attempt to create a polio vaccine?