In the 1930s, Cox joined the U.S. Public Health Service as Principal Bacteriologist at the Rocky Mountain Laboratory in Hamilton, Montana.

In 1947, John Franklin Enders and others demonstrated that monkey tissue provided a suitable medium to grow the virus in the lab.

Salk employed the Enders method, incubating the virus using rhesus monkey kidneys and testicles.

[1] Meanwhile, human trials of Albert Sabin's successful oral vaccine had begun in 1957, and it would be licensed for general use in 1961.

[2] Within Lederle Laboratories, Cox competed with co-worker Hilary Koprowski, as each had developed a successful polio vaccine.