Mao engaged in research on the poliomyelitis virus, bacterin and cell culture technique in the late 1950s and contributed to the development of the poliomyelitis live bacterin. He also studied the effect of D20 towards virus proliferation and its functional mechanism in the 1960s. He established and studied Japanese encephalitis virus in chick embryo cells' interferon production system.
At the end of the 1970s, he separated hepatitis A virus (HAV) and found that red-face monkeys and rhesus monkeys have infection and immune reactions to HAV. He also proved that hepatitis A has a recessive infection and that HAV proliferates in the endoplasmic reticulum of tissue culture cell. One of his most significant accomplishments is that he prepared a safe and effective living vaccine for controlling hepatitis A.