In 1948, Dr. Isabel M. Morgan led a team that successfully inoculated monkeys with a killed-virurs vaccine. From 1945-50, her work at Johns Hopkins defined the level of circulating antibodies needed in the blood stream to protect monkeys from an intracerebral challenge from the poliovirus. Her experiments advanced the knowledge needed to develop a viable vaccine.

These studies supported what Salk suspected from his work with inactivated influenza vaccines: Active infection with a living poliovirus was not required to induce immunity – a killed virus vaccine might work.