In 1976, the U.S. Centers for Disease Control and Prevention (CDC) was involved in a national immunization program for a potential swine flu pandemic. Over 45 million people were vaccinated and there were increased reports of GBS following vaccination.

Guillain-Barre syndrome (GBS) is a rare disorder in which a person’s own immune system damages their nerve cells, causing muscle weakness and sometimes paralysis. While its cause is not fully understood, the syndrome often follows infection with a virus or bacteria. Most people fully recover from GBS, but some have permanent nerve damage.

The National Academy of Medicine conducted a scientific review of this issue in 2003 and found that people who received the 1976 swine flu vaccine had an increased risk for developing GBS. The increased risk was approximately one additional case of GBS for every 100,000 people who got the swine flu vaccine. Scientists have several theories about the cause, but the exact reason for this link remains unknown.