Last year, an advisory panel for the Centers for Disease Control and Prevention (CDC) recommended Americans aged 60 and older get a single dose of a vaccine for the respiratory syncytial virus (RSV). Two RSV vaccine options were made available, one by Pfizer and the other by GSK.

Now, Pfizer's vaccine is under investigation for causing the rare nervous system disorder Guillain-Barre.

CDC officials said they were investigating more than 20 cases of Guillain-Barre syndrome (GBS), a rare illness in which a person's immune system damages its own nerve cells, causing muscle weakness and paralysis, allegedly caused by the RSV vaccines.

"Serious neurologic conditions, including [GBS], have been reported after RSV vaccination in clinical trials of older adults. It is unclear whether the vaccine caused these events," the CDC writes.

An estimated 3,000 to 6,000 people develop GBS in the United States each year, and it's more commonly seen in older people, according to the CDC.

Most people fully recover from the syndrome, but some have permanent nerve damage. Guillain-Barre can occur in people after they are infected with a virus, but in some instances, cases have been linked to vaccinations.

RSV, or respiratory syncytial virus, is a common cause of cold-like symptoms but it can be dangerous for infants and the elderly. The CDC claims RSV is responsible for an estimated 177,000 hospitalizations and 14,000 deaths per year among adults 65 and older.

Two Guillain-Barre cases caused by Pfizer's RSV vaccine confirmed

Two people who received Pfizer's RSV vaccine during a clinical trial were later diagnosed with GBS, and the U.S. Food and Drug Administration (FDA) has asked Pfizer to conduct a safety study.

The pharma giant claims its RSV vaccine, Abrysvo, protects older adults from RSV. Just one dose is supposedly 77.8 percent effective against lower respiratory tract disease associated with RSV.

The two confirmed GBS cases were adults in their 60s who participated in Pfizer's Phase 3 clinical trial for Abrysvo. One person's illness had completely resolved after three months, and another was improving after six months.