(CNN)Over the course of a two-day meeting, vaccine advisers to the US Food and Drug Administration voted in favor of approval for RSV vaccines for adults over the age of 60. The vaccine candidates, made by Pfizer and GlaxoSmithKline, could become the world's first approved vaccines against the common virus.
On Wednesday, committee members voted 10-2 in support of the safety and unanimously in support of the effectiveness of GSK's vaccine for the prevention of lower respiratory tract disease caused by RSV among older adults.
For Pfizer's shot, committee members voted on Tuesday with 7-4 with one abstention that there is adequate data to support the vaccine's safety and effectiveness.
The FDA, which typically follows the committee's recommendations, is expected to decide on approval of the vaccines in May, ahead of RSV's typical winter surge.
Respiratory syncytial virus is a highly contagious virus that causes flu-like illness in people of all ages that increases in severity with age. According to the US Centers for Disease Control and Prevention, RSV is responsible for an estimated 177,000 hospitalizations and 14,000 deaths per year among adults 65 or older.
GSK's RSV vaccine candidate for older adults was 82.6% effective in preventing lower respiratory tract disease defined by either two or more symptoms or and one or more signs or three or more symptoms. The shot was 94.1% effective at preventing severe disease.
The Pfizer vaccine was 66.7% effective at preventing moderated lower respiratory tract illness with two or more symptoms and 85.7% effective at preventing illness with three or more symptoms, according to Pfizer.
Focus on safety
Though a majority of the committee voted in support of the vaccines, some committee members expressed concerns over both vaccines' risk of Guillain-Barre syndrome, a rare neurological disorder in which the immune system attacks its own nerves, causing muscle weakness and sometimes paralysis.
The incidence of Guillain-Barre is about 1.5 to 3 cases for 100,000 adults over age 60 in the US annually, according to the FDA.
Two adults among 20,000 vaccine recipients in Pfizer's Phase 3 clinical trial developed symptoms consistent with the rare neurological disorder within nine days of receiving the shot.
Of roughly 15,000 vaccine recipients of GSK's Phase 3 clinical trial, one man developed Guillain-Barre syndrome within nine days of vaccination.
"It seems to me that one case is a red flag. Two cases is very concerning," said Dr. Marie Griffin, professor of health policy at Vanderbilt University Medical Center, who voted that the data demonstrated that both vaccines are effective but not safe
The FDA has called Guillain-Barre syndrome an "important potential risk" of Pfizer and recommended the company conduct a safety study for further evaluation of Guillain-Barre syndrome and other immune-mediated demyelinating conditions after potential vaccine approval, which the company has agreed to. GSK has said they will closely monitor cases after approval.
While Pfizer did not provide data on co-administration with other vaccines, GSK's data showed potential safety risks when the RSV and flu shots were administered together.
Two adults in their 70s who received both the flu and RSV vaccine developed acute disseminated encephalitis, a rare neurological disorder that involves swelling of the brain and spinal cord, and one of the individuals died. The FDA considers these cases as possibly related to either RSV or flu vaccination, but review is ongoing.