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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.
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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.
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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.
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"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.
"This is a disease with incidence 0.1 in 100,000 usually, the majority being in children and then a scatter in young adults. So two cases in elders within three to four weeks post-vaccine is highly anomalous from a statistical standpoint," said Dr. Hana El Sahly, the FDA advisory committee chair, who voted against the GSK shot based on its safety profile but in favor based on efficacy.
Dr. Adam Berger, who voted in favor of GSK's vaccine based on safety and efficacy, says it seems to be an issue with co-administration and that concerns with acute disseminated encephalitis and Guillain-Barre syndrome won't be answered until larger studies are conducted.
"I suggest a heavy reliance on the postmarketing surveillance and not only just reliance but making sure there is an enforcement around the requirements about this," Berger said. "At the moment, I think the profile seems to be within the acceptable range."
Search for an effective RSV vaccine
Some vaccine advisers wanted to see more data on the Pfizer vaccine's effectiveness at preventing hospitalization or death, especially among high-risk individuals including adults over 80, those with comorbidities, or those who are immunocompromised.
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"I think the data does support the effectiveness of this vaccine, it's just the population was underrepresented by people who could most benefit from the vaccine," Griffin said of Pfizer's clinical trial.
The available safety and efficacy data is from the first season of Pfizer's clinical trial scheduled over two RSV seasons and the first of three seasons for GSK's trial. Some experts want to wait for more data.
"I'm desperately eager to have a vaccine that works for RSV. This has been a terrible disease my whole career. I would love to see it. No doubt about it," said Dr. Jay Portnoy, professor of pediatrics at Children's Mercy Hospital, who voted in support of GSK's vaccine but against Pfizer based on efficacy.
Portnoy said waiting for a second season of data would provide more robust numbers and complete analysis.
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"It's not an emergency use authorization. We can take the time to finish the studies and get the information we need before licensing this product going forward. So I remain a little bit skeptical given the data that we have," he said.
Others think the benefits of the candidate vaccines outweigh the potential risks of deadly RSV infection.
"Despite the challenges, despite the additional hurdles, our obligation is to do what's right for the public," said Dr. Daniel Kim who voted in favor of both shots. "And in this case, we have a bad disease, we have a good vaccine -- so far, anyway, given the clinical trials -- it's a safe vaccine, and the vaccine can be used to prevent disease."
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