• A majority of the FDA's advisors said the safety and efficacy data supports using Pfizer's RSV vaccine in adults ages 60 and older.
• But several FDA advisors said there could be a significant safety issue after two vaccine recipients out of about 20,000 developed Guillain-Barre syndrome.
• Other advisors were frustrated by a lack of efficacy data on people with weak immune systems and nursing home residents.

The Food and Drug Administration's independent advisors on Tuesday recommended what would be the world's first RSV vaccine, a shot from Pfizer for adults ages 60 and older, despite safety concerns after two trial participants developed a rare neurological disorder. 

A majority of the FDA committee members backed the vaccine, but they wrestled with separate votes on whether the safety and efficacy data are adequate to support an approval by the agency. The FDA is expected to make its decision in May.

There currently is no vaccine to protect older adults from respiratory syncytial virus, which kills thousands of seniors every year. Pfizer's shot could become the first to receive FDA approval.

In the first vote Tuesday, seven FDA committee members said the safety data was adequate for an approval, while four said it was not, and one member abstained.

The vote followed concerns at the FDA and among advisory committee members about two cases of Guillain-Barre syndrome in about 20,000 vaccine recipients. Guillain-Barre is a rare neurological disorder in which the body's immune system mistakenly attacks the nerves.

Symptoms can range from brief weakness to paralysis. Most patients, even those with severe cases, recover.

In the second vote, seven committee members said the vaccine effectiveness data was adequate, while four said it was not, and one member abstained.

The shot was about 86% protective against lower respiratory tract illness with three or more symptoms, and 66.7% effective against the same condition with two or more symptoms, according to an FDA review of Pfizer's data. The symptoms included wheezing, shortness of breath, rapid and shallow breathing as well as mucus production.

In adults ages 65 and older, RSV causes 6,000 to 10,000 deaths and 60,000 to 160,000 hospitalizations per year, according to the Centers for Disease Control Prevention. The risk of hospitalization increases with age, and adults ages 70 and older are more vulnerable.

Among adults of all ages hospitalized with RSV, 19% require intensive care and 4% die, according to CDC data from three seasons. Mortality is the highest among seniors.

Pfizer's vaccine contains both strains of RSV, which circulate at the same time during fall and winter. The shot is administered as a single 120 microgram dose.

The Guillain-Barre cases

The FDA considers the two Guillain-Barre cases during the trial as possibly linked to the vaccine, said Dr. Nadine Peart Akindele, an agency official. The FDA has asked Pfizer to develop a safety study that will monitor Guillain-Barre risks after an approval, which the company has agreed to do.

A 66-year-old man in the U.S. developed Guillain-Barre, and a woman of the same age in Japan was diagnosed with Miller Fisher syndrome, a variant of Guillain-Barre. They developed symptoms seven and eight days after vaccination, respectively. The woman has since recovered, and the man's symptoms were resolving as of the last update, according to FDA.

But Alejandra Gurtman, a Pfizer executive, maintained that the company did not identify any safety concerns during the trial and that the vaccine was well tolerated.

Gurtman pointed to other potential causes of the Guillain-Barre cases, noting that the man had suffered a heart attack and the woman had an upper respiratory tract infection. But Dr. Marie Griffin, an FDA advisory committee member, said the cases raise serious safety concerns.

"It seems to me that one case is a red flag. Two cases is very concerning and it's concerning to me that Pfizer doesn't think that there are any safety concerns," said Griffin, a professor of health policy at Vanderbilt University Medical Center

FDA says safety monitoring critical

Dr. Hana El Sahly, who chairs the FDA advisory committee, also said the Guillain-Barre cases raise a significant safety concern. El Sahly said the disease has an incidence of about 1 in 100,000 among people ages 60 and older, but in the trial the rate looks more like 1 in 9,000.

"So this is major if we take it at this level," El Sahly said, while noting that there's significant uncertainty about what the actual rate of the disease would be among vaccine recipients. "But nonetheless, it's significant in terms of incidence," she said.

Dr. David Kaslow, a senior official in the FDA's vaccine division, said safety monitoring will be "critically important" if Pfizer's RSV vaccine is approved.

The FDA is currently reviewing Pfizer's proposal to study the risk of Guillain-Barre syndrome among Medicare beneficiaries. The FDA has the authority to require such a study after approval if the agency determines that is necessary.

Dr. Jay Portnoy, an FDA committee member, said he concluded the safety data was adequate because Guillain-Barre syndrome is rare, and otherwise adverse events in the trial occurred at about the same rate among people who received the vaccine and those who did not.

Adam Berger, an official at the National Institutes of Health, said he was concerned about the Guillain-Barre cases, but said a larger study population is needed to determine whether there's an actual link to the shot. The study planned by the FDA and Pfizer after approval could help answers this question, he said.

"I felt that I could vote yes at this point, with a heavy lean towards the real requirements of that post-market surveillance study," said Berger, who is also an FDA committee member.

Incomplete efficacy data

At the time of Tuesday's meeting, there was not enough data to evaluate the effectiveness of Pfizer's vaccine against severe disease requiring hospitalization, oxygen support or a mechanical ventilator, according to the FDA. Data was also not available on how long protection from the vaccine will last, according to the agency.

Pfizer also did not have data on how effective the vaccine is for elderly individuals with weak immune systems or for those in frail health, according to the FDA.

Griffin, the physician at Vanderbilt University Medical Center, said she was concerned that the vaccine was tested in a relatively healthy population in which hospitalizations were low, and that there wasn't any data on nursing home residents and people in frail health.

Akindele, the FDA official, said real-world evidence after a vaccine rollout could answer how much protection it provides for those in fragile health. The data would then be used to determine whether the FDA needs to make any adjustments to how the shot is used, she said.

Pfizer is conducting its clinical trial over two RSV seasons with more than 34,000 people enrolled. The available efficacy and safety data is from the first season.

When asked what would happen if data from the second season is not as strong after an approval, Akindele said the FDA would reevaluate the shot and meet with the committee again to determine how to proceed.

Pfizer estimates that if 50% of people ages 60 and older receive the shot, the vaccine could prevent more than 5,000 deaths, 68,000 hospitalizations, 51,000 emergency department visits and more than 422,000 outpatient visits.