The GSK vaccine was nearly 83 percent effective in lower respiratory tract illness in a study of about 25,000 patients — half on the vaccine and half on a placebo, according to data the company provided to the F.D.A. No R.S.V. deaths were reported among the patients in the GSK or Pfizer studies.

Several panel members expressed concerns about side effects reported with each vaccine. After seven days, one patient who received the Pfizer vaccine developed Guillain-Barré syndrome, a condition where the immune system attacks the nervous system (but not the spine or brain). The case was considered life-threatening, F.D.A. records show, though a “potentially confounding factor” was that the patient had suffered a heart attack the day before the condition developed.

Another Pfizer vaccine recipient developed Miller Fisher syndrome, which is considered to be a type of Guillain-Barré, eight days after getting the shot. That patient, from Japan, reported double vision and a tingling or burning feeling in her palms and the soles of her feet. Her symptoms were mostly resolved within 41 days, the F.D.A. records said.

The two cases in the Pfizer study put the rate of the condition at about one in 9,000, even though it is typically about one in 100,000, according to Dr. Hana El Sahly, chairwoman of the vaccine advisory committee and a professor of virology at Baylor College of Medicine. “So this is major,” Dr. El Sahly said.

Dr. Marie Griffin, a health policy professor at Vanderbilt University, said the Pfizer vaccine was difficult to evaluate given the low incidence of severe infections among those in the trial.

“I think the benefit for relatively healthy, older people — you have to consider that — is not that great,” Dr. Griffin said. “Compared to a possible high risk of a very severe outcome.”

One recipient of the GSK vaccine, who was 78 and from Japan, also developed Guillain-Barré nine days after getting the vaccine; she went on to spend six months in a rehabilitation hospital. The company and the F.D.A. considered the case to be related to the vaccine.