OAKLAND, Calif., Feb. 13 (UPI) -- An experimental vaccine designed to prevent one of the most common infections people get while in the hospital has shown some promise in clinical trials but researchers said Wednesday there are limitations.
Very ill kidney disease patients on hemodialysis who got the vaccine were less likely, at least for a while, to get a Staphylococcus aureus or Staph infection than patients who did not get the vaccine.
After about nine months, however, the effectiveness of the vaccine diminished, with more vaccinated patients getting infections than nonvaccinated patients.
Study author, John Robbins, chief of the National Institute of Child Health and Human Development's laboratory of developmental and molecular immunity said, "It is likely that the vaccine will be more effective in individuals with less depressed immune systems who are at risk for Staph aureus infections, such as patients with chest and cardiac surgery and with joint replacements."
Kidney dialysis patients, the experimental group, usually have very depressed immune systems.
About a half-million people in U.S. hospitals each year get serious Staph infections and at least 50,000 die as a result. More than 90 percent of Staph infections do not respond to penicillin or ampicillin and 30 percent fail to respond to methicillin. Staph is beginning to show resistance to the antibiotic of last resort, vancomycin.
The study was conducted by investigators from Kaiser Permanente, based in Oakland, Calif., in collaboration with scientists at the NICHD. It was funded by Nabi Inc., a biopharmaceutical company in Boca Raton, Fla.
Nabi is developing the vaccine commercially. The results of the study are reported in this week's New England Journal of Medicine.
The clinical trial involved giving a vaccine injection to about 900 patients with advanced kidney disease who were receiving dialysis treatments. Another group of 900 kidney disease patients on dialysis did not get the vaccine. At the 40-week point, the group that got the vaccine was 57 percent less likely to have gotten an infection. Eighty-six percent of patients who got the vaccine showed antibody production.
From week 40 to week 54, however, vaccinated patients actually were more likely to get Staph infections -- 16 vaccinated patients compared with 11 nonvaccinated patients who got Staph infections during the 14-week period.
The vaccine, called StaphVAX, is specific for Staphylococcus aureus and is made up of molecules from the surface of Staph aureus bacteria attached to a carrier molecule. When the immune system sees the molecule in the vaccine injection it learns to recognize it and prepares to respond quickly if it sees it again. Then, if Staph aureus presents in the blood stream, the immune system can respond quickly with the antibodies it has made.
People most likely to get a Staph infection include surgical patients, particularly cardiac and orthopedic, as well as trauma and burn patients. Newborns, whose immune systems are not fully developed, can be at risk, as well as people in long-term care, on kidney dialysis or who have AIDS. People undergoing hemodialysis treatment have about 1 chance in 25 of contracting a staph infection.
The director of the kidney dialysis program at University of California at Los Angeles, Allen Nissenson, told United Press International the study "addresses a very important clinical problem in dialysis patients."
"I think the immunization approach has a lot of potential," he said, but added more work would need to be done to determine what kind of booster shots might have to be given periodically to dialysis patients. The tendency of Staph aureus to mutate would necessitate continued modification of the vaccine, said Nissenson, who is past president of the Renal Physicians Association.
Gerald Pier, professor of medicine specializing in microbiology and molecular genetics at Brigham and Women's Hospital in Boston and Harvard Medical School, told UPI there was potential evidence for the effectiveness of the vaccine in the short term for hemodialysis patients but offered a cautionary note.
"In reality the vaccine trial didn't work when you look at the entire 54 weeks of data. And it's not clear if you can legitimately go back and look at earlier time points and parse out the part of the trial that gives you a positive answer and ignore the part of the trial that made it not work overall," Pier said.
Changing the point of view from which the data is analyzed after an experiment is completed, called a post hoc analysis, can lead to problems with approvals from the Food and Drug Administration and the trial will need to be repeated, Pier told UPI. "There's no way the FDA would approve this product with this type of a post hoc analysis," he said.
Pier said, however, a vaccine with the ability to act even short-term for patients on hemodialysis was very important.
"The results of this study can certainly be viewed as encouraging but they're not really definitive," he said. The increased Staph infection rate from weeks 40 to 54 among vaccinated patients might be a result of some interaction between the vaccine and the immune system that might cause such a phenomenon in this very ill and immunocompromised group, he suggested.
(Written by Joe Grossman in Santa Cruz, Calif.)
