Making enough vaccine to protect large numbers of people if the H5N1 avian flu strain sparks a pandemic will be more complex that previously thought, a new study reveals.
The scientific paper shows that even at extraordinarily high doses -- 12 times the antigen needed to protect against seasonal flu strains -- nearly half of vaccinated healthy adults did not generate the level of antibodies thought to be necessary to defend against the troublesome strain.
Testing showed only 54 to 58 per cent of people who received two doses of 90 micrograms (mcg) apiece produced the level of antibodies believed to be needed to protect against infection, the researchers reported in this week's New England Journal of Medicine.
"These are lower response rates than what one might have expected for regular flu shots in healthy people," said lead author John Treanor, director of the vaccine treatment and evaluation unit at the University of Rochester in New York State.
Typically, somewhere between 75 and 90 per cent of healthy adults who get a flu shot for seasonal flu would be expected to develop protective levels of antibodies to the strains contained in the vaccine, with some variability depending on the flu strain and the recipient's age and health.
Dr. Treanor said there would be logistical difficulties in trying to produce and administer higher doses of an H5N1 vaccine. And given that this dose is already far too high to be practical during a pandemic, it seems unlikely anyone would consider that option.
With current worldwide flu vaccine production capacity, a dosing regime of two shots of 90 mcg apiece would mean about 75 million people around the world -- or just over 1 per cent of the global population -- could be vaccinated in the first year of a pandemic.
Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said it had been hoped this vaccine would protect about 75 per cent of study participants. And he admitted the dosing requirements were "sobering news."
"Having a vaccine that would require 90 micrograms times two [doses] in and of itself would not and cannot be the answer to where we want to be," said Dr. Fauci in a teleconference for journalists. His institute funded the research.
A vaccine expert who wrote an editorial on the issue for the journal put it in even balder terms.
"The headline here is: Back to the drawing board," said Gregory Poland, of the vaccine research group at the Mayo Clinic College of Medicine in Rochester, Minn. Dr. Poland was not involved in the trial.
"If we had a pandemic this week, this month or this fall, we're not ready in terms of vaccine. This vaccine is not the answer."
Other studies are under way or will soon begin looking for ways to lower dosages by adding boosting chemicals called adjuvants to the vaccine or by injecting it into the skin, instead of muscle.
