Study supports concept of 2-stage H5N1 vaccination

Oct 13, 2006 (CIDRAP News) – The human immune system may respond better to a vaccine for a new strain of H5N1 avian influenza if it is prepared in advance with a vaccine based on an existing H5N1 strain, the preliminary results of a government-sponsored study suggest.

In the study, 37 people who had received an H5N1 vaccine in 1998 were recently given another H5N1 vaccine based on a 2004 strain. They had a much stronger immune response than did another group who received only the newer vaccine, according to the National Institute of Allergy and Infectious Diseases (NIAID).

"These preliminary findings need to be confirmed in larger studies, but they offer the intriguing possibility that pre-pandemic priming with existing H5N1 vaccines may boost the immune response to a different H5N1 vaccine tailor-made years later to thwart an emerging pandemic," NIAID Director Dr. Anthony S. Fauci commented in a news release.

Researchers from the University of Rochester were scheduled to present a preliminary report of their findings today at the Infectious Diseases Society of America's annual meeting in Toronto, the NIAID reported.

When a pandemic flu strain emerges, it will probably take several months to develop a vaccine to match it, and more than one dose will probably be necessary to generate protective immunity, the NIAID said. But providing two doses would be logistically difficult, so researchers have been looking for other strategies. One proposed option is to vaccinate people in advance with a related vaccine in the hope that only one dose of the pandemic vaccine would be necessary.

After the first human cases of H5N1 illness occurred in Hong Kong in 1997, the NIAID funded the production of an experimental vaccine based on the Hong Kong virus and tested it in a clinical trial at the University of Rochester in 1998. The researchers found 37 people from that trial who were willing to take part in the new study. Participants in the earlier trial had received two doses of the vaccine.

This year, the 37 volunteers were vaccinated with one 90-microgram (mcg) dose of an H5N1 vaccine based on a strain that circulated in Vietnam in 2004, according to a study abstract provided by the University of Rochester. The vaccine is made by Sanofi Pasteur and is the one the US government is currently stockpiling in the face of the pandemic threat, according to Fauci. (That vaccine has shown only modest benefits in trials so far, with about half of vaccinees showing a good immune response after two 90-mcg doses, or about a dozen times the dosage used in seasonal flu vaccine.)

The previously vaccinated volunteers had a mean antibody titer (measured by hemagglutination inhibition) of 64.0, with 70% of them achieving a titer of at least 40, according to the study abstract. These findings were compared with results in some volunteers who received 90 mcg of the Sanofi vaccine in a previous study and had never had H5N1 vaccine before. These volunteers had a mean antibody titer of 27.1 after one dose, and only 29% had a titer of at least 40.

"We studied a relatively small group, so that certainly, this issue needs to be studied more thoroughly in a larger group of people," senior author Dr. John J. Treanor, MD, commented in a University of Rochester news release.

If further studies confirm the findings, pandemic response planners might consider giving a "priming" shot to key personnel, such as healthcare workers, said Treanor, who directs the university's Vaccine and Treatment Evaluation Unit.

Fauci told CIDRAP News today, "The data look very encouraging, but the number of people [in the study] is relatively small."

"These data add some scientific credence to the concept that there may be some benefit to priming someone with a potential pandemic strain even though the actual pandemic strain might be somewhat different," he said. "It informs the debate to allow you to have a more scientific basis if you're considering priming."

See also:

NIH news release
http://www.niaid.nih.gov/news/newsreleases/2006/Pages/IDSA.aspx

University of Rochester news release
http://www.eurekalert.org/pub_releases/2006-10/uorm-ebs101206.php

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