Pre-protection against pandemic flu possible, study suggests

One or two shots?

It has been believed that earlier versions of a flu strain wouldn't offer much protection against a later strain, because flu viruses mutate so rapidly. Because that immunologic target is constantly evolving, flu shots are given annually, not once or twice in a lifetime like vaccines against measles or mumps.

With that in mind, countries that hope to vaccinate during a pandemic anticipate that each person would need two shots of vaccine made from the pandemic strain.

Since it will take at least six months to make the vaccine once a pandemic starts, weeks more to administer two doses and more time still for the body to respond, the effect of a pandemic vaccine program would only be expected to start kicking in eight or nine months into a pandemic.

But the study, done by researchers at the University of Rochester in New York, gives weight to the idea that a different, more rapid approach might be possible. The findings were to be presented Friday at the annual meeting of the Infectious Diseases Society of America in Toronto.

The researchers tracked down people who had volunteered to take part in a vaccine trial in 1998. At that time, subjects received two doses of a vaccine containing hemagglutinin (or H) particles from a virus isolated during H5N1's first known foray into humans, a 1997 outbreak in Hong Kong that sickened 18 people, killing six of them.

Of the original participants, the researchers found 37 who were willing to come back to receive a booster shot of H5N1 vaccine made from a virus isolated in 2004, said Dr. John Treanor, senior author on the work and head of the university's vaccine trials unit.

Treanor didn't think the booster would work. All studies to date had shown H5N1 vaccine doesn't induce a strong immune response. So he decided to give the returnees a high dose — 90 micrograms of vaccine, or six times the amount given in an annual flu shot.

Then he and his team compared the levels of antibodies in their blood to those produced by people who had received only one 90 mcg dose of the vaccine.

Dramatic response

The primed participants "had a very, very dramatic response to that single dose. Dramatically different than we're seeing in the unprimed subjects," Treanor said in an interview.

"It looks like the Hong Kong vaccination in 1998 primed them, primed their immune system to respond to a single dose … seven years later."

It should be noted, though, that in total the 37 people received three doses — two in the original study plus the recent booster.

Treanor acknowledged that there is no way of knowing from this work whether a regime of one priming dose followed by a booster some years later would be as successful, or whether two priming doses would be needed.

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