Can Humans Develop Immunity to Bird Flu?

Feb. 14, 2007— -- Science does have a way of changing your mind.

Back in 2003, the idea that some people might have limited immunity from the deadly H5N1 bird flu virus seemed almost impossible. I had a chance to ask one of the leading experts, Dr. Richard Webby of St. Jude's Children's Research Hospital, about the theory, and at the time he had doubts.

"I admit I was skeptical," he said now. "It did not seem likely, and that's one reason it took us so long to do the experiments."

The theory was that exposure to a regular seasonal flu virus might offer some protection from the H5N1 bird flu.

Webby had good reason for his skepticism. It was unlikely that a distant virus would have enough in common with H5N1 to trigger any immunity.

In the H5N1 bird flu, it's the H5 part that is most foreign to humans. The N1 part is more common in typical human flu viruses.

So the question was, if a person had been infected or vaccinated with a nonbird-flu virus that had an N1 component, would that be enough to trigger his or her immune system to fight the H5N1 bird flu virus?

Webby and colleagues investigated this theory in a new study published this week in PLOS Medicine.

"We were surprised at what we found," Webby said in our most recent conversation.

In mice, exposure to an N1-containing virus reduced the death rate from H5N1 bird flu by half.

Preliminary blood work on humans also suggests that yearly flu shots with N1 may offer some weak protection against H5N1 bird flu.

The exact amount of protection a person would have is unknown. "It's not going to prevent infection," Webby said. "But it might reduce the more severe parts of the disease."

All this is good news for flu vaccines, especially those in development that are specific to H5N1. Most of the current vaccines are based on older versions of the bird flu virus that had been circulating in 2004.

Experts have wondered whether these vaccines would do much good against newer forms that aren't an exact genetic match.