STAT Researchers believe immunity to an earlier virus may play a role, but not everyone agrees. The H5N1 bird flu virus has historically extracted a heavy toll when it infects humans, with nearly half of confirmed cases ending in death over the past three decades. But of the 70 cases reported in the United States over the past 18 months, only a single death occurred, leaving experts puzzled at how to explain the phenomenon.
A new study published Wednesday adds weight to an argument that the immunity people have developed to the virus that caused the most recent flu pandemic, an H1N1 virus that emerged in 2009, has induced some cross-protection that may be making it harder for H5N1 to infect people, and mitigating the severity of the ensuing disease when such infections occur.