Wild birds and poultry flocks alike continue to drop dead from the highly pathogenic bird flu that began spreading globally in 2020. Almost 59 million commercial birds have already been culled in the United States. It’s the broadest outbreak of this type of avian flu, known as H5N1, since it was first identified in China in 1996. The virus's proliferation and high fatality rate have prompted questions about two types of possible vaccines: those for birds and those for humans. H5N1 kills almost all the birds it infects; among reported cases in people since 2003, the death rate has been...