Why has this egg method persisted for six decades? The main reason is that its reliable. But even though the eggs are reliable, they have serious drawbacks. One is the long lead time needed to order the eggs. That means its hard to make more vaccine in a hurry, in case of a shortage or unexpected outbreak. And eggs may simply be too cumbersome to keep up with the hundreds of millions of doses required to handle the demand for flu vaccine.
Whats more, some flu strains dont grow well in eggs. Last year, scientists were unable to include the Fujian strain in the vaccine formulation. It was a relatively new strain, and manufacturers simply couldnt find a quick way to adapt it so that it grew well in eggs. We knew the strain was out there, recalls Theodore Eickhoff of the University of Colorado Health Sciences Center, but public-health officials were left without a vaccineand, consequently, a more severe flu season.
Worse, the viruses that pose the greatest threat might be hardest to grow in eggs. Thats because global pandemics like the one that killed over 50 million people between 1918 and 1920 are thought to occur when a bird influenza changes in a way that lets it cross the species barrier and infect humans. Since humans havent encountered the new virus before, they have little protective immunity. The deadly bird flu circulating in Asia in 1997 and 1998, for example, worried public-health officials because it spread to some people who handled birds and killed themalthough the bug never circulated among humans. But when scientists tried to make vaccine the old-fashioned way, the bird flu quickly killed the eggs.
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