单选题
Modern technology has put men on the moon and deciphered the human genome. But when it comes to brewing up flu to make vaccines, science still turns to the incredible edible egg. Ever since the 1940s, vaccine makers have grown large batches of virus inside chicken eggs. New cell-based technologies are in the pipeline, and may finally get the support they need now that the United States is faced with a critical shortage of flu vaccine. Although experts disagree on whether new ways of producing
vaccine (疫苗) could have prevented a shortage like the one happening today, there is no doubt that the existing system has serious flaws.
Each year, vaccine manufacturers place advance orders for millions of specially grown chicken eggs. Meanwhile, Public Health officials monitor circulating strains of flu, and each March they recommend three strains—two influenza A strains and one B strain—for manufacturers to include in vaccines. In the late spring and summer, automated machines inject virus into eggs and later suck out the influenza-rich goop. Virus from the eggs' innards gets killed and processed to remove egg proteins and other contaminants before being packaged into vials for fall shipment.
Why has this egg method persisted for six decades? The main reason is that it's 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 it's hard to make more vaccine in a hurry, in case of a shortage or unexpected outbreak. And eggs may simply be too
cumbersome (大量的) to keep up with the hundreds of millions of doses required to handle the demand for flu vaccine.
What's more, some flu strains don't 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 couldn't ind 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 vaccine—and, consequently, a more severe flu season.'
Worse, the viruses that pose the greatest threat might be hardest to grow in eggs. That's 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 haven't 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 them—although the bug never circulated among humans. But when scientists tried to make vaccine the old-fashioned way, the bird flu quickly killed the eggs.
(选自Time)