With China scrambling to keep a lid on a budding H7N9 bird flu outbreak, authorities in Shanghai said they will close all its live poultry markets beginning tomorrow for the indefinite future, reports CNN. Some 20,000 birds have already been slaughtered at a market where traces of the bird flu were discovered in pigeons. The H7N9 strain has now been found in 14 people, killing six, all near the coast of eastern China; there still are no confirmed cases of human-to-human transmission. Chinese officials say they have discovered genetic similarities between strains found in the infected pigeons and people. In the meantime, international health authorities are prepping for a fight. The CDC has begun to make a seed vaccine against this strain of bird flu, with one virologist admitting the agency is “fairly worried” about H7N9, reports the New York Times. Using genetic information China has posted online publicly, CDC researchers are building a vaccine using synthetic DNA to speed development, and will soon begin testing on ferrets with hope of having a vaccine ready within four weeks. “But some things, like ferrets, you can’t speed up,” says a CDC official.