[video]Top vaccine makers Merck and Pfizer target China and older adults next

During the past few decades, the number of companies producing vaccines has dwindled to just a handful. There are other players in the space, but today, the four big drug companies that now dominate the market are Merck MRK , Pfizer PFE , Sanofi Pasteur SAN-FR and GlaxoSmithKline GSK-GB .