Why are pharmaceutical companies gradually abandoning vaccines?

During the past fifty years, the number of pharmaceutical companies making vaccines has decreased dramatically, and those that still make vaccines have reduced resources to make new ones. Pharmaceutical companies are gradually abandoning vaccines because the research, development, testing, and manufacture of vaccines are expensive and because the market to sell vaccines is much smaller than the market for other drug products. Congressional action could assure both a steady supply of existing vaccines and the promise of vaccines for the future.