Indeed, only four companies today make vaccines, compared to 26 companies 50 years ago. These firms know that the return on their investment will be relatively low, owing to the long lead-in time that results from slow manufacturing processes (though new, faster methods offer some hope).
但另一个原因是制药企业对生产疫苗越来越失去兴趣。事实上，今天生产疫苗的企业只有4家，而50年前有26家疫苗生产企业。这些企业知道生产疫苗的投资回报率较低，因为缓慢的制造过程（尽管新的加速方法让人们有了些许希望）造就了漫长的导入期。