Every year we hear, read and see thousands of people dying as a result of dengue fever in many countries of the world such as Brazil, Cuba, Indonesia, Colombia, Mexico, Honduras, China, Malaysia, Vietnam, Myanmar, Nicaragua, Paraguay, Venezuela, Sri Lanka, Singapore, Philippines, Pakistan, India, Thailand, etc. This dengue virus has now become a permanent threat to these countries. In the past, different scientists have been successful in inventing different vaccines for controlling the spread of many diseases. Why, then, our scientists have failed so far in inventing the vaccine for the dengue fever as well? ABDUL SAMAD SAMO, Karachi, July 5.