Sir: Every year we hear of, read about and witness thousands of people dying as a result of dengue fever in many countries 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 for these and other countries. In the past, scientists have been successful in inventing different vaccines for controlling the spread of many diseases such as hepatitis A and B, influenza, measles, mumps, polio, rabies, smallpox, tetanus, typhoid, varicella (chickenpox), etc. The lives of millions of people are secured annually throughout the world against the above-mentioned diseases by simply injecting vaccines to newly born babies and people of a certain age. Why, then, have our current scientists failed so far in inventing the vaccine needed to combat dengue fever? We can prevent the spread of dengue virus equally in all affected countries if the people concerned were able to invent a vaccine for the same. ABDUL SAMAD SAMO Karachi