Vaccines were developed to provide immunity to a viral infection. which of the following is a likely hypothesis of why the smallpox vaccine is no longer given?
smallpox only infects people who have not been vaccinated for chicken pox
the smallpox vaccine was given for a long period of time and the disease no longer exists
smallpox only exists in some parts of the world so there is no need to vaccinate everyone
the smallpox virus has mutated and so the vaccine is no longer effective