A student was asked to explain why vaccination has successfully eradicated smallpox but not other diseases, such as measles, malaria and sickle cell anaemia. These are the statements made by the student.

1) The antigens of the virus causing smallpox did not change, unlike the antigens of the virus causing measles.
2) Sickle cell anaemia has many different types of mutation, unlike smallpox, so each would require a vaccine.
3) Unlike smallpox, malaria involves animals as part of the transmission cycle and this makes the cycle harder to break.
4) The vaccine against measles often causes a poor primary immune response so that booster vaccines are required, unlike the vaccine against smallpox.

Which statements are possible explanations?