The vaccination programme for smallpox was a medical success story which resulted in the World Health Organisation finally declaring the world free from smallpox in 1980.
Which statement correctly identifies the reasons for the success of the smallpox vaccination programme?
1 )
The virus also infected animals making it easy to stop the transmission cycle.
2 )
The virus remained in the body following infection and could become active later.
The virus was stable and not prone to mutations.
4 )
The virus was unstable and prone to mutations.
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