The two one-year-olds died after receiving the jab against measles, mumps and rubella at Safotu District Hospital on the island of Savai'i.
Samoa uses an MMR vaccine assessed and approved by the World Health Organization and distributed by the UN children's agency UNICEF.
New Zealand's Immunisation Advisory Centre said while the vaccine was extremely safe, medical error or contamination were the two most likely causes of the tragedy.
