The island state of just 200,000, located south of the equator and half way between Hawaii and New Zealand, declared a measles epidemic late in October after the first deaths.
The government has identified 1,644 suspected cases of measles, more than doubling over the past week, with deaths rising to 20 from six, it said on Friday. Children younger than five accounted for all but one of the deaths.
Staff of U.N. agency UNICEF have delivered 110,500 doses of measles containing vaccines this week as they fanned out across Samoa to boost its mandatory immunisation effort, the agency said in a statement.
It has also worked with governments of other tiny Pacific nations to run immunisation campaigns and develop preparedness plans to combat a regional outbreak, it added.
Australia is sending a specialist team of doctors, nurses and public health experts to Samoa, along with equipment and supplies, the foreign ministry said in a statement.