American Samoa has confirmed its second case of measles, a four-year-old boy, and now has 49 probable cases.

The boy, from Tafuna, goes to the same school as the eight-year-old girl who was the first confirmed case announced on 18 April in the US territory, three weeks after she was seen at a community centre with symptoms on March 27.

All 49 probable cases to date are in children aged from two months to 13 years.

Department of Health epidemiologist Scott Anesi said the latest confirmed case would "definitely" be labelled as community transmission.

"The signs and symptoms of the probable cases matching, the new case is linked to the original case as well," Anesi said.

"We know for sure that there is a linkage of transmission locally."

Anesi said the territory is forecasting "sporadic clusters" in highly populated areas and in people who are unvaccinated.

"Not specific surges, as long as we continue to keep up our vaccinations."

Anesi said the Department of Health would more accurately be able to model the outbreak when test results sent to Hawaii earlier this week returned.

Meanwhile in Samoa, Director General of Health Aiono Dr Alec Ekeroma said there are no probable cases in the country, but it was "only a matter of time".

"I'm hoping really that it will be delayed by a few more weeks to get our vaccination rates even higher than what it is now."

Aiono said closing the border between Samoa and American Samoa was "too harsh", but travellers from American Samoa would require proof of their measles, mumps and rubella (MMR) vaccination to enter the country, and would also be subject to screening tests.