Samoa's Government has announced a compulsory mass vaccination campaign, making receiving immunisations a mandatory legal requirement.

Key points: Low immunisation coverage has been attributed as a major factor

Low immunisation coverage has been attributed as a major factor Local media has reported more than 700 suspected cases of measles

Local media has reported more than 700 suspected cases of measles Other neighbouring nations such as Fiji and Tonga are also affected

The move comes amid a deadly measles outbreak in the nation.

Officials said there has been 15 confirmed deaths as a result of the measles crisis, with most of the victims children under the age of two.

Last Friday, the Samoan Government declared a 30-day state of emergency in response to the crisis. All schools were ordered to close, and restrictions were placed on the movement of vulnerable people in order to curb the spread of the disease.

But experts said the outbreak was entirely predictable given the country's low vaccination rate, as well as a range of other factors that helped trigger the emergency.

Here's what you need to know.

How did the outbreak start?

The outbreak is believed to have started in New Zealand, where there have been 2,060 confirmed cases of measles just this year, before spreading to Samoa and other nations across the region — including Australia.

It's believed the New Zealand outbreak, which is largely centred on the Auckland region, may have started due to historically low immunisation rates, creating gaps in vaccine coverage.

In Samoa, approximately two-thirds of the population has been immunised in the island state with a population of just 200,000.

The number of measles cases across the Pacific, as of November 18, 2019. ( ABC News: GFX/Jarrod Fankhauser )

That low immunisation coverage rate has been attributed as the main reason for the deaths seen in Samoa, according to the country's Ministry of Health.

Other nations like Fiji, Tonga and American Samoa, which have higher coverage, have not reported any measles-related deaths despite being in a similar "measles epidemic mode", the ministry said.

However other factors contributed to low vaccination rates in more recent times.

Last year, two Samoan nurses incorrectly mixed and administered a routine mumps, measles and rubella (MMR) vaccination, leading to two infant deaths.

In the immediate aftermath of the deaths, Samoa's Government recalled all MMR vaccines, and placed a temporary ban on administering them.

Experts believe the incident may have further deterred parents in the country from giving their children the injections.

Just how bad is it and what's being done?

The measles outbreak is believed to have started in New Zealand. ( Supplied: WA Department of Health )

Samoan media have reported more than 700 suspected cases of measles, with 48 of those confirmed as measles cases.

Measles is a highly infectious airborne disease that spreads easily through breathing, coughing and sneezing. It typically causes a rash, fever and white spots in the mouth.

More serious complications include blindness, pneumonia, brain damage and severe dehydration.

The Samoan Government has ordered children under the age of 17 to not attend public gatherings, in an attempt to stop the virus from spreading.

"The way it is going now and the poor [immunisation] coverage, we are anticipating the worst to come," Samoa's director-general of Health, Leausa Take Naseri, told a press conference this week.

Meanwhile, officials have also moved to make receiving vaccinations a legal requirement, as part of a mass immunisation campaign.

Children aged six months to 19 years old are to be prioritised, as well as non-pregnant women between 20 and 35.

Which other countries are affected?

In Tonga, all government primary schools and kindergartens have been shut until at least November 25. ( UNICEF Pacific )

Measles cases are rising globally, including in wealthy nations such as the United States and Germany, where some parents shun vaccines for philosophical or religious reasons, or concerns — debunked by medical science — that the vaccine can cause autism.

Western Australia is currently going through one of its worst measles outbreaks in years, which, like the outbreak in Samoa, is also believed to have been triggered by a New Zealand traveller.

Queensland has also recorded 24 cases on Brisbane's southside, Logan and Redland Coast areas.

In Tonga, all government primary schools and kindergartens have been shut until at least November 25, in an effort to limit the spread of measles sweeping through the South Pacific.

American Samoa, which lies to the southeast of Samoa, has also announced a public health emergency over a measles outbreak.

Governor Lolo Matalasi Moliga said everyone travelling from Tonga and Samoa to the US territory must provide proof of measles immunisation as a condition of entry.

Samoa Airways chief executive Tupuivao Seiuli said it was vital all visitors travelling to American Samoa through Samoa comply with the new conditions, in order to reduce further disruption to their travel plans and assist neighbouring countries.

What's next?

UNICEF said the number of deaths and cases "keep climbing". ( AP: Seth Wenig, file )

Australia has sent a team of 34 doctors and nurses to provide assistance to clinical staff at Samoa's main hospital in the capital Apia.

They've also established a temporary eight-bed emergency intensive care unit, and a negative-pressure facility to treat and isolate patients suffering from the worst case of the outbreak.

New Zealand's Foreign Minister has also pledged to send more than 3,000 vaccines and 30 nurses to Samoa to assist.

UNICEF's Sheldon Yett told the ABC the UN agency had already provided more than 110,000 doses of vaccine, and was ready to send additional doses if required.

"Unfortunately the number of deaths keep climbing and the number of cases keep climbing," he said.

"I'm confident that with all parties working together, we'll soon get this under control, but right now we do have an emergency situation."