A WHO spokeswoman said the coronial inquest is due to start today and would examine all available evidence - including the autopsy and lab test results.
Autopsy examinations were conducted by medical staff from Australia about a week after the toddlers died.
"WHO is not privy to the result of these tests. They are sent directly by the forensic pathology and lab teams to the Government of Samoa,'' she said.
"WHO has been providing technical support to the investigation.''
That support included sending an immunisation expert from the organisation's Manila office, identifying independent experts and institutions to carry out the children's autopsies as well as laboratory tests.
Members of the organisation based in Samoa continue to work closely with Ministry of Health authorities there, who have requested assistance.
"Our immunisation expert from Manila returned to Samoa to provide further support to the Government's investigation this week.''
Unicef staff arrived in Samoa shortly after news of the vaccine-related deaths broke.
The world's largest children's charity works with governments in 14 Pacific Island countries to help ensure children receive vaccines to protect against preventable illnesses.
Unicef representative to the Pacific, Sheldon Yett, said: "Obviously, when we have a tragic event like this, it's horrible for everyone involved and we want to make sure that any questions they had - we could answer them for them.
"An event like this is quite traumatic for everybody. Obviously it's a tragedy for the families involved, it's traumatic for anybody involved in the health sector and it does raise questions."
Samoan Police are carrying out its own investigation after the families of Lannacallystah and Lameko made formal complaints.
The nurses who administered the vaccinations have been removed from the hospital for their own safety. No charges have been laid at this stage.
LOCALS AFRAID, BUT AUTHORITIES STRESS THE VACCINE IS SAFE:
Yett told the Herald there was fear among locals. But authorities are pushing the message that MMR vaccinations are safe and that the deaths were very rare.
"The MMR vaccine has been used around the world since the mid-70s. It's one of the safest vaccines in existence," he said.
"We know that the dangers of not vaccinating children are much, much, much greater than that of dangers of vaccination.
"It's something that is extremely rare - all over the world. People in the Pacific are no different from people anywhere else in the world.
"Without vaccinations, some 20 million children would have died. So we know that it's an extremely effective way of keeping people alive and ensuring children can contribute and live productive lives."
