The immunisation, carried out under emergency powers, is a desperate bid to halt measles infection rates that have been inexorably rising since mid-October, with most of the victims young children.
Samoa entered a two-day lockdown on Thursday to carry out an unprecedented mass vaccination drive aimed at containing a devastating measles epidemic that has killed dozens of children in the Pacific island nation.
As the death toll climbed to 62, officials ordered all businesses and non-essential government services to close, shut down inter-island ferries and told people to keep their cars off the roads.
Residents were advised to obey a dawn-to-dusk curfew, staying in their homes and displaying a red flag if any occupants were not yet immunised.
Hundreds of vaccination teams, including public servants drafted in for the operation, fanned out across the nation of 200,000 in the early hours of the morning.
They plan to go door-to-door in villages and towns to administer mandatory vaccinations in red-flagged houses.
The markets on Apia's waterfront, usually packed with tourists buying handicrafts, were silent as stalls stood empty, while there was hardly any traffic in the city centre.
'Very, very quiet'
"It's very, very quiet out here. I can just hear a few barking dogs. The streets are empty. There are no cars," UNICEF's Pacific islands chief Sheldon Yett said.
"People are staying at home waiting for the vaccination campaign. The teams are getting their supplies together and getting ready to go out."
The operation, carried out under emergency powers invoked as the epidemic took hold last month, is a desperate bid to halt measles infection rates that have been inexorably rising since mid-October, with most of the victims young children.
"I've seen mass mobilisation campaigns before, but not over an entire country like this," Yett said.
"That's what we're doing right now. This entire country is being vaccinated."
'Incredibly irresponsible' material
Immunisation rates in Samoa dropped steeply to just 30 percent before the outbreak, the World Health Organization said, blaming an anti-vaccine messaging campaign.
Two babies died after receiving measles vaccination shots last year, which lead to the temporary suspension of the country's immunisation programme and dented parents' trust in the vaccine.
It was later found the deaths were caused when other medicines were incorrectly administered.
Rates of 90 percent are international best practice. Immunisation rates in Pacific island nations Tonga and Fiji are at around 90 percent, and their measles outbreaks have been far milder.
Yett said social media had been used to spread anti-vaccination misinformation in Samoa and the online giants running the platforms needed to clamp down on such "incredibly irresponsible" material.
"It's quite clear that they have a corporate responsibility to step up to the plate and make sure that populations, particularly vulnerable populations, get accurate information that's going to keep children alive," he said.
Samoa's immunisation r ate has risen to 55 percent over the past fortnight and Yett said this week's two-day drive aimed to push it above 90 percent, which should help curb the current outbreak and stop future epidemics.
'Vaccination the only cure'
Even Prime Minister Tuilaepa Sailele Malielegaoi's residence had a red flag fluttering outside it on Thursday, with the leader saying his nephew had recently arrived from Australia and needed a measles shot.
Malielegaoi said he was angered by anecdotal reports some anti-vaccination parents were encouraging their children to hide from the teams to avoid the mandatory injection.
"The only cure for this is vaccination... having your children vaccinated is the only way," he said.
Malielegaoi rejected accusations that his government had contributed to the crisis by not acting more decisively to address the long-term decline in immunisation rates.
"I think you're talking to the wrong people, we did it many, many times," he told reporters.
Children are the most vulnerable to measles, which typically causes a rash and fever but can also lead to brain damage and death.
The latest figures show that 54 of the 62 dead were aged four or less and infants account for most of the 4,217 cases recorded since the outbreak began in mid-October.
A further 19 children are critically ill in hospital.
Samoa has received aid to combat the crisis from Australia, New Zealand, France, Britain, China, Norway, Japan, American Samoa, the United States and the UN.
But efforts so far have failed to stop infection spreading, prompting this week's drastic escalation in the immunisation campaign.
Samoans can expect no immediate relief even if the national shutdown succeeds in boosting vaccination rates as the injection typically takes 10-14 days to become effective.
There have also been measles epidemics in neighbouring Fiji and Tonga, but higher immunisation rates mean they have been more easily contained, with no fatalities.