The United States on Friday authorized the Pfizer Covid vaccine for children aged five-to-11, paving the way for 28 million young Americans to soon get immunized.

The decision came after a high-level medical panel advising the government this week endorsed the shots, ruling that the known benefits outweighed the risks of side-effects.

The United States follows only a handful of other countries including China, Chile, Cuba and the United Arab Emirates that are inoculating younger children with various vaccines.

"As a mother and a physician, I know that parents, caregivers, school staff, and children have been waiting for today's authorization," said acting Food and Drug Administration chief Janet Woodcock in a statement.

"Vaccinating younger children against Covid-19 will bring us closer to returning to a sense of normalcy."

The vaccine rollout should begin in earnest after the Centers for Disease Control and Prevention convenes a panel on Tuesday to further discuss clinical recommendations.

Pfizer and its partner BioNTech announced this week the US government had bought 50 million more doses as it works to protect children, including eventually those under five.

In a clinical trial involving more than 2,000 participants was found to be more than 90 percent effective at preventing symptomatic disease.

The vaccine's safety was also studied in more than 3,000 children, and no serious side effects have been detected in the ongoing study.

In this age group, the vaccine is given as two shots three weeks apart, dosed at 10 micrograms -- a third what is given to older age groups.

Severe Covid is rarer in children than adults, but far from non-existent.

According to the CDC, there have been 8,300 Covid hospitalizations of children aged five-to-11 since the start of the pandemic, and 146 deaths.

There have also been more than 5,000 pediatric cases of multisystem inflammatory syndrome in children (MIS-C), a rare but highly serious post-viral complication, including 46 deaths.

- Ongoing safety monitoring - 

Health authorities will continue to monitor for potentially highly rare side-effects, such as myocarditis and pericarditis (heart inflammation and inflammation around the heart).

The clinical trials were too small to detect these, but the hypothesis is they will be exceedingly rare, because the effect is thought linked to testosterone levels.

In male teens and young adults, the most affected group the effects occur mostly after the second dose of an mRNA vaccine at a rate of a few dozens per million. Most of the cases have fully resolved.

Covid itself can cause more severe forms of myocarditis, potentially more frequently, depending on the level of transmission within a community.

Beyond protecting childrens' own health, epidemiologists think vaccinating this group will help bring an end to disruptions to school and other activities.

But most of the panelists at a meeting of experts called by the FDA on Monday said they would not support mandates in this age group. 

Instead, the decision whether to get vaccinated should depend on factors such as the child's risk factors and should be left to families, they said.

The United States is emerging from its latest coronavirus wave, driven by the Delta-variant. But cases remain high, particularly in colder northern states that are lagging in their vaccination rate. 

Almost 58 percent of the total population is now fully vaccinated.

Tonga faces lockdown after first Covid case detected

Tongans flocked to vaccination centres Saturday after the government warned the main island Tongatapu might be plunged into lockdown next week after recording its first case of Covid-19.

The infected person was among 215 people on a repatriation flight from the New Zealand city of Christchurch.

A routine test on arrival Thursday, while in compulsory managed isolation, returned a positive result the following day. 

Prime Minister Pohiva Tuionetoa warned islanders Saturday to prepare for the possibility of a lockdown if more cases emerge but said there was no need for immediate action as it could take "more than three days" before a person with the virus becomes contagious.

"We should use this time to get ready in case more people are confirmed they have the virus," he said.

The tiny Pacific kingdom, about 1,800 kilometres (1,100 miles) northeast of New Zealand, had been among only a handful of countries to escape the virus that has affected billions worldwide and claimed nearly five million lives.

Only about a third of Tonga's population of 106,000, most of whom live on Tongatapu, have been double vaccinated against Covid-19. 

But news that the first case had been detected in managed isolation prompted thousands to rush to get vaccinated.

"We've been to the community several times but only a few come to the site but yesterday and today was really good, really full, very best," Tonga's national immunisation coordinator Afu Tei said.

"Almost 2,000 last night and today you can see the turnout is very good."

Health officials confirmed the infected Tongan had been double vaccinated, having received the second jab in mid-October.

Tonga health ministry chief executive Siale Akau'ola said that although it would be another two weeks before the second vaccination was fully effective, "we believe the person would not get seriously ill and reach a dangerous level."

All health workers, police and airport staff who were on duty when the flight arrived have been put into quarantine as a precaution, Akau'ola said. 

New Zealand's health ministry confirmed the infected person had tested negative before the flight left Christchurch, where there are only four known cases of Covid, all of them in the same household.

The repatriation flight included members of Tonga's Olympic team who had been stranded in Christchurch since the Tokyo Games.

The athletes were double vaccinated before they departed for the Olympics.