The debate has been especially contentious in Britain. Just two weeks ago government advisers said that the "minimal benefits" of vaccinating children under 18 didn't outweigh the potential risks. Yet on Wednesday, those same advisers said 16- and 17-year-olds should get jabbed as soon as possible. Though only one dose for now. And younger children should still wait. Unless they are between age 12 and 15 and medically vulnerable or living with immunosuppressed adults.
The new guidance represents a confusing 90-degree turn for British teens and their families. Even pro-vaccine parents would be forgiven if their heads were spinning with the ever-changing messaging from this government.
Britain's Joint Committee on Vaccination and Immunization said its revised recommendation reflected both the state of the virus in Britain and additional safety data.
"In the last few weeks, there have been large changes in the way COVID-19 has been spreading in the UK, particularly in younger age groups," the committee said in a statement on Wednesday.
It did not specify what about the spread had influenced the change in thinking, but Britain was hit early and hard by the highly contagious delta variant of the virus, and because so many older people are vaccinated, many of those getting infected are young.
The vaccination committee also cited data suggesting that one dose would provide young people with 80 percent protection against hospitalization. The committee had previously cited concern about serious side effects involving inflammation of the heart muscle or the membrane around the heart, but on Wednesday it offered further assurance that these were "extremely rare" and more of an issue after a second dose.
At a Downing Street news conference, government officials were repeatedly pressed on how much had changed in just two weeks.
"I think people are a bit confused about this changing advice," said one reporter.
Wei Shen Lim, chair of the vaccination committee, said the advisers now had a greater certainty of data that has influenced their recommendations.
"There is no time to waste in getting on with this," said Jonathan Van-Tam, England's deputy chief medical officer, noting that children will start returning to school within weeks. "I want us to proceed as fast as is practically possible."
But health policy experts who have been critical of the government's hesitancy to inoculate teenagers said officials have been wasting time while getting to even this halfway conclusion, undermining what is otherwise one of the world's most successful vaccination programs.
"The idea of allowing a pretty much uncharacterized virus tear through our children is utterly reckless and irresponsible, especially because it's a vaccine-preventable disease," said Stephen Griffin, a virologist at the University of Leeds.
"I'm very pleased that they are doing this U-turn," Griffin said. "I hope once the safety data is in they go even younger."
Peter Kyle, the opposition Labour Party's point person on schools, tweeted that the government reversal on vaccines for older teens "is too late to make a difference to education when terms starts next month. Government have squandered the opportunity summer offered."
Britain's resistance to jabbing the young has made it an outlier in the developed world, where most rich countries are trying to get a needle into the arms of those under 18 as quickly as possible.
The United States and Canada have been vaccinating 12- to 17-year-olds since May. France, Italy and other countries in Europe began in June. Germany was something of a holdout, but this week it decided to move forward on vaccinations for that age group.
Trials are underway to test the vaccines in even younger subjects. Last week, President Biden expressed optimism that children under 12 would become eligible for inoculations in the United States "soon."
In Britain, regulators authorized use of the Pfizer-BioNTech mRNA vaccine in June for those as young as 12. But when the experts on the government advisory panel delivered their assessment two weeks ago, they concluded: "Until more safety data is available and has been evaluated, a precautionary approach is preferred."
The government advisers have emphasized the benefits and risks for individual teens. With the new advice on Wednesday, Lim said: "While covid-19 is typically mild or asymptomatic in most young people, it can be very unpleasant for some and for this particular age group, we expect one dose of the vaccine to provide good protection against severe illness and hospitalization."
But other public health experts say it's not just a question of risk to the individual but risk for the whole society. Teens can readily become infected and spread the virus to other children and to adults, including those who cannot be vaccinated for medical reasons.
Some of the most comprehensive studies on the effects of the coronavirus on children and teens have come out of Britain, and those studies have shown it is very rare for children to get severe covid.
Within England, from the beginning of the pandemic through February of this year, studies calculated that there were about four million cases of covid identified in children, with fewer than 6,000 hospitalizations and 25 deaths.
Children can suffer from so-called long covid, but a review in the journal Nature reported estimates of how common it is in children "vary wildly."
A new study published Tuesday by researchers at King's College London followed 1,734 children, aged 5 to 17, who developed symptoms and tested positive for the coronavirus between September 2020 and February 2021, before the delta variant became dominant. The researchers report that one in 50 children with had symptoms that lasted for more than eight weeks. Two percent - of millions of cases - could be worrisome.
It is well-established that the delta variant of the virus is highly contagious, but researchers are not certain that the strain makes anyone - including young people - more ill.
Some front line health-care workers in the United States are reporting anecdotal evidence from their hospital wards that the delta variant makes for "younger, sicker, quicker" patients. But in Britain, the scientists are awaiting for more evidence.
In England, hospital admissions of those aged 15 to 24 years reached their highest rate since the peak in January 2021, but researchers note that the most likely to be unvaccinated today are the young.
A preliminary study by Aziz Sheikh and colleagues at Public Health Scotland suggested that the risk of hospitalization was doubled for those infected by the delta variant, when compared to the alpha strain, originally detected in England. But when the alpha version of the virus was rampaging, vaccines were just beginning to be deployed, so comparisons are tricky.
Some commentators in Britain have wondered aloud if the government's reluctance to inoculate younger people has been driven by concerns about vaccine supply. Health officials insist that supply has never been a limiting factor, though the government has declined to release numbers of available jabs.
Others have argued that the morally responsible move would be to focus on vulnerable people worldwide - not kids in Britain or America.
Andrew Pollard, a leader of the team that developed the Oxford-AstraZeneca vaccine, told lawmakers in May that "it feels completely wrong" to prioritize children in rich countries over older populations in poor countries that don't have access to vaccines.
"Children have near-to-zero risk of severe disease or death," Pollard said.
While it is true that children are less likely to become seriously ill or die, the risk is not zero.
"Kids are not invulnerable to covid, we have long covid to consider, and hospital admissions data show that between 6 and 8% of people in hospital in England are under 18," said Griffin, of the University of Leeds.
He added that this age group has a lot more social mixing and contacts, "so, in terms of achieving population immunity, you need to target them."
Vaccination of children under 16 in Britain requires parental approval, and a survey conducted in April and May by the U.K. Office of National Statistics found that almost 9 in 10 parents would definitely, or probably, vaccinate their children against the coronavirus if they could. In a more recent YouGov survey, 53% of parents with underage children said they would get them a shot if the vaccine was available, while 18% said they would not. Twenty-nine percent were unsure.
Educators are also keen to see students vaccinated before their return to school in the fall, to help reduce outbreaks and minimize disruption.
The National Education Union, the biggest teachers union in Britain, said the decision will "help to protect young people, their families and communities and minimise disruption to education next academic year. No one wants thousands of pupils to miss out on school."
Before the recent term ended, more than 1 million children in England were out of school for covid-19-related reasons in a single week in July, a record. This comes in the wake of schools being closed for months because of the pandemic.
Lawrence Young, a virologist and professor of molecular oncology at Warwick Medical School, said infection among youths is high - which translates into quarantine and missed school days and knock-on mental health impacts.
He warned that Britain may never reach population immunity or herd immunity if children are not vaccinated. People under 18 make up about 20 percent of the British population, and this pool of unvaccinated humans could keep the pandemic going on and on, increasing the risk of new variants. Some estimates suggest 85 percent of a population must be vaccinated or have had a previous infection to stop the pandemic, which would be hard to reach without including children and teens.
"The benefits of vaccination for the young strongly outweigh any risks," Young said.
Anthony Harnden is a professor of primary care at the University of Oxford and deputy chair of Joint Committee on Vaccination and Immunization. In a statement, Harden stressed that "the primary aim of the vaccination program has always been to prevent hospitalizations and deaths."
Meaning: the point of vaccines is not ending the pandemic, which many might argue with.
"The benefits of reducing transmission to the wider population from children are also highly uncertain, especially as vaccine uptake is very high in older people who are at highest risk from serious covid-19 infection," he said.