The Marin County, California elementary school in the US had been conscientious about following Covid-19 protocols.

Masks were required indoors, desks were spaced six feet apart, and the students kept socially distant. But the Delta variant found an opening anyway.

On May 19, one teacher, who was not vaccinated against the coronavirus, began feeling fatigued and had some nasal congestion. She dismissed it as allergies and powered through.

While she was usually masked, she made an exception for story time so she could read to the class.

By the time she learned she was positive for the coronavirus two days later, half her class of 24 had been infected - nearly all of them in the two rows closest to her desk - and the outbreak had spread to other classes, siblings and parents, including some who were fully vaccinated.

"The mask was off only momentarily, not an entire day or hours.

“We want to make the point that this is not the teacher's fault - everyone lets their guard down - but the thing is Delta takes advantage of slippage from any kind of protective measures," Tracy Lam-Hine, an epidemiologist for the county, said in an interview.

The case study, published by the Centres for Disease Control and Prevention and highlighted by CDC director Rochelle Walensky during a briefing on Friday, highlights the potential danger for children under the age of 12 - the only group in the United States ineligible for coronavirus vaccines as a hyper-infectious variant tears across the country.

THE GROWING NUMBER OF INFECTED CHILDREN

Just this month in Brevard County, Florida, 1,623 children were infected and more than 8,000 students were quarantined.

And in the Atlanta area, thousands of positive cases were confirmed in schools with 23,000 students and staff have been quarantined. The situation has turned the nation's schools into ideological battlegrounds - with one angry parent ripping off a mask from a teacher's face in a Texas school this month, and parents both for and against masks filing lawsuits against their children's school districts.

Without concerted efforts to curb Delta's transmission, things are likely to get worse in coming months.

A simulation posted this month by a CDC-funded lab predicted that in elementary schools without either masks or regular testing, 75 per cent of children might be infected with the coronavirus in the first three months.

The Delta variant-fuelled surge has put new pressure on the Food and Drug Administration to authorise the vaccine for younger children as soon as possible.

It has thrown school reopening plans into disarray, with some officials scrambling to impose vaccines mandates for staff, as well as universal mask mandates. And it has frightened and bewildered many parents, unsure how to protect their kids.

"It's hard to put our heads around this," Julie Swann, an expert in mathematical modelling at North Carolina State University who leads the team that published the school transmission study and a mother to a 10-year-old. "As parents, we are having to wrestle with these really hard notions of expected risk."

Vaccines for children ages 5 to 11 had been widely expected to be available in the early fall, but to the surprise of many, federal regulators asked vaccine companies in late July to double the number of trial participants to include several thousand more children.

The FDA is seeking to better understand the vaccines' link to a rare but potentially serious inflammation of the heart muscle known as myocarditis and pericarditis that has predominantly affected younger males, and to learn whether it might affect younger children as well.

National Institutes of Health Director Francis Collins and vaccine makers have indicated that the expansion of the paediatric testing means a vaccine for younger children is unlikely before the end of the year, or perhaps even early 2022.

That forecast has spurred alarm among some public officials and health providers, with more than 180,000 new child Covid-19 cases confirmed in the week ending August 19 - an up to 20-fold increase over weeks in June when summer breaks began.

This week, Maryland Republican Gov. Larry Hogan called on regulators to expedite approval for 5- to 11-year-olds. "Getting our children vaccinated is critical to giving parents greater peace of mind, but we are being told approval is still months away," he said.

The American Academy of Family Physicians warned that "the risk for severe and long-lasting impacts on health outcomes in unvaccinated children is increasing."

And the American Academy of Pediatrics has urged the FDA to use a two-month follow-up time frame for safety studies rather than six months, which would "significantly hinder the ability to reduce the spread of the hyper infectious Covid-19 delta variant among this age group."

"In our view, the rise of the Delta variant changes the risk-benefit analysis for authorising vaccines in children," AAP President Lee Savio Beers wrote in a letter, urging the agency to make the shots available for younger children "as swiftly as possible."

The FDA said it could not comment on its discussions with manufacturers but stressed that it is working to "ensure the number of participants in clinical trials are of adequate size to evaluate a product's safety and effectiveness in the intended population."

HOW PARENTS FEAR A FOURTH WAVE

The fourth wave of the coronavirus is hitting children and families faster and harder than before, raising new questions for doctors and researchers.

In Southern states, paediatric ICUs are at or near capacity with record numbers of severely ill children. They include newborns just weeks or months old and previously healthy children - almost unheard of in previous waves - reinforcing the idea that this is a virus that can strike anyone.

"Is it that we have more cases overall and this is a more transmissible virus? Or is it something about Delta? It's too early to tell, and if anyone is making assumptions, they are not basing it on rigorous data, as there are not rigorous data," said Adrienne Randolph, a researcher at Boston Children's Hospital who is leading a nationwide study on Covid-19 in children. "However my colleagues in ICUs have reported many more severe cases."

Doctors are also speculating about anecdotal reports of unvaccinated young parents getting seriously ill, and what that might say about transmission in families.

In Arkansas just outside Little Rock, Tate Ezzi, 44, and his pregnant wife Christine, 39, parents to five young children, have been urging the vaccine-wary to re-evaluate their stance since both were hospitalised and she lost the pregnancy after attending a birthday party at a skating rink.

In Texas, Lydia Rodriguez, 42, died this month of Covid-19, two weeks after her husband Lawrence's death from the same disease, orphaning their four children. And in Florida, a 52-year-old mother died and the father was still in the hospital as of this week.

A recent technical paper out of Britain suggested the Delta variant does not cause more serious illness than its predecessors, but the analysis did not specifically break out children.

David Rubin, a researcher at Children's Hospital of Philadelphia who has been studying US hospitalisation data, said that in recent weeks, 1,200 to 1,400 children were inpatients at the peak, and while those numbers may be large, the rate of hospitalisation remains the same as in the past at 0.8 to 0.9 per cent.

"What you are seeing is many more kids are getting Covid now because our country is open, and they are being exposed," he said.

THE PERILS OF BEING UNMASKED

What is indisputable is that the virus can spread like wildfire in settings where children are unmasked and unvaccinated, such as schools and homes.

And there are fresh worries about the impact the initial dose of exposure may play in disease severity for parents and caregivers, who may be more vulnerable to severe illness.

One early paper, published in August 2020 in the Journal of Pediatrics, found that the viral load of some children in the first two days of symptoms could be higher than in severely ill adults, implying a high degree of potential infectiousness.

Another found that the virus was detectable for a mean of 6.7 days in infected children - whether or not they had symptoms. More recently, researchers found that people are testing positive for the Delta variant at a peak of 3.71 days after exposure, as compared with five to six days for previous variants - showing the illness can hit very quickly.

An important new study published in JAMA Pediatrics on August 16 found that infants and young children appeared to spread the coronavirus more aggressively than those in their early teens - likely because of how they and their caretakers interact.

Researchers note that babies cannot cover coughs or wash their hands themselves, and they require more touching. Preschoolers and early elementary children may be more likely to be in close contact with each other, whether it's whispering during circle time, holding hands in the hallways or wrestling at recess.

"Our interactions with young children are physically very different than [with] others, even in the same family," Randolph said. "You hold them and cuddle them, and they are usually not masked."

Swann, the North Carolina mum and scientist, set out to try to simulate what could happen with so many children mixing in one building breathing the same air.

As part of one of six CDC-funded simulation groups that are designed to help local school officials make decisions, she teamed up with Pinar Keskinocak, a systems engineer at the Georgia Institute of Technology and others to look at transmission over time.

Many of the assumptions they made were conservative based on the extent of spread in many parts of the United States today. They assumed that when school began, there were already a few kids and teachers with asymptomatic infections, that masking might drop the infections by 50 per cent, and that in elementary schools, most of the children were vulnerable to the virus.

Each week, they imported one new case, which they imagined might come from a sibling, or perhaps from a student who had been at soccer practice, church or with another community group.

They used a transmission rate of four - which means that each infected person would spread it to four others, a number that is lower than the six to seven some studies have estimated for the Delta variant, but which they felt was reasonable given that children are only in school for part of the day.

The models showed that more than 75 per cent of susceptible students - meaning those who were not vaccinated, or who had previous immunity due to natural infection - would become infected within three months.

With masking, the infection rate would decrease to 50 per cent for elementary schools, 35 per cent for middle schools and 24 per cent for high schools, based on average vaccination rates. Testing further drops infections to 22 per cent, 16 per cent, and 13 per cent.

"Parents in California are freaking out that my model shows that, even with masks, there would be a lot of infections," Swann said.

But she also emphasises that "we have an incomplete picture of what's happening," and different communities have widely different levels of susceptibility based on vaccination rates and levels of natural immunity. She also said she has had to remind herself that the relative risk of coronavirus complications in children is low.

WHY THE FDA IS BEING CAUTIOUS

With more than 80 million children and adolescents in the United States, public health officials and researchers believe the pandemic will not end until coronavirus vaccines are approved for all ages.

The FDA's rollout of the vaccines for adults occurred in record time, but there are additional steps involved for children, and for good reason. Younger children's immune systems tend to be more robust and vary greatly in size - think of a 5-year old versus a 15-year-old.

Medical historians point to cautionary tales about rushed approvals: In the 1960s, thousands of children in the United States who got a vaccine developed atypical measles, which resulted in lung inflammation that often sent them to the hospital.

That vaccine was later recalled. And several years ago in the Philippines, a school-based program for dengue fever had to be stopped after the drugmaker discovered it could lead to more severe illness in some children.

Clinical trials typically involve looking separately at children in various age groups, moving from oldest to youngest - ages 16 and up, 12 to 15, 5 to11, and under 5. Pfizer's coronavirus vaccine is currently available for those 12 and above, and the Moderna vaccine, for those 16 and older.

Yvonne Maldonado, a professor of paediatrics and population health at Stanford who heads the AAP committee on infectious diseases, said one big challenge for the 5-to-11-year-old group right now has to do with titrating down the dose. This isn't necessary for all vaccines, she said, but it is something that is being studied for the messenger-RNA shots.

The cardiac complication in some adolescents and young adults after receiving the second shot has been well publicised. But Maldonado, an investigator on the Pfizer vaccine trials for children, said the issue is so rare that adding a few thousand more children to the studies is unlikely to provide insight, and that you'd probably need to add millions to be able to identify those patients with the reaction.

She said researchers have not seen signals of other concerning side effects, and she and her colleagues were not informed the FDA's authorisation might be delayed - until they heard it from the media.

"If there had been a valid reason to slow down the authorisation, we want to understand that," she said. "But based on what we've seen and heard there's no specific other issue."

Julie Morita, a vaccine expert who was on the Biden transition team on Covid and a former member of the Advisory Council on Infection Protocols for the CDC, said that as the Delta variant has surged, the calculus for public health officials should change.

"If Delta wasn't making children sick and hospitalising them, it might make sense to take more time to look at the safety profile," said Morita, executive vice president for the Robert Wood Johnson Foundation. "But when you have a virus that is surging, expediency may become more important."

THE NEED FOR ONGOING VIGILANCE

The outbreak at the school in Marin County is the first Delta case in young children published by the CDC in the United States. Epidemiologist Lam-Hine remembers that as the cases mounted, he and his colleagues commented how "this strain is really different" - even before they had confirmed it was Delta.

Among the most puzzling aspects, he said, is how the virus jumped from the initial class to a second class three grades apart, where six children also tested positive.

The school, which has 205 students in pre-kindergarten through eighth grade, had taken multiple measures to combat viral spread. All the classrooms contained portable air filters, and doors and windows were kept open. The two rooms in question were separated by a large courtyard, which had been blocked by lunch tables with yellow tape on them.

The students in the two classes did not seem to share siblings, carpools, sports teams, or other extracurricular activities, he said. Yet sequencing showed their virus was genetically indistinguishable. Perhaps, Lam-Hine speculated, the kids had passed each other in the hallway, or had some other close contact.

The contact tracing team also found five additional people in the community who had infections with the same virus, but were unable to find a link with the school cases.

The findings included some good news: While more than 80 per cent of the infected children, or 22 out of 27, had symptoms such as fevers, coughs, headaches and sore throats, none was hospitalised. And the county saw no obvious further spread. Lam-Hine speculated the county's mask compliance and vaccination rate - the highest in the state - made the difference.

"This is not a story about a teacher and her class," he said. "It's about the need for all of us to be super vigilant."