Children and young people are vulnerable to the new coronavirus, but the Covid-19 disease appears to be less severe in children than in some older adults, and scientists aren’t sure why.

This age disparity of severe cases has become a critical question as scientists race to slow the virus’s spread. Some clues to this mystery are starting to emerge, and the answers could help establish what kinds of infection control measures are actually effective, as well as point the way toward treatments.

“We do know that children tend to have more mild infection, have more mild disease, but we have seen [at least one child] die from this infection,” Maria Van Kerkhove, the Covid-19 technical lead at the World Health Organization, said in a press conference on March 16. “We can’t say universally that it’s mild in children, so it’s important that we protect children as a vulnerable population.”

In the United States, doctors say they have seen milder symptoms — like a fever and a dry cough — in children among those that have sought treatment than the adults who have.

“At our hospital, Seattle Children’s, we’re a little bit further along in the pandemic; we are not seeing serious disease in children,” said Janet Englund, a professor of pediatric infectious disease at the University of Washington Seattle and a physician at Seattle Children’s Hospital. “For us, it’s relatively indistinguishable from flu, but we can’t tell the difference unless we test, which we can do here.”

Right now, the data on Covid-19’s effects on the young are sparse, but recent studies show that even with children, some groups are more vulnerable than others depending on age and health conditions. Some children with Covid-19 can still experience serious lung disease. Kids with simultaneous respiratory infections can be more vulnerable to Covid-19. Infants without mature immune systems can get sick from the virus. Yet compared to adults, Covid-19 appears to be less severe in most kids.

“I think it is clear that children are not as seriously impacted as older adults,” Englund added.

The twist is that even if they don’t get very sick, young people infected with the virus can transmit it. And the particular concern is that they spread it to someone who might be immunocompromised or otherwise at higher risk. That means epidemic control measures — from proper hand-washing to social distancing — are just as important for the young and healthy as they are for people who are older or sicker.

What we know about the age disparity in severe cases of Covid-19

Countries that have experienced more extensive outbreaks of Covid-19, including China and Italy, have seen the worst impacts of Covid-19 on older adults, with the number of people hospitalized and killed by the disease rising with age. Underlying health problems like high blood pressure, a depressed immune system, and diabetes have also worsened the outcomes among the infected.

However, a small percentage of younger people, from babies to young adults, have also suffered serious harm.

A study published in February in the Journal of the American Medical Association of more than 72,000 infected people in China showed that just 2 percent of the infected in the sample were under the age of 19. Similarly, a February report from the WHO’s mission to China showed that only 2.4 percent of the infected were 18 years old or younger. Out of those infected young people, 2.5 percent developed severe disease and 0.2 percent developed critical disease.

But the WHO report also noted that “it is not possible to determine the extent of infection among children, what role children play in transmission, whether children are less susceptible or if they present differently clinically (i.e. generally milder presentations).” Which is to say few children are being tested for the virus, so there still isn’t much good information about how many children are getting infected overall. And from there, it’s hard to gauge the rate of severe illness for the young.

Another element is that Covid-19 seems to play out differently in children compared to what health officials have seen with other infections, such as influenza.

Influenza, also caused by a virus that infects the airways, can be a very serious illness in children. Complications from influenza, like a subsequent bacterial pneumonia infection, can be fatal. According to the Centers for Disease Control and Prevention, since September, 144 children have died in the US from influenza in the current season as of March 7. Meanwhile, there have been no reported deaths from Covid-19 among children in the US so far.

“Everything we know about other viral infections would suggest that children are at higher risk” of infection, said Bria Coates, an assistant professor of pediatrics at Northwestern University and an attending physician at the Ann & Robert H. Lurie Children’s Hospital of Chicago. “They don’t tend to wash their hands very well. They don’t give each other space.” Which makes the lower prevalence of severe illness for Covid-19 among children all the more remarkable.

Why the disease varies so much among age groups, especially compared to influenza, remains a mystery. “The reason why children are less affected than adults is potentially a very interesting question,” said Steven Zeichner, a professor of pediatrics at the University of Virginia School of Medicine who studies infectious disease. “I don’t think anybody knows the answer.”

The overall risk to children is low, but infants can experience more severe illness from Covid-19 compared to older children

While children seem less likely to experience severe symptoms of Covid-19 than adults, the risk isn’t zero. A study published March 16 in the journal Pediatrics of more than 2,100 children in China found that children of all ages were vulnerable to Covid-19, though the vast majority experienced mild symptoms, and some experienced none at all. A caveat for this study is that only one-third of the children in the sample were tested and confirmed to have the Covid-19 virus, SARS-CoV-2. The rest were presumptive Covid-19 cases, which means there’s a possibility that another pathogen could have caused the observed symptoms.

Zeichner, who co-authored a commentary article about the findings, noted that the worst outcomes in children were often among infants. The study showed that about 30 percent of childhood Covid-19 cases deemed “severe” and more than half of Covid-19 cases deemed “critical” were among children less than 1 year old. Though the overall numbers were small — 7 infants had critical illness and 33 suffered severe illness — it did show that younger children faced a higher likelihood of more dangerous outcomes.

That higher risk for babies may be because babies are still building up their immune systems. When a baby is born, it retains some infection resistance in the form of antibodies from its mother. That protection wanes over the first few months of life as the baby builds up its own defenses.

“The immune systems in children are not weaker than adults,” Coates said. “Instead, they are untrained.”

However, for a new virus like SARS-CoV-2, there isn’t immunity a mother can pass on, since the mother likely would not have had a chance to be infected with the new virus. There’s also no Covid-19 vaccine right now that can coach a child’s immune system to fight the virus.

At the same time, babies are being exposed to all sorts of environmental stimuli for the first time — bacteria, pollen, dust. To prevent their bodies from overreacting to otherwise harmless things, their immune reactions are tamped down. “In general, infants’ immune systems err on the side of a decreased response to infections,” Coates said.

The results is that infants, particularly in their first year of life, are more vulnerable to dangerous complications from infections. But during the first year, the baby’s immune system matures, becoming more effective at fighting off disease as it learns to identify threats.

Similarly, children who are immunocompromised or have other cardiac, metabolic, or respiratory problems are also at higher risk of complications from Covid-19, just as they would be from other infections.

On the other hand, there may be factors that reduce risks for older children. One hypothesis is that children face more frequent exposure to viruses that are related to SARS-CoV-2 but are much less harmful, so they have some cross-reacting immunity that helps them fight it.

“SARS-CoV-2 is a coronavirus, but it’s far from the only coronavirus that’s out there, and a substantial fraction of the common cold [cases] are caused by other coronaviruses,” Zeichner said. Children may be exposed to these viruses more frequently than adults at school and in playgroups, which may be helping their immune systems detect and fight off the new virus.

Another hypothesis is that the immune system in children is less likely to overreact to an invader, according to Coates. Many of the older people who were hospitalized by Covid-19 suffered from severe inflammation and fever, immune responses that ended up causing more damage than the virus itself. With a lower risk of severe immune response, children experience milder symptoms.

Right now these ideas are just hypotheses with scant evidence to back them up. But figuring out why Covid-19 is playing out this way could yield dividends for everyone. “If you could find out why children are protected, you could use that information to devise a strategy to protect adults,” Coates said.

What is the best strategy to protect children from Covid-19?

Preventing infections is the best way to protect everyone, regardless of age. Hand-washing, for instance, remains an important tactic for young and old alike to control the spread of the coronavirus.

But across the country, schools, colleges, offices, and sporting events have all been shut down to limit the spread of the infection. Canceling gatherings is a key component of social distancing, the practice of limiting exposure to other people to control the spread of the virus. Many schools have no scheduled date to resume.

However, school closures come with trade-offs, including for public health. Schools are also the main source of food for many students, and the supervision they provide allows the parents of children to go to work.

Some researchers also question the effectiveness of such measures in protecting children. “From the evidence that we are seeing, we are not seeing transmission in settings like schools where we would worry about amplification of transmission,” said the WHO’s Van Kerkhove.

As Vox’s Anna North has reported, inconsistent guidelines for school closures are also undermining their effectiveness:

The CDC has said that school closures of under four weeks likely will not have a meaningful effect on the spread of the virus. In fact, the CDC is somewhat equivocal on whether school closures are effective at all, stating that closures of 8 to 20 weeks may have some effect, but that other interventions like hand-washing and isolation are more effective, according to the Washington Post.

However, other experts argue that now is exactly the time to pursue such drastic measures. “I think we’re at the very beginning of [a potentially] exponentially increasing problem, and if you want to get ahead of an exponentially increasing problem, you have to intervene at the beginning,” Zeichner said.

Coates agreed. “I think that closing schools is a very effective way of stopping transmission of infectious diseases,” she said.

Experts are also divided on continuing smaller gatherings of children like play dates amid the outbreak.

Zeichner also emphasized keeping kids current with their vaccinations and making sure they get regular flu shots. While the influenza vaccine can’t prevent Covid-19, it can prevent influenza, which in turn reduces the stress on the health care system, and on families.

And if you suspect your child may be infected with SARS-CoV-2, the best thing to do is to call your doctor for advice. Seattle Children’s Englund said that parents should seek medical attention if they notice their children are having difficulty breathing — wheezing, persistent coughing, breathlessness, and difficulty talking. “If the child is having trouble breathing, they certainly need to be seen urgently,” she said.

But ultimately, some degree of exposure is inevitable, among children and between children and adults. “It’s hard to conduct a program of social distancing with your toddler,” Zeichner said.