Children typically are “superspreaders” of respiratory germs, which makes the fact that they don’t seem to be major transmitters of the coronavirus that causes COVID-19 puzzling. They’re relatively absent among hospitalized patients, which initially was thought to be because they’re less likely to become seriously ill once infected. Later studies indicate that those of primary school age, at least, may be less likely to catch the virus in the first place. With schools and universities in the Northern Hemisphere considering reopening, scientists and public health authorities are trying to determine the role of young people in spreading the pathogen and how best to mitigate that threat.

1. To what extent are kids getting infected?

Estimates by midyear indicated that only 2% to 5% of individuals with laboratory-confirmed COVID-19 were under 18 years old. One survey in the U.S. found infections among children grew 40% in the last half of July, bringing the total number to 8.8% of all U.S. cases. That’s all well below that age group’s share of the global population, which hovers around 30%. Compared to adults, children with COVID-19 typically have milder symptoms that are predominantly limited to the nose, throat and upper airway, and they rarely require hospitalization.

2. Are the risks spread evenly?

Perhaps not. Children younger than 10 are significantly less susceptible to the virus than teenagers and adults, according to research by scientists at the Harvard T.H. Chan School of Public Health in Boston and the University of St Andrews in Scotland that was released ahead of peer review and publication in July. Susceptibility for children older than 10 was similar to adults apart from those over 60 years, who are at greater risk, they said. The findings are supported by a study published in June that used antibody tests to survey 2,766 people in Geneva. It found teenagers were almost as likely to have been infected by the virus as adults age 20 to 49, while children age 5 to 9 trailed well behind.

3. Why might younger kids be less susceptible?

There are several theories. Scientists have posited that COVID-19 might be sparing children because they are less exposed to the virus, with school closures and other distancing measures largely isolating them. It’s possible children mount a more robust initial immune response to the virus, giving the body a better shot at fending it off (and avoiding some complications adults get). It’s also been suggested that the receptor the virus uses to invade human cells is less mature in children, making it harder for it to cause an infection.

Younger people are less commonly afflicted by hypertension, type-2 diabetes and other chronic conditions known to increase the risk of severe illness from COVID-19. A low infection rate and mild symptoms among children were also seen during earlier outbreaks of two other novel coronaviruses, one in 2002-2003 that causes severe acute respiratory syndrome (SARS) and another starting in 2012 that causes Middle East respiratory syndrome (MERS).

4. Do kids spread the virus?

It’s not yet clear, but it may depend on if they’re sick. Children younger than 5 with “mild-to-moderate” COVID-19 have higher concentrations of the virus in their upper airway than older children and adults, doctors in Chicago reported in July. This could make them more infectious. However, another study published in Nature found the vast majority of infected kids in this age group don’t have symptoms, which reduces their likelihood of transmitting the virus.

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In another, much-cited study of 5,706 coronavirus patients and their contacts in South Korea, researchers concluded that children younger than 10 spread the virus within a household at the lowest rate, but those age 10 to 19 were more likely to spread it than even adults. That study had limitations. As with similar studies, the researchers first identified an infected person and then tested that person’s contacts, which means they can’t be certain who started the chain. Plus, the study was conducted while schools were mostly closed.

In the U.S., school closure was associated with a 62% drop in COVID-19 cases and 58% decline in deaths, researchers in Cincinnati reported in July. They acknowledged that the trends were largest in states with a low cumulative incidence of COVID-19 at the time schools were shut, and that it’s possible some of the reduction was due to other measures.

5. What’s happened where schools reopened?

The evidence is preliminary and mixed. Denmark and Norway reopened schools in April and avoided subsequent outbreaks. Health specialists connect their success both to mitigation strategies including smaller classes and increased hand washing, and to the fact that overall cases were low at the time. Germany brought older students back to schools in small groups in early May when overall cases were moderately high and saw increased transmission among students, though not school staff.

In Israel, schools were fully reopened without restrictions on May 17. Ten days later, a major outbreak occurred in a high school, followed by a significant wave of infections in the general population, prompting the government to shut down parts of the economy again.

6. How sick do children get?

Scientists at the London School of Hygiene & Tropical Medicine in June found that clinical symptoms manifest in 21% of infections in 10- to 19-year-olds, rising to 69% in people older than 70. Death is extremely rare in children, although it can occur in those who are already very sick with cancer or other serious conditions.

An uncommon but serious blood disorder has been associated with SARS-CoV-2, as the virus is called. Known as pediatric inflammatory multisystem syndrome (PIMS) or multisystem inflammatory syndrome in children (MIS-C), it’s a potentially lethal condition, similar to Kawasaki disease. It occurs at a rate of about two per 100,000 people younger than 21 years — much less than the 322 per 100,000 in which coronavirus infection is diagnosed in that age group.