What we know — and don't know —about kids and coronavirus

Children typically escape COVID-19's most severe complications, presenting a host of questions scientists are just starting to be able to answer.

Why it matters: As schools and day cares look to reopen in the U.S. and grandparents long to reunite with their grandchildren, parents and policymakers are trying to assess the risk to kids, their families and their communities.

Driving the news: Children are 35%–60% less likely to become infected with SARS-CoV-2, the virus that causes COVID-19, compared with people over the age of 20, according to a study published this week in Nature Medicine.

What's known:

• As of early April, 1.7% of detected COVID-19 cases in the U.S. were in children under the age of 18, per the CDC. (The age group makes up 22% of the population.)
• Many who are infected don't have severe complications. 5.7% of pediatric cases in the CDC survey were hospitalized (versus 10% in adults 18–64 years old) and three children died.

Yes, but: There are some who do experience severe complications. COVID-19 is linked to a rare but severe condition in children called multisystem inflammatory syndrome (MIS-C) in which the heart, lungs and other organs become inflamed. In most cases, children recover with anti-inflammatory therapy although long-term implications are unknown.

• Luigi Notarangelo, chief of the NIAID Laboratory of Clinical Immunology and Microbiology, is studying MIS-C. He says it appears to be a post-infectious manifestation that appears in some children for unknown reasons — genetic biomarkers, environment, or "molecular mimicry" or cross-reactivity are possible factors.

What's unknown:

• Whether children are resistant to being infected in the first place, become infected and are less likely to develop symptoms compared to adults, or both.
• What is protecting children. A recent study suggests it may be the health of their blood vessels that prevents them from having serious complications from the disease. (Adults with severe COVID-19 can have excessive clotting. Children with the disease don't.) Other researchers suspect children have heightened immunity due to exposure to other coronaviruses, like those that cause the common cold, or their immune systems are better tuned to fight infection.
• Their role in spreading the virus. Children may make up a small percentage of reported COVID-19 cases, but it is unclear whether they are significant spreaders of the virus, even if they don't have symptoms. It comes back to how much virus they carry —what's known as viral load — and whether that load is infectious, both of which are actively being studied (and debated).
• "Baked within that question is, how much time does someone need to spend around that person to become infected? And how large of a dose needs to be transmitted [for infection]?" says José Ordovas-Montañes of Boston Children's Hospital and the Broad Institute. "We don't understand what the actual numbers are on any of that."
• The long-term effects. It's too soon to know how COVID-19 affects children and others in the long run.

What to watch: The NIH is enrolling thousands of kids and their families in a six-month study (called HEROS) to better understand the infection in kids, says Alkis Togias, chief of NIAID's Allergy, Asthma, and Airway Biology Branch.

• "The data so far from all the epidemiology that has been published or has been available to us, is very difficult to discern. It's not clear at all," Togias says.
• HEROS will examine if children transmit the infection to their family, whether asthma or allergies play a role (including prevention), and what percentage actually develop COVID-19 disease, he adds.

Between the lines: How the immune system is working in kids remains a giant question mark and is crucial to vaccine development too, as there are concerns a vaccine developed for adults could affect the young differently.

• "The idea of making a vaccine that is given across the age range is something that is quite challenging, especially when you're seeing these differences and without a good understanding of how severe it is in children, and how necessary a vaccine might be for younger individuals," Ordovas-Montañes says.
• Researchers must be sure children don't mount an inflammatory response to vaccines that is similar to MIS-C, says Notarangelo. "We have to be absolutely sure that any vaccine strategy is actually helpful and preventing disease but not causing post-infections in some patients."

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