By David Olson, October 23, 2023 10:50 Newsday

Young children rarely get severely ill from COVID-19 because their immune systems seem to block the coronavirus from spreading beyond the nose, a new study suggests, and experts say kids’ response to the virus could help develop a more effective vaccine for people of all ages.

Scientists long have been perplexed by why young children are among those least likely to get severe COVID-19, even though they are more vulnerable to serious illness from other viruses like the flu and RSV, and their immune systems are not fully mature.

Researchers from several U.S. and German institutions studied 54 children under 4 years old, with a median age of 9 months, who were infected with COVID-19 and found that the coronavirus did not appear to migrate beyond the upper respiratory tract.

“In order to get severe disease, you need spreading of the virus, not just at the nose, but then down to the lower respiratory tract,” said lead researcher Bali Pulendran, co-director of Stanford’s University’s Institute for Immunity, Transplantation and Infection and a professor of pathology, microbiology and immunology. “Here the virus seems to be nipped in the bud.”

WHAT TO KNOW

• A new study found that the immune systems of very young children, with a median age of 9 months, apparently were able to block the coronavirus from migrating beyond the nose.
• That may help explain why young children are among those least likely to get severe COVID-19, even though they are more likely to get seriously ill from viruses like the flu and RSV.
• Experts say if scientists can find a way to mimic children’s immune response in a nasal spray vaccine, the nasal spray could provide more effective protection against COVID-19 for people of all ages.

The study was published Oct. 12 in the journal Cell.

Scientists found much higher levels of the molecule interleukin 17 — which attracts disease-fighting white blood cells called neutrophils — in the babies’ and toddlers’ noses after they were infected, compared with adults, Pulendran said.

Dr. Andrew Handel, a pediatric infectious diseases expert at Stony Brook Children’s Hospital, who was not involved with the study, said the research “gives us a lot of insight into what it is about these young kids that protects them from getting so sick from COVID. … Their body was able to respond to the virus and kill it and shut it down right away, before it was able to reproduce and spread through the body.”

Unlike most virus-caused diseases, it’s not COVID-19 itself that causes the most damage to the body, but the “tremendous inflammatory response” of the immune system to COVID-19 that can lead to severe illness or death, he said. That is much less likely to happen if the coronavirus is blocked from migrating past the upper respiratory area, Handel said.

Children’s immune systems aren’t able to prevent COVID-19 infection. More than 90% of children 6 months to 17 years old have been infected with COVID-19, according to Centers for Disease Control and Prevention estimates.

Yet although kids under 5 make up 6% of the U.S. population, they make up only about 0.1% of COVID-19 deaths, the CDC says. Similar gaps exist for other age groups of children.

“They get infected at the same rate, but they don’t get sick at the same rate,” said Dr. Jessica Justman, an associate professor of medicine in epidemiology at Columbia University who has researched infection rates and was not involved in the new study.

Follow-up study on flu, RSV

Although the study provides “a plausible explanation” why children rarely get seriously ill from COVID-19, “I’m still left with the same question of why that doesn’t apply to influenza, why that doesn’t apply to RSV,” she said.

Pulendran said he’s already part of a follow-up study on children’s immune responses to the flu and respiratory syncytial virus.

One other key finding of the study: Children’s immunity lasts far longer than adults’, Pulendran said. Adults’ antibody levels peak about a month after infection and then decline rapidly. The antibody levels of children in the study kept rising for as many as 300 days, Pulendran said.

“It raises the question, how is it that these babies are launching such durable responses?” he said. “What is the immunological mechanism? It has implications for vaccine design, because if you could design a vaccine that could induce this level of durability, the antibody response to that would be remarkable.”

Scientists have been trying to create a nasal-spray vaccine that is more effective than current ones. Young children’s apparent ability to stop the coronavirus from spreading beyond the nose could offer important clues in vaccine development, Pulendran said.

“If there’s a nasal spray vaccine that mimics the kind of immune responses that we see in these COVID-infected kids, this protection could be useful in achieving the same results — that is, [apparently] nipping the virus in the bud and not allowing the virus to go beyond the site of entry, which is the nose,” he said.

The trick, Handel said, is putting what happens naturally in children’s noses into the form of a vaccine.

“We just haven’t figured out yet how to do that,” he said.