How Children’s Immune Systems Can Evade COVID

Young children account for only a small percentage of COVID-19 infections¹ – a trend that has puzzled scientists. Now, a growing body of evidence suggests why: Children’s immune systems appear to be better equipped to clear SARS-CoV-2 than that of adults.

“Children are highly adapted to respond, and very well equipped to respond, to new viruses,” says Donna Farber, an immunologist at Columbia University in New York City. Even when infected with SARS-CoV-2, children are more likely to experience mild or asymptomatic illness.².

Another clue that children’s response to the virus differs from that of adults is that some children develop symptoms of COVID-19 and specific antibodies to SARS-CoV-2, but never test positive for the virus on a standard blood test. RT-PCR. In one study, three children under the age of ten in the same family developed antibodies against SARS-CoV-2³ – and two of them even experienced mild symptoms – but neither tested positive by RT-PCR, despite being tested 11 times over 28 days while in close contact with their parents, who tested positive.

Fast answer

Your immune system sees the virus “and it just generates this really fast and effective immune response that shuts it down, before it has a chance to replicate to the point that it tests positive on the diagnostic swab test,” says Immunologist Melanie Neeland. who studied the family, at the Murdoch Children’s Research Institute in Melbourne, Australia.

Even in children who experienced the serious but rare complication called multisystem inflammatory syndrome in response to SARS-CoV-2 infection, studies report that the rate of positive RT-PCR results ranges from only 29% to 50%^4,5,6.

Farber says that the types of antibodies children develop offer clues as to what is happening. In a study⁷ Out of 32 adults and 47 children 18 years or younger, she and her colleagues found that the children primarily produced antibodies targeting the SARS-CoV-2 spike protein, which the virus uses to enter cells. The adults generated similar antibodies, but they also developed antibodies against the nucleocapsid protein, which is essential for viral replication. Farber says that the nucleocapsid protein is generally released in significant amounts only when a virus is spread in the body.

The children lacked specific antibodies to the nucleocapsid, suggesting that they are not experiencing a generalized infection, says Farber. Children’s immune responses appear to be able to clear the virus before it replicates in large numbers, he says.

Adaptive vs innate immune system

Farber suggests that the reason children can neutralize the virus is that their T cells are relatively naive. T cells are part of the body’s adaptive immune system, which learns to recognize pathogens it encounters throughout life. Farber says that because children’s T cells are mostly untrained, they may have a greater ability to respond to new viruses, a phenomenon he is studying in more detail.

But other evidence suggests that the situation is not so simple: a study⁸ Of the people with COVID-19 who included 65 children and youth under 24 years of age, along with 60 adults, they found that adults had a stronger T-cell response to the spike protein of the virus than did children and youth. But Farber says the study measured the responses of memory T cells, which are much less developed in children, rather than the activity of naive T cells.

Children’s ability to neutralize the virus could also be related to the fact that they have a strong innate immune response from birth, says Alasdair Munro, who studies pediatric infectious diseases at the University Hospital of Southampton, UK. “There has been some suggestion that the speed and scale of your innate immune response might protect against the onset of infection,” he says. But this effect is difficult to study and raises the question of why it is not seen with other viruses that can cause serious illness in children, he says.

Other factors

Children are also the main reservoir for seasonal coronaviruses that cause the common cold. Some researchers have suggested that antibodies to these coronaviruses might confer some protection against SARS-CoV-2, but the evidence is mixed.^9,10says Munro.

Meanwhile, there is evidence that when children are exposed to the virus, they receive a lower dose than adults, because their noses contain fewer ACE2 receptors.^eleven, which the virus uses to access cells. This could also explain why COVID-19 is less common in children than adults, the researchers say.

Munro says there is unlikely to be a single explanation for why COVID-19 seems to affect children less than adults. “Biology is rarely that simple.”