The true scientific reason behind children's better resistance to the coronavirus and its variants compared to adults remains unknown. This is one of the mysteries surrounding the novel virus. Unlike most other respiratory viruses, which generally manifest more severe disease in children, the coronavirus and its variants appear to behave contrary to this trend.
Children, who often show either no symptoms or mild symptoms of COVID-19, may carry significant viral loads, leading to their potential to harbor and spread the virus, creating a reservoir for the transmission and evolution of the virus's genetic variants. With evolving guidelines on social distancing and mask-wearing even after vaccination for older individuals, understanding the dynamics of disease infection in children is crucial for the proper development of public health policies and vaccination strategies to mitigate the disease's impact.
In research published in the journal "Nature," Betsy Herold, a pediatric infectious disease physician leading a virology lab at Albert Einstein College of Medicine in the US, notes that data collected by the Centers for Disease Control and Prevention from hospitals across the country show that individuals under 18 represent less than 2% of COVID-19 hospital admissions. This makes the coronavirus relatively atypical compared to most other viruses, such as influenza and respiratory syncytial virus, where young children and the elderly are usually at higher risk of severe illness.
Research has begun to reveal that the reason children perform better against COVID-19 may lie in their innate immune response, which is the body's rapid reaction to pathogens. Children appear to have a ready-to-go "rapid response" akin, according to Herold. However, she adds that more studies are needed to fully support this hypothesis. The emergence of the Delta variant has made finding answers more urgent, and reports indicate that in the United States and elsewhere, children are beginning to make up a larger share of reported infections. This trend might stem from increased virus transmission due to the Delta variant and the fact that many adults are now protected by vaccines.
Currently, there is no clear evidence that children are more susceptible or more affected by the Delta variant compared to earlier strains. However, like all viruses, COVID-19 continually evolves and becomes better at evading host defenses. Understanding the protective benefits associated with childhood may become increasingly important, says Lyle Yonker, a pediatric lung specialist at Massachusetts General Hospital in Boston, in a preprint paper on "medRxiv".
Contradicting Ideas
Omar Irfan from the Global Child Health Centre at Toronto's Hospital for Sick Children in Canada questions in his research published in the "Journal of Global Health": "Why are children better than adults at controlling COVID-19?" Researchers initially believed that children were less frequently infected, but data show that, particularly for those under ten, they might actually be less at risk of infection. The American Academy of Pediatrics found that as of late last month, approximately 15% of all COVID-19 cases in the US were among individuals under 21 years old, which translates to over 4.8 million youths. Another survey conducted in India tested individuals for antibodies against COVID-19—typically produced following infection or vaccination—and found that over half of children aged 6 to 17, as well as two-thirds of the general population, possessed detectable antibodies. It was evident that children were also getting infected, indicating that the virus may not replicate in their bodies as it does in adults.
Consequently, some researchers proposed that children might have fewer ACE2 receptors, which the virus uses to enter and infect cells, with conflicting evidence regarding age-related differences in the expression of this enzyme within the nose and lungs. However, scientists measuring viral load—the concentration of viral particles in the upper airways—did not observe a clear distinction between children and adults, says Karl Pierson from the Department of Microbiology and Immunology at Albert Einstein College of Medicine in the Bronx, New York. Yonker adds that infants and even teenagers can have high viral loads shortly after infection. Another hypothesis is that children, who seem to be exposed to various respiratory viruses throughout the year, may exhibit cross-immunity from other coronaviruses that cause colds, thus possessing a readiness of antibodies that can partially attach to the pandemic coronavirus. Yet much evidence suggests that adults also have this immunity.
Childrens’ Blood Antibodies
Amy Chong, an immunologist at the Peter Doherty Institute for Infection and Immunity in Melbourne, Australia, and her colleagues, in research published in "Nature Communications," identified some evidence in a broad study of antibodies in the blood of hundreds of children and adults, including 50 infected with COVID-19. They found that adults have reactive antibodies targeting viral parts similar to those of other coronaviruses, while children tend to produce a broader array of antibodies against all sections of the virus. Experts are also investigating other factors known to worsen with age, such as the ability to control inflammation and heal damaged tissue; additionally, children are less prone to form blood clots in vessels, which may provide some protection, says Vera Ignatovich, a biochemist studying blood diseases in children at the Murdoch Children’s Research Institute (MCRI) in Melbourne, Australia.
On the other hand, Michael Levin, a pediatrician and infectious disease specialist at Imperial College London in his research published in the "New England Journal of Medicine," expresses concern that as the pandemic continues, researchers worry the virus could evolve in ways that undermine part of children's innate immunity. Some researchers found that the Alpha variant, which was prominent in certain parts of the world for some time, developed strategies that allowed it to suppress the body's innate immune response. They are currently worried that the Delta variant could do the same. It appears that the rising number of children in hospitals in areas where Delta spreads is due to its contagiousness across all ages, in addition to the fact that many adults have been vaccinated or previously infected with COVID-19, although researchers are carefully monitoring these developments.
Herold mentions, "Almost all viruses have developed ways to evade the innate immune response, and COVID-19 is no exception." "So far, children continue to win with their innate immunity," but to what extent? This is what we do not know.
