Worldwide, we have seen COVID-19 affect far fewer children than adults. Similarly, those children who do experience COVID-19 generally have much milder symptoms and are less likely to experience a severe COVID-19 infection. A recent review published in the journal BMJ Archives of Disease in Childhood has compiled a list of potential reasons behind this phenomenon. Researchers reviewed both protective mechanisms in children, as well as factors that specifically put adults at higher risk. The authors of the study admit that it is still not clear why children are less prone to severe COVID-19 infection. But reviewing various theories will help focus future research.
Protective Factors in Children
Melatonin Levels
Melatonin is a hormone traditionally known to promote sleep. It is also anti-inflammatory in nature, which is important in this instance because the severity of COVID-19 symptoms may be linked to an overactive inflammatory response. Melatonin has also been shown to reduce acute respiratory distress syndrome (ARDS) in previous viral illnesses. Additionally, this hormone is thought to increase the number and replication speed of specialized immune cells in both the innate and adaptive immune systems. Because melatonin levels decline as we age, the higher levels of this protective anti-inflammatory hormone may be why children are less likely to have severe cases of COVID-19.
Immune System
The body’s immune system has two parts: innate immunity and adaptive immunity. Innate immunity is when the body fights off generalized invading bacteria, viruses, or other microorganisms that may cause disease (collectively known as pathogens). Adaptive immunity, meanwhile, is when the body recognizes a specific invading organism and attacks it. Children have a stronger innate immune system, so they can fight off illnesses they have never been exposed to. Melissa Gallagher, ND, explains, “Literally, children’s immune systems are more adaptive in that they are built to respond to novel viruses as every pathogen is a novelty to their immature immune systems.” As we age, we’re exposed to more pathogens, which our adaptive immune system remembers, subsequently creating specific antibodies to fight these. This is how our adaptive immune system becomes stronger against specific targets. In addition to the innate and adaptive is the “trained” immune system. All this means is that when a child (or person) is exposed to an illness, the innate immune system cells alter in their make-up and become stronger and faster at fighting off future general exposures. The theory here is that because COVID-19 is such a new disease, those with a stronger innate immune system will mount a better response against COVID-19. Thus, if children have a stronger innate immune system, they will fight the pathogen better than adults who have a weaker innate immune system.
Vaccine Schedules
Most vaccinations involve providing a small amount of a pathogen so the immune system fights and builds antibodies against a specific disease. This process develops the adaptive immune system. The trained immune system is also activated by a vaccine and improves its efficiency and reaction time to fight off many other general or new pathogens. One theory suggests that because children who follow a standard vaccine schedule have had their immune systems activated by vaccination more recently than many adults, their trained immunity is potentially in a heightened state. Therefore, they’re better able to fight off COVID-19.
Higher Frequency of Illness
Children often have more frequent illnesses, such as mild colds and flu. These frequent illnesses help to trigger the trained immune system to fight stronger and faster, protecting the children from COVID-19. Gallagher explains, “Frequent infection exposure in infancy and early childhood may build up the memory pool of immune cells… and be more reactive than an adult immune system to a novel viral exposure.”
Microbiota
The microbiota of our nose, mouth, lungs, and gut all help to fight invading pathogens. The studies that have looked at strains of microbiota in the nose, mouth, and lungs of COVID-19 patients are so far inconclusive. However, gut bacteria research is showing some promising results. COVID-19 patients generally seem to show a low diversity in their gut bacteria, which researchers think may be significant. There is one strain that seems to be naturally higher in concentration in children, which could provide some insight. Christine Bishara, MD, an integrative internal medicine practitioner, reports that Bifidobacterium, a probiotic strain that normally lives in the intestines and stomach, naturally occurs at high concentrations in babies and children. Bifidobacterium could make up as much as 80% of young children’s gut bacteria. Levels decline as we age; in older people, only 5% or less of gut bacteria is Bifidobacterium. Bishara explains that this particular strain is considered so vital because “it works on the same immune pathway that COVID-19 attacks.” “Bifidobacterium functions as an immune regulator and helps to inhibit IL-6—the same pro-inflammatory cytokine that initiates the deadly cytokine storm,” in COVID-19, Bishara says. It is important to remember that many things impact the gut microbiota, including diet, hospital admission, and antibiotic use.
Why Older People are More at Risk of Severe COVID-19
Inflammation
Generalized inflammation can increase with age. It results in inflammatory disorders, such as arthritis, inflammatory bowel disorders, and auto-immune disorders. One of the key chemicals in this inflammatory process are cytokines. In severe COVID-19 cases, a “cytokine storm” can occur. A cytokine storm is a sudden increase in a collection of inflammatory chemicals that can lead to organ shutdown and subsequent demise. In people over 65, cases of chronic inflammation and its associated conditions are higher than those in children, putting them at an increased risk of severe complications of COVID-19.
Blood Vessels
The COVID-19 virus can infect the cells of the lining of blood vessels. In older people who have pre-existing conditions affecting the blood vessels such as cardiovascular disease, hypertension, or diabetes, the risk of adverse events related to a COVID-19 infection increases. Complications can include the formation of blood clots which can lead to stroke or pulmonary emboli.
Lower Levels of Vitamin D
Vitamin D levels have been found to be lower in many COVID-19 positive people versus those who tested negative. Vitamin D is thought to reduce the risk of respiratory tract infections, have anti-inflammatory properties, and reduce the rate of cytokine production. The research authors suggest that children may be more protected because many children are supplemented with vitamin D in the first few years of life. However, this is not universal and therefore may not be a strong argument for the differences between children and the older population. The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.
