Question: Why aren’t children as severely affected by COVID-19? If it’s about an adaptable immune system, would someone whose immune system has been exposed to lots of new things for their life have a similarly adaptable immune system? Or does it just not work that way?

Answer: The short answer is that we still don’t know. Here’s a quick run-down of what we current know (to my knowledge!). First, preliminary evidence shows that children are are less likely to be symptomatic or to develop severe symptoms as compared with adults (yay!). The preliminary evidence is, however, mixed when it comes to three other important issues: 1) whether children are just as likely as adults to become infected SARS-Cov-2; 2) whether children are just as likely as adults to transmit the virus; and 3) why children are less likely to be symptomatic or develop severe symptoms. As you’ll so often read in scientific publications, more research is needed. Even so, we can partially answer your question by exploring the hypotheses scientists have about why children are less likely to be symptomatic or develop severe symptoms. At this stage of our understanding, it seems like children’s highly adaptive immune systems are protecting them. In this case, it’s actually their lack of exposure to other viruses and infections that’s helping them (their immune systems aren’t stuck in old ways of doing things)!

• Quick refresh on our immune system: Our immune system has two parts — innate (e.g. the response we’re born with) and adaptive (e.g. the response we learn as we come into contact with our world). Both parts work together to mount an immune response.
  • The innate system is the first responder when your body encounters an invader (virus, bacteria). Immune cells called phagocytes engulf the invader and in essence eat it up (like pac-man)! These phagocyte cells (macrophages and neutrophils) are made in our bone marrow and are produced throughout our life. They move through our blood system, ready to destroy viruses and bacteria they come into contact with. But sometimes, a virus or bacteria can overwhelm our innate system. So our adaptive system kicks in!
  • The adaptive system produces antibodies to protect your body from various invaders. These antibodies are developed by cells called B lymphocytes a few days after first exposure to a new virus or bacteria. The two kinds of lymphocytes are B lymphocytes and T lymphocytes. Lymphocytes begin in the bone marrow and either stay there and mature into B cells, or go to the thymus gland to mature into T cells. B lymphocytes identify invaders and basically mark them for other immune cells to destroy (like military intelligence). T lymphocytes are either Helper T-cells, which stimulate B-cells to make antibodies and help killer cells develop, or Killer T-cells, which directly kill the cells that B lymphocytes have identified.
• When it comes to children, here are the current hypotheses:
  • Children have fewer environmental exposures (ex: smoking) and underlying conditions (ex: cardiovascular disease) and are therefore less likely to have severe outcomes: This is compelling, but insufficient. For example, this hypothesis does not explain why children are less likely to have symptoms.
  • Children have fewer ACE2 cells that the virus can use to replicate itself (angiotensin-converting enzyme-2 (ACE2): Interesting idea, but so far “there is no evidence of a lower degree of expression or function of the SARS-CoV-2 receptor (namely ACE2) in children.”
  • Children have stronger innate immune response: Scientists are exploring the hypothesis that the natural antibodies children make in abundance are better at effectively reacting to unknown invaders.
  • Children have more adaptable adaptive immune response: Scientists are also exploring the hypothesis that the types of memory B cells that children develop are far more adaptable than the types of memory B cells of adults, especially the memory B cells of the elderly, which are very good at recognizing known invaders, but very bad at recognizing unknown invaders. “With ageing, malnutrition, immunosuppression, and co-morbid states, our immune system loses the ability to adapt to novelty.” These two innate and adaptive immune response factors may also be the reason why adults are more likely to mount an overly-aggressive immune response.
  • Finally, some scientists earlier hypothesized that because children are more exposed to coronaviruses in their school and play environments, and therefore have an immune system more primed to respond to SARS-CoV-2. This hypothesis doesn’t seem to hold much water given that newborns are also less likely to have symptoms or severe outcomes but haven’t been exposed to other coronaviruses. Something else is going on, which makes me think that it’s the way that children’s immune systems are primed for novelty that is protecting them.