Shannon Hall writes:

As the new coronavirus continues to burn through populations, studies are beginning to shed light on its impact on infants. And so far the findings have been promising for parents and researchers alike.

The initial data suggest that infants make up a small fraction of people who have tested positive for COVID-19. A Centers for Disease Control and Prevention study released in April reported 398 infections in children under one year of age—roughly 0.3 percent of all U.S. cases at that time for which age was known. In addition, most of these cases appear mild in nature: a recent review published in the Italian Journal of Pediatrics that looked at infants up to the age of six months found that those who were infected would typically exhibit only a slight cough, runny nose or fever, which disappeared in a week or so. Other studies have suggested similar minor reactions. The question is: Why?

One of the favored hypotheses focuses on how easily the new coronavirus can gain access to the body’s tissues. Infection occurs when particles of the virus, SARS-CoV-2, enter human cells through a receptor called ACE2 and hijack those cells’ machinery to make copies of themselves. These copies then invade new cells. The thinking is that infant cells have only a few ACE2 receptors, whereas those of an elderly person might harbor thousands. With fewer available points of entry in a baby, it could be harder for the virus to break in. Alternatively—and perhaps counterintuitively—an infant’s immune system might simply be too immature to attack SARS-CoV-2. Given that most of the damage in severe COVID-19 cases seems to be caused by strong immune responses, that immaturity may work in babies’ favor.

The latter possibility could even explain a subtlety within the data: although infants appear resilient to COVID-19, they might be at a marginally higher risk than older children. Early data from China suggested that 10.6 percent of infected children younger than one had severe or critical illness—a rate that decreased dramatically with age. “It’s a dance that takes place between the virus and our own immune system,” says Rana Chakraborty, a pediatric infectious disease specialist at the Mayo Clinic. If the body’s defenses react too little, the virus will be able to take over. An overreaction can be equally deadly, however. So children older than about one year of age might be in a sweet spot between infants, whose immune system has not yet fully kicked in, and adults, whose defenses are sometimes overzealous. [Continue reading…]