There are seven types of coronaviruses that naturally infect humans. Three of them, SARS-CoV-2, SARS-CoV and MERS-CoV can cause severe acute respiratory illnesses. By contrast, the rest four coronaviruses with the names: HKU1, NL63, OC43 and 229E usually cause much milder upper respiratory tract infections, and are therefore considered low-pathogenic human coronaviruses. Intriguingly, low-pathogenic human coronaviruses are more prevalent and manifest their symptoms in young children, contrary to the new coronavirus Sars-CoV-2, which might infect young children but it does not affect them with severe clinical symptoms (of course there are exceptions).
Given that these low-pathogenic human coronaviruses are continuously circulating among the global population and especially among young children (<5 years old), one can understand that the young population has very high chances of carrying antibodies against these coronaviruses. Keep in mind that globally, about 5% of acute respiratory tract infections are attributed to these pathogens! This means that because:
1. the incidence of respiratory infections is worldwide at multiple billion episodes per year and
2. antibodies can live in the body for quite some time,
a substantial proportion of the global population and particularly children are expected to carry antibodies against these well-known human coronaviruses. Genetically, these viruses show a moderate similarity to SARS-CoV-2. It was therefore of no surprise, when it was found that some individuals’ antibodies against OC43 and NL63 (low-pathogenic human coronaviruses) were able to recognize SARS-CoV-2 too. This is the so-called SARS-CoV-2-cross-reactive immune response and this might be a reason why young children — who are very often confronted with OC43 and NL63 in their early life and have thus antibodies against these coronaviruses — might be able to combat so well SARS-CoV-2.
Microbiologist and immunologist Donna Faber from Columbia University investigates antibody response of children and adults to SARS-CoV-2. According to her reporting to DW, ‘children can contract the virus without showing any symptoms.’ They therefore pose a risk of unknowingly spreading the virus. Faber adds a second reason why young individuals might be so good in ‘fighting’ SARS-CoV-2. As she explains, this has partly to do with their so-called ‘naive T cells’. Children — according to Faber — are constantly producing these new ‘weapons’; they have an entire arsenal of them and every time they are confronted with a new pathogen, they produce these naive T cells. Adults, on the other hand, gradually lose the ability to produce new ones.
These cells circulate between blood vessels and peripheral lymphoid organs. After coming into contact with an antigen (part of a pathogen, such as the Spike protein on SARS-CoV-2), they begin multiplying, launching an immune response.
The majority of T cells found in adults, in contrast, are specific to infections that the body has already endured in the past, such as influenza viruses. This means adults’ immune system responses can be very effective against infections that the body ‘knows’ from a past infection (see: immunologic memory article). Now, with the new coronavirus SARS-CoV-2, both children and adults are facing a new pathogen. It seems that the former can cope with this new virus better (also) because of their naive T cells, or in other words because they have a ‘fresher’ immune system.
In a nutshell, the majority of young children infected with SARS-Cov-2 will manifest mild symptoms of the COVID-19 disease or no symptoms at all. This is good news for them but does not completely eliminate the risk of spreading the virus. What gives us hope regarding children’s potential to transmit the virus is that early data show that young individuals (up to ~10 years old) carry mostly a low viral load. This is encouraging, as — in general — a low viral load means lower transmissibility.
Nevertheless, SARS-CoV-2 is a new pathogen and the most data we have on the correlation of viral load and transmissibility come from other viruses. We can therefore conclude that taking preventive measures, such as ventilating classrooms, teaching in small groups, testing more children when this is possible and last but not least, prioritizing the vaccination of the teaching personnel can prove critical in handling this pandemic.