Read on Twitter
You may have heard my conversation with @jasonbnpr on @NPR this weekend as to why kids make up a relatively low percentage of #COVID cases overall, <5% in the US, despite being 24% of the population. Here are a couple thoughts as to why: https://www.npr.org/sections/goatsandsoda/2020/06/20/880983822/coronavirus-mystery-are-kids-less-likely-to-catch-it-than-adults-are
One theory is that children are more likely to have pre-existing immunity to endemic #CoV (ie HKU1 and OC43). Neutralizing Ab target Spike protein on the viral membrane. SARS2 Spike is most similar to SARS1 (76%) but 30-37% similar to endemic CoVs, so cross protection unlikely.
Antibodies to endemic CoV are not even always protective against other endemic CoVs, despite much closer genetic similarity! https://www.sciencedirect.com/science/article/pii/S1386653211004732?via%3Dihub
Another idea is that due to mild symptoms in children, they are less likely to receive symptom-based testing for #COVID, and therefore would be under counted. I return to this fantastic @NEJM from Iceland where they tested 6% of the entire population https://www.nejm.org/doi/full/10.1056/NEJMoa2006100
One arm was symptom based, which revealed 38/564 (6.7%) positive children. Of 848 children in the population screening group, NONE were positive! Suggesting that symptom based testing does not under count infected children.
Another theory is that children have a lower viral load than adults, but that also seems to be false. In this study from @c_drosten, a wide range of viral loads were found in infected people of all ages.
https://www.medrxiv.org/content/10.1101/2020.06.08.20125484v1
https://www.medrxiv.org/content/10.1101/2020.06.08.20125484v1
Final and most plausible: ACE2 receptor expression is less in children than adults. Analysis of microarray data demonstrated decreased ACE2 expression in noses and lungs of children, potentially explaining differences in infectivity.
https://www.cell.com/molecular-therapy-family/methods/fulltext/S2329-0501(20)30100-5
https://www.cell.com/molecular-therapy-family/methods/fulltext/S2329-0501(20)30100-5
