Children have been susceptible to Covid-19 all along. More than 4.1 million children have been diagnosed with Covid-19 since the beginning of the pandemic, accounting for 14.3 percent of all cases, according to the latest data from the American Academy of Pediatrics.

From July 15 to July 29, that percentage rose to 19 percent of weekly reported cases.

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https://www.msn.com/en-us/news/us/never-seen-anything-like-it-delta-is-making-a-growing-number-of-kids-very-sick/ar-AAMZ7nT?ocid=BingNews
Most of the people getting sick and dying are between zero and 50 years old now because 85% of those above
50 got vaccinated already (as an average) in all 50 states.
The increased number of children being put in hospitals likely also has to do with that the delta Variant came 
from India where the average age of people there is about 30 years old (a very young country). So, it is more
used to attacking people under 30 than older versions
of the virus a year ago. So, children are being struck
down worldwide by the Delta Virus now.
However, the good news is because of the vaccinations
we are only losing about 358 people per day instead 
of around 2000 per day if there were no vaccinations
right now in the U.S.