Until the delta variant laid siege this summer, nearly all children seemed to be spared from the worst ravages of covid, for reasons scientists didn’t totally understand. Although there’s no evidence the delta variant causes more severe disease, the virus is so infectious that children are being hospitalized in large numbers — mostly in states with low vaccination rates. Nearly 30% of covid infections reported for the week that ended Sept. 9 were in children, according to the American Academy of Pediatrics.