Since Americans first began rolling up their sleeves for coronavirus vaccines, health officials have said that those who are immunized are very unlikely to become infected, or to suffer serious illness or death. But preliminary data from seven states hint that the arrival of the Delta variant in July may have altered the calculus. Breakthrough infections are also likely to be most severe among older adults or those who have conditions like obesity or diabetes.

For more than a year, we’ve worked with North Carolina school districts and charter schools, studying the rate of new Covid cases, the efficacy of mitigation measures such as masking and the increased risks of participating in school-sponsored sports. We have learned a few things for certain:

Doctors across the country have been seeing a striking increase in the number of young people with the condition Braden had, which is called Multisystem Inflammatory Syndrome in Children or MIS-C. Even more worrisome, they say, is that more patients are now very sick than during the first wave of cases, which alarmed doctors and parents around the world last spring.

A flurry of recent studies has shown that people with extra weight are more susceptible than others to severe bouts of disease. And experiments in animals and human cells have demonstrated how excess fat can disrupt the immune system.