Your questions, answered "My soon-to-be 12-year-old son is genuinely looking forward to being vaccinated, but based on the expected timeline to approval of the vaccine for children 5 to 11, he may be 12 by the time it is available…Since my very-late-blooming child is only 78 pounds, I'd prefer him to receive the lesser dose. But if he turns 12 prior to it being available, will we even have an option, or would he have to take the adult dose?" — Glen in Connecticut First of all, if you have concerns specific to your child, please consult your doctor, pediatrician or other trusted medical professional for advice. That said, this is a good question – especially because you're correct the dosages for young children's coronavirus vaccines and adult vaccines are likely to be different. Pfizer-BioNTech submitted an application to the Food and Drug Administration this week for emergency use authorization for children age 5 to 11. In clinical trials cited in that application, the dose those companies tested in children was 10 micrograms per shot, a third of the dosage people 12 and older receive. Even if 12-year-olds are on the smaller side, it's generally a good idea to get the vaccine dosage designed for that age. These vaccines have been rigorously tested — first in clinical trials, and then in a multitude of real-world observations — for their respective age groups. There are biological reasons for this. Vaccines do not follow the same dose-response rules many familiar drugs do. Pills that contain aspirin or other painkillers are like buses full of commuters, eager to go where they need to be and get to work. In these drugs, the active agents and their byproducts are doing the heavy lifting. To oversimplify a little, the more delivered, the more work gets done. People with more mass are also likely to need higher amounts as those drugs diffuse through the body. That's why, all things equal, it takes more alcohol to intoxicate a larger person. Vaccines don't have this same relationship with people's bodies. Instead, they kickstart an immune system that's already part of us. You can imagine a vaccine like throwing a switch, turning on microscopic factories within our bodies that begin to churn out immune defenders. Our own immune systems dictate the strength of a vaccine. Immune systems, crucially, change with age, so much so that immunologists and pediatricians have an axiom: Kids are little humans, but not little adults. It's less about how children's body sizes react to the vaccine, and more how their developing immune systems will. You can also think of it like this: There's wide variance in adult body sizes, but adults get the same vaccine amount whether they are petite grandparents or massive weightlifters. In fact, if those weightlifters or grandparents are older than 65, then they may be eligible for even more vaccine given as a booster — reflecting that age of a person's immune system, not body size, is key to a vaccine's effect. | 
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