Question: Given the recently released Pfizer data and probable emergency use approval to vaccinate kids under 12, can you explain why vaccination guidance is given by age and not height/weight like car seats? My daughter turns 12 in early February. She is taller (and weighs more) then most of her close friends, who have been getting the vaccines since this summer when they started turning 12. The new guidance says that children 4-11 should receive 1/3 of the dose of adults. I have not seen if the data indicates a stronger response for younger vs older children with that 1/3 dosage. The question is.. do I hold off a few months until she turns 12 to get the full dose, or, I assume, the quicker the shot, the better the probability of diminishing risk? And I also assume that, should she need a booster 6 months down the road, she would get a full dose booster?
Answer: Usual caveat– I am not a medical doctor and you should definitely raise these questions with your pediatrician. In the meantime, here’s a bit of what the scientific literature says on your excellent questions. First off, vaccines are not like medicine– vaccines are generally *not* dosed based on weight (or height or body mass index for that matter), whereas medicines generally are. Instead, vaccines are generally dosed based on a person’s age— and by proxy, the age of their immune system. To use an analogy– the vaccine is teaching the immune system to react to a given virus; and the lesson (e.g. dosage) may vary based on how much the immune system already knows. I encourage you to read more on vaccine dosing from this excellent synthesis offered by the Children’s Hospital of Philadelphia.
Turning specifically to the COVID-19 vaccine, CDC’s guidelines for vaccination among children and teens is explicit, “There are no patient weight requirements for COVID-19 vaccination, and COVID-19 vaccine dosage does not vary by patient weight.” Infectious disease specialist at Boston’s Children’s Hospital, Dr. Kristin Moffit further describes in this interview that “dose will still be based on age and not on weight, and in the Phase 3 trials that included over 1,000 12-to-15 year-olds [who received the vaccine], it included people of various weights, including people who were small for their age; so the safety profile was still very reassuring in those individuals.” If you’re interested about weight in older ages, the Phase 3 trial of adults also included people with varying weights (ranging from underweight to obese).
Finally, when it comes to your last question– assuming that the data support it (we’ll know once we see the data!), if it were me, I’d choose to have my child get the first dose as soon as possible rather than holding off. In my opinion, it is better to be protected sooner rather than later, and if the vaccine at a lower dosage shows efficacy and safety among 11 year-olds (which it sounds like it will!) then I would jump!