The Pfizer vaccine trial for 6 months to 4 years old is being delayed after the early immunogenicity data was inferior to the teen/adult controls. They are going to test a third dose 2 months after the primary series in this age group. Now expected to apply for an EUA in the second quarter of 2022.
The dosage for this group is only 3 μg, compared to 10 μg in 5-11 year olds, and 30 μg in 12+. So Pfizer is trying to determine the right balance in terms of efficacy and side effects. Too high of a dose and you risk too many side effects. Too low of a dose and you don't get an adequate response. This age group was probably already going to get a third dose due to the Omicron variant, so it isn't a huge change.
The coronavirus risk for this age group is fairly small, so that's why it is important to get the dosage right.
For 6 months to 11 year olds the trials are only immuno-bridging trials, so instead of comparing infection/disease rates between vaccinated and control, they compare antibody levels between vaccinated and an older control group that we already have clinical efficacy data for. If the antibody levels are similar or better, the trial meets its endpoints.
This age group has tended to have lower rates of infection than older kids, but at this point it is quite likely ~40-50% or more have been infected with the virus already.
Pfizer is also testing a 10 μg third dose in 5-11 year olds and testing both 10 μg and 30 μg third doses in 12-17 year olds.