In the clinical trials for Pfizer's Covid-19 vaccine in children aged 6 months to 4 years, 6 children who received the vaccine experienced a severe case of Covid-19, compared to only 2 who received the placebo. Because 2 children received the vaccine for every 1 who received the placebo in the trial, those figures imply more parity than the raw numbers reveal. But even after accounting for this by doubling the placebo cases, the rate of severe Covid-19 among children who received the vaccine was 50% higher than those who received the placebo. However, the result is not statistically significant because the trial was underpowered, a medical term for clinical trials that don't enroll enough participants to detect important effects. All mRNA Covid vaccine clinical trials have been relatively small and short, and none of them show any statistically significant and clinically meaningful benefits like preventing hospitalization or death.