Per a 2022 BMJ article out of Oxford university:
“To prevent one COVID-19 hospitalisation over a 6-month period, we estimate that 31,207–42 ,836 young adults aged 18–29 years must receive a third mRNA vaccine. Booster mandates in young adults are expected to cause a net harm: per COVID-19 hospitalisation prevented, we anticipate at least 18.5 serious adverse events from mRNA vaccines, including 1.5–4.6 booster-associated myopericarditis cases in males (typically requiring hospitalisation).”
https://jme.bmj.com/content/early/2022/12/05/jme-2022-108449