From their Lancet article

there could be risks if boosters are widely introduced too soon, or too frequently, especially with vaccines that can have immune-mediated side-effects (such as myocarditis, which is more common after the second dose of some mRNA vaccines,3 or Guillain-Barre syndrome, which has been associated with adenovirus-vectored COVID-19 vaccines4)

But there is plenty of word circus in there, about how the vaccine is not effective against mild infection, but great against serious infection, but also that its hard to interpret observational data.

The authors are twisting their own conscience to tell themselves that what they have done so far is great, but they dont want to kill more people with boosters. Or the threshold of murder at 2 doses is acceptable for them, but not what happens at booster.