Effectiveness of a Third Dose of Vaccines in Preventing Covid-19 Hospitalization

> Second, VE [vaccine effectiveness] associated with newly emergent variants, including Omicron, was not assessed.

Insofar as the Omicron variant is the dominant strain, it leaves open the question of VE *today*. There's reason to expect improvement in VE of a 3-dose strategy over a 2-dose strategy, but perhaps the magnitude of the Δ would be less. Hard to say.

It's the difficult thing about studying vaccine efficacy with this virus: by the time the study has recruited enough participants, enough time has elapsed to measure an effect and the data are analyzed and published, a new variant emerges that casts uncertainty on the applicability of the data.