Has any of the main cast been absent for whatever reason? I was also wondering has any advanced absences been reported. Just curious! It's such a long run that there must be some, I would also be interesting to know who the covers are.
It's just finishing its third moth of performances. I've worked many plays on Broadway, and in my experience actors in plays tend to have significantly better attendance than actors in musicals. Most shows I've worked had gone months without a single absence. With a show this high profile (all the celebrity visits), I wouldn't be surprised if no one has missed.
Having just seen HARRY POTTER parts 1 and 2 in London, I don't think that it matters if one or two understudies are performing. The production is overwhelming and awesome and the cast is huge, so no single understudy performance is going to alter the equation.