I think it's a function of assumptions related to the evolution of the virus and the extent to which new variants affect vaccine effectiveness or escape the immune protection that's generated by the existing vaccines. This is definitely something we have to look at and follow very closely. I think it's too early to state definitively that indeed this will be the case. However, I think there's enough evidence that we need to continue to follow it very closely into the future.
On May 3rd, 2021. See this statement in context.