Discussions within the administration are focused on whether a third shot should be considered part of the original Covid-19 vaccine regimen. That would change what it means to be fully protected against the virus and immediately raise questions about the need for more shots, just as breakthrough cases are confusing many people about what vaccines can and cannot do.

There is “no doubt in anyone’s mind” that the original regimen plus a booster is ideal, especially with Omicron looming, said one health official familiar with the discussions. But “[Biden officials are] not going to change that anytime soon, because there’s too many legal aspects hanging on this issue of what is fully vaccinated.”

Vaccine manufacturers are focusing on the same questions. Pfizer officials said Friday that they are exploring whether to make a booster dose part of their regimen. CEO Albert Bourla has even floated the potential need for a fourth dose, though administration officials dismiss that prospect as premature.