The CDC took questions on Wednesday after announcing that booster shots would be available and pushed to the American public starting the week of September 20, pending FDA approval of the plan. In so doing, they said the concept of what it means to be "fully vaccinated" against Covid-19 would change.

A reporter asked "What will it mean to be fully vaccinated once people are eligible for boosters? Will it be two shots or three shots?"

In response, Surgeon General Vivek Murthy said "Well so on that question right now I think what's very important for people to know is that if you've gotten both shots of your mRNA vaccine, you are fully vaccinated right now, you have full, you have high degree of protection against the worst outcomes of Covid-19.

"But our recommendation, down the line, again pending the advice and the review of the FDA and ASIP, the Advisory Society on Immunization Practices, is that we believe that that third dose will ultimately be needed to provide the fullest and continual extent of protection that we think people will need for the virus."

Prior to the recommendation of the booster shot, the concept of being "fully vaccinated" meant that a person had received either two doses of a two dose vaccine, or one dose of the Johnson&Johnson one dose vaccine, more than two weeks in the past.

It is these recommendations that have been used in the American cities of New York, San Francisco, and New Orleans to require people to show proof of vaccination before permitting them to engage in public life.

The CDC did not address how many booster shots may be required past this first round.