As the U.S. prepares to roll out booster shots, scientists question the need for a third COVID-19 shot and whether it is necessary to become an annual thing.

This week, the Centers for Disease Control and Prevention released a joint letter approving a third shot for Americans who received either the Pfizer or Moderna vaccine. However, CDC Director Dr. Rochelle Walensky said Thursday that this COVID-19 booster shot may not become an annual recurrence since a third shot may sufficiently strengthen individual long-term protection.

“This virus has been humbling, so I don’t want to say never, but we are not necessarily anticipating that you will need this annually,” Walensky said on the CBS program “This Morning.” “It does look like after this third dose, you get a really robust response, and so we will continue to follow the science both on the vaccine side but also on the virus side.”

Yet, some scientists question whether a third dose is even necessary at all.

CNBC said that some scientists have sharply criticized the Biden administration’s push to widely distribute Covid-19 vaccine booster shots starting Sept. 20, saying the data provided by federal health officials this week wasn’t compelling enough to recommend third shots to most of the American population right now.

Scientists argue that the administration’s booster shot roll out is a premature move since recent studies published by the CDC show that though protection against the coronavirus given by vaccines declined in recent months, vaccinated people still had sustained protection against hospitalization and severe infection.

The administration is also receiving sharp criticism from global leaders, who have extensively called for a moratorium on COVID-19 booster shots to address the vaccine supply shortage in poor countries.

Despite this criticism, the U.S. booster shot plan will remain in place.

Starting next month, U.S. health care workers, nursing home residents and immunocompromised individuals will be first in line to receive the third dose of either Pfizer or Moderna.

Meanwhile, countries such as the Congo, Chad, and Haiti have less than one percent of their population vaccinated.