Rochelle Walensky, director of the Centers for Disease Control and Prevention (CDC), on Friday signed CDC’s Advisory Committee on Immunization Practices’ (ACIP) recommendation for certain individuals to receive another dose of the two-dose mRNA vaccines.

According to the release, the CDC is recommending those with “moderately to severely compromised immune systems” get an additional shot after the completion of the two-dose Pfizer or Moderna series. The move coincides with the Food and Drug Administration (FDA) ruling that “transplant recipients and other similarly immune-compromised patients can get a third dose of either the Pfizer or Moderna vaccine.”

“Today’s action allows doctors to boost immunity in certain immunocompromised individuals who need extra protection from COVID-19,” the FDA’s acting commissioner Dr. Janet Woodcock said in a statement.

Walensky’s release estimates the percentage of immunocompromised people to comprise roughly three percent of the U.S. adult population. It also cites “small” studies suggesting that immunocompromised people have accounted for 40 to 44 percent of breakthrough cases requiring hospitalization:

Per Walensky’s release:

While people who are immunocompromised make up about 3% of the U.S. adult population, they are especially vulnerable to COVID-19 because they are more at risk of serious, prolonged illness. Included in CDC’s recommendation are people with a range of conditions, such as recipients of organ or stem cell transplants, people with advanced or untreated HIV infection, active recipients of treatment for cancer, people who are taking some medications that weaken the immune system, and others.  A full list of conditions can be found on CDC’s website. The additional dose of an mRNA COVID-19 vaccine should be the same vaccine as the initial series and administered at least four weeks after completing a primary mRNA COVID-19 vaccine series.  While vaccination is likely to increase protection in this population, even after vaccination, people who are immunocompromised should continue follow current prevention measures (including wearing a mask, staying 6 feet apart from others they do not live with, and avoiding crowds and poorly ventilated indoor spaces) to protect themselves and those around them against COVID-19 until advised otherwise by their healthcare provider. CDC does not recommend additional doses or booster shots for any other population at this time.

“At a time when the Delta variant is surging, an additional vaccine dose for some people with weakened immune systems could help prevent serious and possibly life-threatening COVID-19 cases within this population,” Walensky added.

This is not the first time officials have floated additional doses of the two-dose vaccine series. Last month, Pfizer representatives met with top U.S. health officials to outline plans to request federal authorization for an additional dose of its vaccine. Months prior to that, Pfizer CEO Albert Bourla said it was like a third shot would be needed for fully vaccinated people.

“Every year, you need to go to get your flu vaccine,” Bourla said at the time. “It’s going to be the same with COVID. In a year, you will have to go and get your annual shot for COVID to be protected.”

Dr. Anthony Fauci followed suit, suggesting a third jab is “likely.”

“We know that the vaccine durability of the efficacy lasts at least six months, and likely considerably more, but I think we will almost certainly require a booster sometime within a year or so after getting the primary,” Fauci said.