• The CDC adopted a recommendation from its Advisory Committee on Immunization Practices to clear Covid booster shots for people with weak immune systems.
• The decision follows the FDA's approval Thursday of the booster shots for immunocompromised patients.
• With both agencies' OK, booster doses could start being administered immediately.

The Centers for Disease Control and Prevention gave final approval Friday to start administering Covid-19 booster shots to Pfizer and Moderna vaccine recipients hours after a key panel unanimously voted to endorse third doses for immunocompromised Americans.

"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," CDC Director Dr. Rochelle Walensky said in a statement.

The CDC's decision and recommendation by the Advisory Committee on Immunization Practices followed the Food and Drug Administration's approval late Thursday of the booster shots for immunocompromised patients. With both agencies' OK, booster doses could start being administered immediately.

"Over the past almost year and a half I have taken care of many patients with life threatening disease, and including deadly disease, and even after a vaccination" who are immunocompromised, Dr. Camille Nelson Kotton, a transplant and infectious disease specialist at Massachusetts General Hospital, told the panel in strongly supporting boosters for patients with weak immune systems. "They're just suffering from a lack of good vaccine protection, we know that vaccine efficacy is diminished in this population."

The FDA's approval cleared third doses of Pfizer and Moderna vaccines for "solid organ transplant recipients or those who are diagnosed with conditions that are considered to have an equivalent level of immunocompromise."

"Emerging data suggest some people with moderately to severely compromised immune systems do not always build the same level of immunity compared to people who are not immunocompromised," Walensky said. "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."

The agencies didn't clear boosters for other fully vaccinated individuals or for recipients of Johnson & Johnson's vaccine manufactured under its Janssen vaccine division.

"Currently there are not data to support the use of an additional mRNA COVID-19 vaccine dose after a primary Janssen Covid-19 vaccine in immunocompromised people. FDA and CDC are actively working to provide guidance on this issue," the CDC's Dr. Neela Goswami wrote in her presentation to ACIP.

The CDC recommended a third dose for vulnerable Americans 28 days or more after completing the initial two rounds of shots. Booster doses are also recommended for cancer patients and HIV patients after data showed that immune responses after the initial two doses in those patients did not provide adequate protection against Covid-19 and its variants.

The extra shots were recommended for Pfizer recipients who are 12 or older and Moderna recipients who are 18 and older. The Advisory Committee on Immunization Practices said it would revisit the shots for younger Moderna recipients after the FDA clears the shots for kids.

Immunocompromised patients make up roughly 2.7% of the U.S. adult population and 44% of hospitalized breakthrough infections, where someone gets infected even after they've been fully vaccinated.

Studies suggest that a third vaccine dose might help patients whose immune systems don't respond as well to a first or second dose. Five small studies cited by the CDC showed that 11% to 80% of people with weakened immune systems didn't have detectable antibodies to fight Covid after two shots.

Among immunosuppressed patients who had no detectable antibody response, 33% to 50% developed an antibody response after receiving an additional dose, according to the CDC.

Vulnerable patients are also more likely to experience prolonged Covid infections, the panel said. Data also suggests they are likely to shed more virus and potentially infect more people than those who are not immunocompromised.

Early data from small studies of the effects of booster doses in immunocompromised patients didn't show any severe adverse effects from a third shot of an mRNA vaccine and symptoms beyond those already identified after the initial two-dose regimen.

Several countries including Israel, the Dominican Republic, France, the United Kingdom and Germany have either already begun administering booster doses of Covid-19 vaccines or are considering it.

Immunocompromised patients who receive a third dose should still wear a mask and social distance, the panel said.

Survey data from hesitant immunocompromised patients indicate that many are still worried about side effects from the vaccines and the speed in which the shots were developed as well as general distrust of the vaccines, according to a panel presentation by the CDC's Dr. Kathleen Dooling.

About 10% of immunocompromised patients say they will "definitely not" be receiving a Covid vaccine, another 9% say they are "unsure" or will "probably not" receive a shot and 44% said they "definitely will" receive a vaccine. Those who are hesitant tend to be younger, part of an ethnic or racial minority or female.