Advisers to the Centers for Disease Control and Prevention voted unanimously to recommend that people with weakened immune systems get a third shot of either the Pfizer-BioNTech or Moderna vaccine.
The vote follows Thursday night's authorization of such use by the Food and Drug Administration.
The CDC is expected to endorse the committee's action later Friday. Providers generally wait for a CDC recommendation on vaccine use, even if the FDA has approved or authorized a vaccine.
Immunocompromised people make up about 2.7 percent of U.S. adults, or around 7 million people. They are more likely to get severely ill from COVID-19, and they are at higher risk for prolonged coronavirus infection and transmission. They also have a lower antibody response to the initial vaccine regimen and are more likely to transmit the virus to household contacts, according to studies CDC officials presented to the committee.
People with conditions that weaken the immune system or who are taking treatments that suppress immunity are also far more likely to have breakthrough infection than people in more normal health. One U.S. study shows 40 percent to 44 percent of hospitalized breakthrough cases are in immunocompromised people.
Studies have also shown the vaccine is less effective in these people, ranging from 59 percent to 72 percent. That compares with 90 percent to 94 percent among people without serious immune deficiencies. People who've received a solid organ transplant have the lowest immunity after the standard vaccine regimen.
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The CDC will further clarify the conditions for which the recommendation is being made once the agency formally accepts the committee's advice.
Some of the situations presented to the committee for consideration, which are likely to be officially recommended by the CDC, include:
Active or recent treatment for solid tumor and hematologic malignancies.
Receipt of solid organ or recent hematopoietic stem cell transplants.
Severe primary immunodeficiency.
Advanced or untreated HIV infection.
Chronic medical conditions such as asplenia and chronic renal disease that may be associated with varying degrees of immune deficiency.
Receipt of CAR-T-cell or hematopoietic stem cell transplant (within two years of transplantation or taking immunosuppression therapy).
Moderate or severe primary immunodeficiency (for example, DiGeorge, Wiskott-Aldrich syndromes).
Active treatment with high-dose corticosteroids (for example, greater than or equal to 20 milligrams of prednisone or equivalent per day), alkylating agents, antimetabolites, transplant-related immunosuppressive drugs, cancer chemotherapeutic agents classified as severely immunosuppressive, TNF blockers, and other biologic agents that are immunosuppressive or immunomodulatory.
The recommendation is limited to adults 18 and older for the Moderna vaccine because that vaccine has not been authorized for adolescents yet. That authorization is expected from the FDA in the next few weeks. The Pfizer vaccine is authorized for adolescents 12 and older and adults.
People with other conditions are not being advised to get an additional dose at this point. "This would not include long-term care facility residents, persons with diabetes, persons with heart disease — those types of chronic medical conditions are not the intent here," the CDC's Dr. Amanda Cohn said. Additional studies of how long immunity lasts in healthy people are underway and will determine the timing of any additional doses for the general population, both FDA and CDC officials said.
The CDC said attempts should be made to match the additional dose type to the one used in the original set of shots someone receives. But if that is not feasible, an additional dose with the other vaccine is permitted. The additional dose should be administered at least 28 days after completion of the primary vaccine series, according to the data reviewed by the committee.
On Thursday, the FDA said that the agency was unable to extend the authorization for an additional dose to the Johnson & Johnson vaccine due to insufficient data. Representatives of both agencies said they are "actively engaged" to determine the best course of action for recipients of the Johnson & Johnson vaccine.
In a presentation to the committee, the CDC's Dr. Kathleen Dooling said immunocompromised people, including those who receive an additional dose, should continue to follow prevention measures, including wearing a mask, staying 6 feet apart from others they don't live with and avoiding crowds and poorly ventilated indoor spaces until their health care provider advises otherwise.
Close contacts of immunocompromised people should be strongly encouraged to be vaccinated against COVID-19, she said.
Separately, the CDC said about 1.1 million people who do not necessarily have immune system dysfunction have already received one or more additional doses of the Moderna or Pfizer vaccines. About 91,000 have received one or more additional doses of the Johnson & Johnson vaccine.
The committee's action comes during an intense new surge driven by the delta variant. Since July 1, the number of confirmed cases has gone up 700%, according to the CDC.
According to the CDC, emerging experimental and observational data in adults suggest that an additional COVID-19 vaccine dose with the Moderna or Pfizer vaccine enhances antibody response and increases the proportion of immunocompromised patients who respond to the vaccine.
In her review of the evidence, Dooling said that small studies of an additional dose of the Pfizer or Moderna vaccine found no serious adverse events. However, the standard dose regimen of these vaccines has been associated with rare but serious adverse events, including anaphylaxis and myocarditis and pericarditis in young adults. The impact of immunocompromising conditions on these rare events is unknown, she said.
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