WASHINGTON — Federal regulators are expected to authorize a third shot of coronavirus vaccine as soon as Thursday for certain people with weakened immune systems, an effort to better protect them as the highly contagious Delta variant sweeps the nation.
The decision to expand the emergency use of both the Pfizer-BioNTech and Moderna vaccines is meant to help those patients with immune deficiencies who are considered most likely to benefit from an additional shot. It covers people who have had solid organ transplants and others whose immune systems are similarly compromised, according to an official familiar with the plan.
The development will give physicians latitude to recommend additional shots for those patients. At least 3 percent of Americans have weakened immune systems for a variety of reasons, from a history of cancer to the use of certain medications such as steroids.
Many scientists argue that the immunocompromised population is too diverse to uniformly recommend additional shots of coronavirus vaccine. Some may be protected by the standard vaccine dosage, despite their conditions. Others may be poorly shielded by the vaccines, but unable to benefit from an additional shot.
Studies suggest that patients such as organ transplant recipients are in between — often showing little immune response to the standard vaccine regimen, but benefiting from a third shot. One recent randomized, placebo-controlled study by Canadian researchers found that a third dose of the Moderna vaccine improved the immune response of people in that group.
Dr. Dorry Segev, at the Bloomberg School of Public Health at Johns Hopkins University, has said about half of transplant patients have no antibody response to the currently authorized vaccine dosage. His team studied 30 transplant patients who were vaccinated but had negative or low-positive antibody titers, which suggested a poor response by their immune systems to the shots. After an additional shot, 14 of them had higher antibody titers.
The Food and Drug Administration’s decision to authorize a third shot for organ transplant recipients and those with similarly compromised immune systems will be considered by an advisory committee to the Centers for Disease Control and Prevention, scheduled to meet on Friday. Although the F.D.A.’s action is independent of the panel’s recommendation, in practice many physicians wait to act until the C.D.C. weighs in.
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If the committee votes to endorse the shots, as expected, the C.D.C. could issue a recommendation the same day. That could give further guidance to physicians and pharmacists about how to proceed.
France has offered additional vaccine doses to certain people with poor immune responses since April, and Germany and Hungary recently followed suit. After considering several regulatory pathways, health officials in the Biden administration decided that expanding the emergency use authorizations for Pfizer and Moderna was the fastest route to reach those most in need. The F.D.A. is also expected to grant full approval to Pfizer-BioNTech’s vaccine by early next month, a move that also would give physicians more freedom to prescribe additional doses.
Although the vast majority of Americans who have been vaccinated received Pfizer or Moderna shots, it was not immediately clear how those with immune deficiencies who received Johnson & Johnson shots were to proceed.
