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. About 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.
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.
Understand the State of Vaccine and Mask Mandates in the U.S.
- Mask rules. The Centers for Disease Control and Prevention in July recommended that all Americans, regardless of vaccination status, wear masks in indoor public places within areas experiencing outbreaks, a reversal of the guidance it offered in May. See where the C.D.C. guidance would apply, and where states have instituted their own mask policies. The battle over masks has become contentious in some states, with some local leaders defying state bans.
- Vaccine rules . . . and businesses. Private companies are increasingly mandating coronavirus vaccines for employees, with varying approaches. Such mandates are legally allowed and have been upheld in court challenges.
- College and universities. More than 400 colleges and universities are requiring students to be vaccinated against Covid-19. Almost all are in states that voted for President Biden.
- Schools. On Aug. 11, California announced that it would require teachers and staff of both public and private schools to be vaccinated or face regular testing, the first state in the nation to do so. A survey released in August found that many American parents of school-age children are opposed to mandated vaccines for students, but were more supportive of mask mandates for students, teachers and staff members who do not have their shots.
- Hospitals and medical centers. Many hospitals and major health systems are requiring employees to get a Covid-19 vaccine, citing rising caseloads fueled by the Delta variant and stubbornly low vaccination rates in their communities, even within their work force.
- New York. On Aug. 3, Mayor Bill de Blasio of New York announced that proof of vaccination would be required of workers and customers for indoor dining, gyms, performances and other indoor situations, becoming the first U.S. city to require vaccines for a broad range of activities. City hospital workers must also get a vaccine or be subjected to weekly testing. Similar rules are in place for New York State employees.
- At the federal level. The Pentagon announced that it would seek to make coronavirus vaccinations mandatory for the country’s 1.3 million active-duty troops “no later” than the middle of September. President Biden announced that all civilian federal employees would have to be vaccinated against the coronavirus or submit to regular testing, social distancing, mask requirements and restrictions on most travel.
The F.D.A. decided not to broaden emergency use of the single-dose vaccine, at least in part because health officials wanted to see Johnson & Johnson’s clinical trial data on the safety and efficacy of two doses first, according to one official. Johnson & Johnson is expected to release the results of its two-dose study this month.
Some Biden administration officials are debating whether vulnerable segments of the general population will need booster shots to shore up their protection against Covid-19 — a decision that a number of scientists and public experts argue cannot be justified by current data.
Any such strategy would be based on a different risk-benefit calculus because the vaccines have proven extremely effective against severe disease or hospitalization for those without immune deficiencies. Some ordinary citizens are already seeking booster shots on their own, without waiting for an F.D.A. decision or a C.D.C. recommendation.
Dr. Dan Barouch, a virologist at Beth Israel Deaconess Medical Center in Boston who worked with Johnson & Johnson as it developed its vaccine, said the F.D.A.’s move to make third shots available to some with weakened immune systems made sense. But he said a similar approach could be attractive to physicians and patients: getting a different kind of vaccine altogether.
Dr. Barouch said there was a long and successful history of mixing and matching different vaccine platforms for other viruses. Because of the similarities in how coronavirus vaccines were produced, he said, mixing was particularly appealing, since it could potentially boost and broaden one’s immunity.
There is no data yet to determine whether protection against the virus is enhanced by such an approach, Dr. Barouch said. But researchers at the National Institutes of Health have a study underway that is expected to deliver some answers by early fall.