The advisory panel next week plans to focus on the 2 to 4 percent of U.S. adults who have suppressed immunity, a population that includes organ transplant recipients, people on cancer treatments and people living with rheumatologic conditions, HIV and leukemia.

The Advisory Committee on Immunization Practices, which makes vaccine recommendations to the Centers for Disease Control and Prevention, is scheduled at a July 22 meeting to discuss the clinical considerations involved with giving additional doses to immunocompromised patients.

Pfizer and the German firm BioNTech announced last week they planned to seek regulatory approval for a booster from the FDA because they anticipated some people would need a third shot of the companies’ two-dose regimen six to 12 weeks within being fully inoculated.

Next week’s discussion won’t address the request for regulatory approval. Instead, it will focus on updating clinical considerations for coronavirus vaccines, which doesn’t require an advisory panel vote. Because the vaccines were authorized last year, committee members have regularly updated guidelines for health-care professionals and public health officials based on disease trends and data.

Federal health officials are in discussions to work out legal and regulatory issues about how such a change could take place. Health officials are making a distinction between boosters for people who already have an immune response after a primary series vs. additional doses for immunocompromised individuals who don’t have much of a response after initial doses.

Francis S. Collins, director of the National Institutes of Health, said he does not view giving immunocompromised people an additional dose of vaccine as a booster.

“I wouldn’t call that a booster,” Collins said. “I would call that trying to get the primary immunization into the effective place.”

Transplant physician Kotton said: “From my perspective as someone who takes care of immunocompromised people, including organ transplant recipients, who seem to have a diminished response to vaccination, and based on the data we have seen recently … it does seem like an additional dose of vaccine would be appropriate for this population. And as far as we can tell, it would be safe and likely to augment immunity.”

She cited a June 23 New England Journal of Medicine study of 101 organ transplant patients in France that showed the rate of antibodies against the coronavirus was 4 percent after the first dose of the Pfizer-BioNTech vaccine, 40 percent after the second dose and 68 percent after the third dose, showing the third dose dramatically increased the likelihood of developing a disease-fighting response.

For people who showed no protection after the second dose, 44 percent developed an antibody response after the third dose. The study author still recommends infection-control measures until there is better understanding of the extent of protection, Kotton said, “but we would anticipate that their protection would be more robust after the third dose of vaccine.”

Patients who did not develop an antibody response were older, had a higher degree of immunosuppression and had lower kidney function than patients who had an antibody response, according to the study. Kotton said clinicians may need to look at measures beyond a third dose for some of these more vulnerable patients, including medications to prevent infections and standard infection-control measures such as masks, social distancing and hand hygiene.

Medical experts said the data is limited on whether a booster shot confers a substantial increase in protection against the virus.

“I haven’t seen evidence that a booster would be indicated for anybody, including the immunocompromised,” said Helen Boucher, an infectious-disease physician at Tufts Medical Center in Boston.

“A lot of that data is still emerging,” Boucher said, noting that studies that focus solely on disease-fighting antibodies don’t provide a full picture. Clinical data and long-term follow-up will provide critical information, she said. The immune system also includes B cells, which make antibodies, and T cells, which can recognize pathogens and kill infected cells.

The best way to improve protection for immunocompromised individuals, Boucher said, is to surround them with vaccinated people.

“Vaccinating as many people as we can, and as fast as we can, is how we’re going to stop this thing,” Boucher said.

Although health officials remain split on the necessity of a booster, Pfizer has continued to move forward with its plans. The company’s announcement last week that it would seek an emergency-use authorization for a booster prompted health officials to declare that Americans do not need a third shot at this time and that science will dictate when it might be appropriate to recommend one.

Israel announced this week it would begin administering a third dose of the Pfizer vaccine to severely immunocompromised adults, and Britain has announced a plan for booster shots to begin in September, targeted first to groups that include people who are immunosuppressed, people older than 70 and front-line health-care workers.

Pfizer met with U.S. health officials Monday to explain why it believed some Americans would need a booster in coming weeks. It cited data from Israel showing a rise in infections among those who are fully vaccinated, and interim data from the company’s trial of its booster shot showing a third dose stimulates an antibody response five to 10 times the level seen after the second dose of its vaccine.

But scientists remain mixed on whether available data indicates a booster is effective and provides substantial additional protection.

Joel Achenbach and Ben Guarino contributed to this report.