Federal officials have twice rejected requests to discuss the proposal, and pharmaceutical companies — even acknowledging the likely efficacy of the plan — have declined to design or manufacture the shots, according to a Times investigation. The lack of interest in launching development of immunity shots comes amid heightened scrutiny of the federal government’s sluggish pandemic response.

There is little disagreement that the idea holds promise; the dispute is over the timing. Federal health officials and industry groups say the development of plasma-based therapies should focus on treating people who are already sick, not on preventing infections in those who are still healthy.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the NIH, said an upper-arm injection that would function like a vaccine “is a very attractive concept.”

However, he said, scientists should first demonstrate that the coronavirus antibodies that are currently delivered to patients intravenously in hospital wards across the country actually work. “Once you show the efficacy, then the obvious next step is to convert it into an intramuscular” shot.

But scientists who question the delay argue that the immunity shots are easy to scale up and should enter clinical trials immediately. They say that until there’s a vaccine, the shots offer the only plausible method for preventing potentially millions of infections at a critical moment in the pandemic.