According to two federal officials familiar with the situation, the Food and Drug Administration is expected to announce this week that people can receive coronavirus vaccine booster shots that differ from their initial doses.

The agency is rushing to make the announcement as part of its approval of boosters for the Moderna and Johnson & Johnson vaccines on Wednesday. According to two federal officials who spoke on the condition of anonymity because they were not authorized to discuss the issue, the FDA may also say that people should generally stick to the same vaccine if possible.

Because Johnson & Johnson’s vaccine has been shown to be less effective than the Pfizer-BioNTech and Moderna vaccines, researchers are working hard to improve protection for those who receive the single-shot Johnson & Johnson regimen: Should it be the Johnson & Johnson shot or the other vaccines that use a different technology?

A key point of contention: What should the correct dose be if Moderna is used as a booster for Johnson & Johnson? According to some officials, it should be half the dose of the regular shot — the dosage that will be permitted for Moderna boosters in general — while others argue that it should be the full dose, which was tested as part of a National Institutes of Health study published last week.

“Generally, people should get the same vaccine as their initial series,” said one federal official familiar with the situation. However, the official stated that people in nursing homes may not have had access to their original vaccine, or that some people may have had a negative reaction to the mRNA vaccines, which were used to create the Pfizer-BioNTech and Moderna shots.

Last week, an FDA advisory committee discussed NIH mix-and-match data, but the number of people in the studies was small and they were only followed for a short period of time. The FDA provided committee members with documents that included an agency review of Johnson & Johnson’s tests of a second dose of its own vaccine and a separate preprint study that tested mixing booster doses from different companies. The data could provide a roadmap for the 15 million people in the United States who received the Johnson & Johnson vaccine, many of whom have felt left out because the vast majority of vaccine recipients in the United States received either the Pfizer-BioNTech or Moderna vaccine.

According to the findings, recipients of the Johnson & Johnson coronavirus vaccine may benefit from a second dose of the original vaccine, they may benefit from even greater protection if the boost is provided by a different vaccine technology. The study found that a second dose of the Moderna vaccine resulted in the greatest increase in virus-neutralizing antibodies in Johnson & Johnson recipients, with a 76-fold increase in antibody levels. Antibody levels were increased 35-fold by a Pfizer booster. Only a fourfold increase was elicited by a matching Johnson & Johnson booster.

The association of the Johnson & Johnson vaccine with a very rare clotting side effect that tended to be more common in women under 50 years old is one issue that may influence real-world decisions about how to use boosters. If health-care providers have the freedom to recommend any booster, they could recommend a messenger RNA dose as a second shot to people who may be at increased risk for an adverse event.