Without robust data on the long-term effectiveness of the Johnson & Johnson vaccine, it is difficult for health officials to recommend booster vaccinations, said John Moore, a virologist at Weill Cornell Medicine. “If you are making data-driven decisions and you don’t have the data, what can you do?” He said. “It’s kind of a dilemma. Public trust in vaccines generally depends on how the sausage is made, as it is a data-driven, transparent process. “

Clinical studies conducted prior to the distribution of the Delta variant found that the Johnson & Johnson vaccine was 72 percent overall effective in the United States, less than the approximately 95 percent effectiveness of the Pfizer and Moderna vaccines. However, direct comparisons between vaccines tested in different places and at different times are difficult.

All available vaccines seem to be becoming less effective against Delta, which may evade some antibodies of the immune system. The Johnson & Johnson vaccine is no exception. “You would expect there to be some resistance to Delta because there always is,” said Dr. Moors.

Small laboratory studies have produced conflicting results about how well the Johnson & Johnson vaccine protects against Delta. Last month, Johnson & Johnson said a single dose of its vaccine produced a strong immune response against Delta and that the reaction lasted for at least eight months.

Updated

Aug. 20, 2021, 8:20 p.m. ET

But data from another recent laboratory study suggested that a single dose of the vaccine produced a relatively weak antibody response to Delta, which could make boosters more important.

The first real data on the vaccine’s effectiveness against the variant were released this month. The data, which are preliminary results from a clinical study of nearly 500,000 healthcare workers in South Africa, suggest that a single dose of the vaccine has an effectiveness of up to 96 percent against deaths and 71 percent against hospitalizations due to infection Delta had.

It was “a very extensive analysis and very clear results that showed that the single-shot J. & J. Vaccine provided significant protection against the Delta variant, ”said Dr. Dan Barouch, a virologist at Beth Israel Deaconess Medical Center in Boston who conducted studies for Johnson & Johnson but was not involved in the South Africa study.