• Johnson & Johnson said Wednesday a booster shot of its Covid-19 vaccine generated a promising immune response in early stage clinical trials.
• J&J recipients who received a booster dose of the shot generated virus-fighting antibodies "nine-fold higher" than those seen four weeks after a single dose.
• While promising, the company's press release made no mention of the booster shots potential impact on the delta variant or on safety.

Johnson & Johnson said Wednesday a booster shot of its Covid-19 vaccine generated a promising immune response in early stage clinical trials – though the information provided by the company in a press release was light on some details.

J&J's vaccine requires only one dose and recipients are considered fully vaccinated two weeks after receiving the shot. The company said Wednesday that J&J recipients who received a booster dose of the shot generated virus-fighting antibodies "nine-fold higher" than those seen four weeks after a single dose.

Increases in antibody responses were observed in vaccine trial participants between ages 18 and 55, the company said, and in those 65 years and older who received a lower dosage of the booster shot.

The results are based on two Phase 1/2 studies, according to the company.

"We have established that a single shot of our COVID-19 vaccine generates strong and robust immune responses that are durable and persistent through eight months," Dr. Mathai Mammen, head of research and development at J&J's Janssen vaccine arm, said in a statement.

"With these new data, we also see that a booster dose of the Johnson & Johnson COVID-19 vaccine further increases antibody responses among study participants who had previously received our vaccine," he added.

While the new data is promising, the company's press release made no mention of the booster shots' potential impact on the coronavirus delta variant or on safety.

When asked about data on delta, J&J referred CNBC to a report in July that showed a single dose of the vaccine generated a promising immune response to the variant.

It also raises questions about why J&J recipients need booster shots – especially after the July report showed that a single shot of its vaccine provides immunity that lasts at least eight months and appears to deliver adequate protection against the fast-spreading delta variant.

To be sure, the Centers for Disease Control and Prevention has said J&J recipients will probably need a booster dose but added it doesn't have enough data right now to support a formal recommendation.

The company said Wednesday it is engaging with the Food and Drug Administration and other health authorities regarding booster shots.

The new data comes less than a week after J&J announced that Alex Gorsky was stepping down as CEO. Gorsky, 61, who was chairman and CEO for nine years, will become executive chairman.