A booster shot of the Johnson & Johnson COVID-19 vaccine generates a “rapid and robust” increase in antibodies, the company said Wednesday, citing early study data.

Johnson & Johnson said its Phase 1/2a studies found that people who received a booster had spike-binding antibodies nine-fold higher than what was seen 28 days after the initial shot. The company previously said that studies showed its single-shot vaccine still provided strong protection against COVID-19 eight months after initially being administered.

In anticipation of the possibility that the vaccine’s effectiveness would decline over time, Johnson & Johnson launched two booster shot studies. 

“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, global head of Janssen Research & Development at Johnson & Johnson, said in a news release. “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.”

The company said it is engaging with regulators in the United States and Europe to discuss offering the boosters.

Last week, President Joe Biden announced plans to offer booster shots starting in September to all Americans who received the Pfizer and Moderna vaccines, pending approval from the Food and Drug Administration and the Centers for Disease Control and Prevention. Federal health officials said at the time they were awaiting data from Johnson & Johnson.