Box of Johnson & Johnson vaccine doses. Credit: CBS Johnson & Johnson said Wednesday that a second shot of its COVID-19 vaccine eight months after the initial dose was found in a study to increase people’s immunity to the disease. The drugmaker has been testing the efficacy of boosters in people who were previously injected with its single-dose vaccine. The latest findings show that individuals who received an additional shot saw a ninefold increase in antibody levels, according to J&J. The company said researchers observed a “rapid and robust” increase in antibodies in people ages 18 to 55 and in those 65 years and older who received a lower booster dose. While the company’s single-shot vaccine stimulates a strong immune response, “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,” said Dr. Mathai Mammen, global head of research and development at Johnson & Johnson’s Janssen Pharmaceuticals, in a statement. “We look forward to discussing with public health officials a potential strategy for our Johnson & Johnson COVID-19 vaccine, boosting eight months or longer after the primary single-dose vaccination,” he added. J&J said it is working with the Food and Drug Administration, U.S. Centers for Disease Control and Prevention, European Medicines Agency and other health agencies on rolling out booster shots. The Food and Drug Administration announced Monday that it has granted full approval to Pfizer and BioNTech for their COVID-19 vaccine to be given to Americans as young as 16. Roughly 13.8 million Americans have received J&J’s coronavirus vaccine. Although studies suggest that the degree of immunity conferred by the Pfizer and Moderna vaccines fades over time, those studies have excluded J&J’s shot. Yet several studies — ranging from antibody research that has yet to be peer-reviewed to anecdotal reports of more hospitalizations in outbreaks among vaccinated people — have raised concerns that J&J’s vaccine offers weaker protection to the highly contagious Delta variant. The White House said last week that Americans who received Pfizer and Moderna shots should get booster shots eight months after their second dose, and at the same time advised that J&J vaccine recipients would eventually also need an additional shot. With the Delta variant ravaging large swaths of the southern U.S. and COVID-19 cases rising in other states, more employers and government entities are moving to require that workers get vaccinated. Attorneys say the FDA’s approval of the Pfizer-BioNTech vaccine this week will likely spur many businesses to mandate vaccines.