WASHINGTON - Johnson & Johnson said Wednesday a second dose of its COVID-19 vaccine boosted immunity beyond the first dose.
In two clinical studies that have not yet been peer-reviewed, the additional shot evoked a nine-fold increase in antibodies to the coronavirus, the company said in a news release.
President Joe Biden's administration has announced plans for all Americans vaccinated with mRNA vaccines -- Pfizer and Moderna -- to receive a third dose eight months after the second in the series.
The Centers for Disease Control and Prevention (CDC) has said it expects a second dose of the J&J vaccine will be required, but that it needs more data before making a firm recommendation. Wednesday's announcement will likely help inform that strategy.
The company's news release did not specify when the second dose was taken, but information about the trials on government websites shows the company was testing multiple intervals, beginning at six months.
The news comes after the CDC announced the effectiveness of COVID-19 vaccines overall have fallen after the hyper-contagious Delta variant became dominant.
In an ongoing study of US health workers who were overwhelmingly vaccinated with the Pfizer and Moderna shots, effectiveness against all forms of infection fell from 91 percent prior to Delta to 66 percent.
But protection against severe COVID-19 and deaths has remained more stable, exceeding 90 percent according to most studies.
© Agence France-Presse