Johnson & Johnson has said 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In two clinical studies that have not yet been peer-reviewed, the additional shot evoked a nine-fold increase in antibodies to the coronavirus, the company said in a news release.

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, according to AFP.