From PopSci.com (Sept. 23, 2021):
Johnson & Johnson just released new data about a booster shot regimen for their single-dose vaccine, including important initial efficacy stats. The FDA and CDC have yet to evaluate this information, and there are still plenty of questions unanswered, but here is what we know right now.
Do I need a booster if I got the J&J shot?
You are still protected against COVID-19 with just that first shot, but new evidence released directly from J&J indicates that a second booster shot protects you a whole lot more. A first dose had a recorded 75 percent efficacy rate of protection against severe/critical COVID-19 globally, but after a second dose, this efficacy extends to moderate symptomatic cases, too. If you have reason to suspect you had a depressed immune system response to your first shot, perhaps as an elderly or immunocompromised individual, getting a second dose can only help.
How much does a booster shot of J&J help?
The data we have currently is still preliminary, and based solely on J&J’s press release. Based on that information, it seems that the efficacy of a two-dose regimen—with the second shot given 56 days after the first—rises to 94 percent against moderate to severe/critical COVID-19 and 75 percent worldwide. However, it’s worth noting that the participants were followed for around 36 days each, versus the roughly four months used for the initial trial. More time would likely yield more COVID cases.
It’s also unclear how the timing of the second dose might impact the efficacy. Booster shots given to study participants six months after their initial dose prompted a much greater increase in antibody levels than did boosters given two months after the first, which could suggest that waiting longer could yield better results. But it’s not clear how much antibody levels relate to protection against disease. [read more]