Q: I feel like I’m doing something wrong with the 1-dose J&J vaccine… like I’m cheating. Is it REALLY as good as the 2-dose vaccines?
A: Yes, it’s really as good! The J&J vaccine prevented hospitalization and death from COVID-19 extremely well in the trials.
The decision to go with 1 dose was based on scientific observations, not just guessing. Vaccine developers went with the 1-dose schedule after early testing showed great immune responses after one dose. Specifically, drug developers found good immunity in the weeks after one dose, and even stronger immune response as time went on–suggesting that a single dose is all that is needed. This may be due to the way the vaccine works. It uses a different way into the body than the mRNA vaccines.
When scientists are developing a new vaccine, one of the early testing stages often explores the results of different dosing strategies. The J&J developers found that 1 dose was best during early testing. In fact, they found a very strong immune response that went up in the weeks after the first shot. So that was the best dosing, and the one they picked for the larger phase 3 efficacy trial–and ultimately what was authorized by the FDA.
It doesn’t make sense to compare the dosing schedules of different vaccines with different technologies and convince ourselves that if 1 is good, 2 must be better. But just in case, J&J is now performing a trial of a 2-dose regimen, and we’ll see if a 2nd dose can provide improved protection. This may make a difference especially for non-severe infections, new variants, or both–or, it may not. If they do find that a 2nd dose provides more protection, those of us who got the J&J vaccine may need to get need a booster shot down the line. But that’s all a lot of if’s right now. They picked 1 dose in the first place because 1 dose worked great.
The next stage of development was a large phase 3 clinical trial to test how well the vaccine prevented severe COVID-19 outcomes. Johnson & Johnson-Janssen measured how well the vaccine worked at 28 days after the one dose. There’s a discrepancy between what the CDC used to define “fully vaccinated” and the timing of the clinical trial data from J&J. CDC may have been trying to simplify the (already very difficult) guidance for fully vaccinated people by using 2 weeks across the board. We are left to resolve this ourselves. Personally, this Nerdy Girl (Malia) will consider herself “fully vaccinated” at 28 days after getting the J&J shot.
And remember that it doesn’t make sense to compare the trial results of different vaccines head-to-head either. They were tested in different places, at different times, with different study designs and even different definitions for key outcomes like what “severe disease” means.
In a nutshell: 1 dose of the J&J shot was very good at preventing severe outcomes of COVID-19 and pretty good at preventing symptomatic illness completely. Avoid the temptation to compare it directly to other vaccines that are also working really well. It just isn’t a sensible comparison and won’t lead us to sound conclusions.