What to Know About Johnson & Johnson One-Shot Covid-19 Vaccine

The Food and Drug Administration is expected to soon grant an emergency use authorization (EUA) to Johnson & Johnson’s covid-19 vaccine, possibly as early as this weekend. That decision would make it the third such vaccine to become available to Americans. So it’s as good a time as any to go through the similarities and differences between J&J’s vaccine and the two already out from Moderna and Pfizer/BioNTech.

The most important similarity between all three is their high effectiveness at preventing life threatening illness from covid-19. In a FDA analysis of clinical trial data from around 40,000 volunteers released Wednesday, J&J’s vaccine was found to be 77% effective at preventing severe to critical illness 14 days after vaccination and 85% effective at preventing severe to critical illness 28 days later. Overall, it was deemed to be 66% effective at preventing moderate to severe illness 14 days or more out.

The topline numbers aren’t quite as impressive as what we saw from clinical trials of the mRNA vaccines developed by Moderna and Pfizer/BioNTech, both of which are upwards of 90% effective at preventing any symptoms. But the most crucial goal of any vaccine should be to keep people alive and free from serious complications of its target illness—a goal that J&J’s version seems to accomplish very well. In the clinical trials, there were no covid-related hospitalizations or deaths seen in vaccinated people after 28 days. It’s likely that none of these vaccines will be 100% effective at preventing hospitalizations or deaths, but they will all greatly reduce the chances of it happening.

The J&J vaccine does have its own clear advantages. Namely, it’s only one shot, unlike the two-dose schedule over a month recommended for either mRNA vaccine. There is ongoing research testing whether a second shot, taken two months later, could boost its effectiveness. But for now, it’s the one-shot version that people will first get access to, assuming it’s authorized by the FDA. It’s also more stable and durable at warmer temperatures, meaning it can be stored in a typical fridge for longer and without the immediate worry of spoilage once taken out. (That said, Moderna and even Pfizer’s vaccine don’t seem to need ultra-cold storage as originally thought.)

If you’re worried about the novelty of the mRNA technology behind the other vaccines (which, to be clear, is actually three decades old), the J&J vaccine might make you feel more comfortable due to its familiarity. It works by using a neutered adenovirus (meaning it can’t make more of itself like normal) to carry the DNA for the coronavirus spike protein, the key the virus uses to invade cells. The dummy virus gets inside a cell and prompts the body to marshal an immune response specific to the spike protein—one that should train it against the real thing.

The use of viral vectors as a delivery system in medicine is older than the mRNA method and comes with a longer track record of safety. Though, like the mRNA vaccines, this would be the first adenovirus-based vaccine to see wide use. Adenovirus vaccines do have their potential limitations, such as preexisting immunity to the virus used that could weaken vaccine effectiveness. In the case of the J&J vaccine, it’s using a type of adenovirus that less commonly infects people, which is meant to work around those limitations. And because it was tested in different parts of the world, with similar success, that should relieve concerns that its effectiveness will wildly vary from country to country.

The J&J vaccine may slightly win out when it comes to potential side effects too. The most common adverse events linked to the vaccine were injection site pain (48.6%), headache (38.9%), fatigue (38.2%), and myalgia (33.2%). By contrast, more than 84% of people given the Pfizer/BioNTech vaccine experienced soreness around the injection site after the first or second shot, and 60% experienced fatigue. Potentially severe adverse events were rare for the J&J vaccine, and there was no difference in frequency between people given the vaccine or placebo, after accounting for potential covid-19 symptoms (0.4% in each group), which should further indicate its safety.

As with other vaccines, people under the age of 60 were more likely to experience any symptoms at all, which is probably because of a stronger upfront immune response. There was a possible but still rare risk of allergic reaction to the vaccine, but only five patients in the study who received the vaccine had any reaction shortly after the shot and none experienced a severe reaction that’s known as anaphylaxis. (One patient in the placebo group had a reaction.)

In its review, the FDA stated that more data would have to be collected before it’s clear whether allergic reaction is a real risk from the vaccine. But otherwise, it endorsed the vaccine as safe and effective, stating that their analysis “supported a favorable safety profile with no specific safety concerns identified that would preclude issuance of an EUA.”

Another consideration is that J&J’s vaccine was tested in the U.S., South America, and South Africa, during a time when important new variants of the coronavirus have started to turn up. The data suggests that the vaccine should still provide good, if slightly diminished, protection against the variant first found in South Africa, likely the most worrying variant of them all. And it may also offer some protection against asymptomatic infections, which is a great sign that it will cut down on transmission.

On Thursday, the FDA’s Vaccines and Related Biological Products Advisory Committee will meet to discuss the trial data and provide their recommendation for an emergency use authorization or not. And should things go as expected, the FDA will authorize it soon after. Once authorized, J&J has said they’ll be ready to ship almost 4 million doses immediately. By the end of March, it’s said it will have 20 million doses available—a timeline similar to Moderna/Pfizer’s early planned distribution. By then, all three companies have pledged to make 240 million more doses in total available to the public. Currently, 1.4 million Americans on average a day are being vaccinated.

In short, the road to mass vaccination is likely to get less bumpy over time, especially if other candidates enter the picture later this year as expected. And you shouldn’t worry too much about which specific vaccine to get when you’re finally eligible, since they’ll all protect you from the worst covid-19 can do (though J&J’s vaccine might be the most convenient for some). More importantly, they’ll all help end this pandemic as soon as possible.