Dr. Fauci has a stunningly simple way to explain how Johnson & Johnson's COVID-19 vaccine differs from Pfizer and Moderna shots

• There are now 3 different COVID-19 vaccines authorized for use in the US.
• Two are mRNA-based (Pfizer, Moderna), while one is an adenovirus (Johnson & Johnson).
• Dr. Fauci explained how they differ in action, but said "the ultimate end game" is the same. 
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The US now has three authorized vaccines to fight the pandemic: two mRNA options from Pfizer and Moderna, and one adenovirus shot from Johnson & Johnson. 

While it's true that both Moderna's and Pfizer's shots were more effective, overall, than J&J's in their trials, experts have stressed that all of these vaccines share two very important statistics in common: zero hospitalizations, and zero deaths among fully vaccinated trial participants. 

And when asked which shot might provide people with the best long-term protection from infections, including protection against worrisome new virus variants like B.1.351, leading virus expert Florian Krammer told Insider on Monday:

"There's little that can be said, for now." 

But, we do already know there are key differences in the way these two types of vaccines work. 

During a White House COVID-19 briefing on Monday, President Biden's Chief Medical Advisor, Dr. Anthony Fauci, laid bare exactly how each shot takes effect.

Shots from Pfizer and Moderna give the body genetic instruction manuals to learn how to fight the coronavirus, safely 

Pfizer and Moderna's vaccines train the body to learn how to fight against COVID-19 by injecting mRNA (messenger RNA) into a person's deltoid - the rounded muscle that hugs your upper arm and shoulder.

"The mRNA that's injected into the muscle codes for the spike protein," as Fauci explained it on Monday.

The coronavirus's spike protein is what would otherwise allow the virus to latch on to and invade our cells. But the mRNA vaccines train up our bodies to immunologically say, "not so fast."

"The body sees that [protein] and makes an immune response against that, giving you the protection that has been shown with both of the mRNA vaccines," Fauci added. 

J&J inserts a harmless cold virus to do the same job

The J&J shot, in contrast, injects viral DNA (not mRNA) into a person's arm. This is a key reason why J&J's vaccine is so much easier to manufacture and to store in the fridge: the DNA inside is not as fragile as the single-stranded mRNA in Pfizer and Moderna's shots. 

That J&J DNA is encapsulated inside a "harmless, non-replication competent virus," Fauci said.

The virus in question, called Adeno26, is a common cold virus that has had its illness-causing genes removed, so it can't get you sick. 

Once the vaccine is injected into a person's arm, the virus injects its DNA into cells. There, they are copied into messenger RNA. That mRNA then codes for coronavirus spike proteins in the same way that Pfizer or Moderna's shots would have.

Both types of vaccines result in the same 'end game' for the virus

So, "the ultimate end game" of both adenovirus and mRNA COVID-19 vaccines is the same, Fauci said. 

"Both of the vaccines ultimately result in a spike protein in the right conformation that gives the body the opportunity to feel that this is the actual virus that it's seeing."

To be clear, "it's not [the virus], it's the protein," Fauci said. 

That all prepares a vaccinated person's body to fight off any eventual COVID-19 infection, if the vaccinated person ever comes into contact with the virus again. 

The good news is that both mRNA and adenovirus vaccines appear to fight against the most severe COVID-19 infections very well: both Pfizer and Moderna's 2-shot courses were more than 94% effective in their trials, while J&J's was 85% effective against severe disease and death.

There's also some speculation that adenovirus vaccines like J&J's may give people a broader, more robust form of immunity against viral variants, with both a demonstrated antibody and a T-cell response (mRNA vaccines may only provide more narrow antibody protection.) But that all still remains to be seen, as more people get vaccinated, and virus variants continue to spread. 

Either way, experts agree that all three authorized vaccines need to work in concert, in order to blunt the spread of the virus across the US, and help end the pandemic.