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

• There are now three COVID-19 vaccines authorized for use in the US.
• Pfizer's and Moderna's are mRNA-based, while Johnson & Johnson's is an adenovirus.
• Dr. Anthony Fauci explained how they differ but have the same "end game."

The US now has authorized three coronavirus vaccines: two mRNA options from Pfizer and Moderna, and one adenovirus shot from Johnson & Johnson.

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

When asked which shot might provide people the best long-term protection from infection, including protection against worrisome virus variants like B.1.351, Florian Krammer, a leading virus expert, told Insider on Monday, "There's little that can be said, for now."

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

During a White House briefing on Monday, President Joe Biden's chief medical advisor, Dr. Anthony Fauci, explained exactly how each shot takes effect.

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

Pfizer's and Moderna's vaccines train the body to fight COVID-19 by injecting mRNA, or messenger RNA, into a person's deltoid, the rounded muscle that hugs the upper arm and shoulder.

"The mRNA that's injected into the muscle codes for the spike protein," Fauci said.

The coronavirus' spike protein is what allows the virus to latch on to and invade our cells. But the mRNA vaccines train our bodies to say, immunologically, "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

J&J's shot injects viral DNA, not mRNA. This is a key reason 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's and Moderna's shots.

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

That adenovirus, called Ad26, 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, where it's copied into messenger RNA. That mRNA then codes for coronavirus spike proteins in the same way as Pfizer's or Moderna's shots.

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

The "ultimate end game" of 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," he said.

To be clear, he added, it's not the virus, "it's the protein."

That prepares a vaccinated person's body to fight off a COVID-19 infection if the person comes into contact with the virus.

Both the mRNA and the adenovirus vaccines appear to prevent the most severe COVID-19 infections very well: Pfizer's and Moderna's two-shot courses were more than 94% effective in their trials, while J&J's was 85% effective in preventing severe disease and death.

Adenovirus vaccines like J&J's might give people a more robust form of immunity against viral variants, with antibody and T-cell responses. (mRNA vaccines may provide only more narrow antibody protection.) But that remains to be seen as more people get vaccinated and 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 to help end the pandemic.