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The Johnson & Johnson vaccine introduces a weakened virus to the body to protect individuals from COVID-19.

March 15, 2021

One and Done: The Science Behind the Johnson & Johnson COVID-19 Vaccine

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With only one required dose, the newly authorized Johnson & Johnson COVID-19 vaccine has the potential to dramatically accelerate nationwide vaccine distribution, joining the likes of the Pfizer-BioNTech and Moderna two-dose vaccines. 

Clinical trials of the Johnson & Johnson vaccine yielded 72 percent efficacy against moderate to severe illness from COVID-19, compared to Pfizer’s and Moderna’s roughly 95 percent efficacy rates.

This difference in efficacy — though concerning for some Americans at face-value — doesn’t reflect the whole truth, according to Prof. John Moore, microbiology and immunology at Weill Cornell. 

Moore explained that it’s hard to directly compare efficacy among the three vaccines since the clinical trials for each used different populations at different times throughout the pandemic. But the main takeaway, Moore emphasized, is that all three of the vaccines are similarly effective at preventing hospitalizations and deaths.

How Johnson & Johnson gets away with one dose

Unlike the mRNA vaccines that train the body to produce antibodies against the coronavirus, the Johnson & Johnson viral vector vaccine introduces the body to DNA inside a genetically-modified, weakened common cold virus called an adenovirus.

The main step in eliminating pathogens from the body, antibodies are vital proteins that identify and counteract foreign substances.

After infecting a cell, the weakened adenovirus allows cells to produce pieces of the coronavirus antigen — the spike protein — using the cell’s machinery to make and display the protein on the cell’s surface. This modified virus cannot replicate inside cells, cause illness or harm cells.

An important mechanism in immune defense, antigens provoke an immune response in the body, triggering the production of antibodies.

According to Moore, the Pfizer and Moderna COVID-19 vaccines require two doses because the double dose produces a stronger antibody response than a single one. For coronavirus variants, strong protection is particularly important against the more contagious strains of COVID-19. 

“The concern about one dose is how long its protection will last,” Moore said. “We don’t yet know because we can only really tell in the long run.”

Concern over the Johnson & Johnson vaccine’s efficacy

Moore explained that all approved vaccines, including Johnson & Johnson, prevent serious illness and death. However, there is only preliminary data suggesting that the vaccines also protect against asymptomatic infection.

Although individuals with asymptomatic COVID-19 don’t feel ill, Moore explained that they still can transmit infection to others. If the goal is herd immunity, people must be both protected from infection and unable to pass it on. 

But since asymptomatic infection is difficult to track, Moore said that most vaccine trials primarily measure whether a vaccine prevents symptomatic infection, as well as severe disease and death. On the latter measures, the Pfizer, Moderna and Johnson & Johnson vaccines are relatively comparable.

In its clinical trials, though, the Johnson & Johnson vaccine was reported to be 72 percent effective at preventing symptomatic COVID-19 infection, while Moderna and Pfizer were roughly 95 percent effective. 

Vaccine efficacy is determined by monitoring infections in clinical trials, comparing patients who received placebo and vaccine doses. If a trial reported 100 infections and 72 of those were participants who received a placebo, the vaccine would be deemed 72 percent effective.

“That’s quite a lot of people who are going to get symptomatic infection,” Moore said. “Mild, sure, but potentially transmissible to others.”

Moore argued that one benefit of the Johnson & Johnson vaccine is that since it only requires one dose, it requires less discipline from recipients. This could be significant for children — if the vaccine is eventually approved for them — and young adults, who may be less reliable in following up with a second appointment and could have difficulty securing access to transportation.

Moore added that a vaccine with slightly lower efficacy is generally of less concern to younger people than older people. The younger population is at lower risk of severe illness from COVID-19, so widespread use of the Johnson & Johnson vaccine among young people could potentially accelerate the world’s journey toward herd immunity.

Reaching herd immunity 

While the Pfizer and Moderna vaccines require ultracold freezers for storage and transport due to the fragility of mRNA, the Johnson & Johnson vaccine’s simple handling is a major advantage.

According to Moore, the Johnson & Johnson vaccine can be stored at a typical refrigeration temperature of 4 degrees Celsius, making it easier to distribute to harder-to-reach areas.

For people in rural areas and underserved communities, traveling a far distance to a vaccine site once is much easier than traveling twice, Moore said.

“The downside is that some people are saying ‘It’s not as potent, don’t give me this, I want the good stuff,’ which is a human reaction,” Moore said. “You can understand why people choose to say that, even if they’re not being terribly accurate on the science.”

Moore emphasized it is important for all people to receive a vaccine when they become eligible. Society’s best chance at getting fully back to normal, Moore said, is getting a substantial percentage of the population vaccinated.

“The key message,” Moore said, “is if you’re offered a vaccine, take a vaccine. Each of them is better than nothing, and a lot better than contracting COVID-19.”