Here’s Why Johnson & Johnson’s Vaccine Only Requires One Dose

Unlike vaccines from Moderna and Pfizer, this vaccine uses a viral vector

Photo: Pavlo Gonchar/SOPA Images/LightRocket via Getty Images

We may soon have a third Covid-19 vaccine. On Wednesday, the Food and Drug Administration (FDA) released a new analysis showing that Johnson & Johnson’s vaccine is both safe and effective at preventing severe symptoms of coronavirus infection.

The vaccine’s efficacy was 72% in the United States and 64% in South Africa, where a more contagious variant has taken hold. This Friday, an FDA advisory committee made up of outside experts will vote on whether the agency should authorize the shot for emergency use. Its approval is highly anticipated because it only requires one dose.

So how does Johnson & Johnson’s vaccine work?

Whereas Moderna and Pfizer’s vaccines use mRNA technology, Johnson & Johnson’s vaccine uses a viral vector — a modified virus that can’t cause infection. Viruses are good at getting inside cells, making them an attractive way to deliver a vaccine. Specifically, Johnson & Johnson’s vaccine is made with a virus called Adenovirus 26.

Scientists first modify the adenovirus by removing its genetic material. Then, they replace it with a gene that instructs cells to make the “spike” protein of the coronavirus. This spike protein is found on the surface of the SARS-CoV-2 and helps it gain entry into our cells. Once our cells start making the spike, the immune system recognizes it as foreign and starts making antibodies against it.

Johnson & Johnson partnered with Beth Israel Deaconess Medical Center in Boston last year to develop the vaccine after early tests led by Dan Barouch, MD, PhD, looked promising.

AstraZeneca’s Covid-19 vaccine, which was developed with researchers at the University of Oxford, uses a similar approach, except with a modified version of a chimpanzee adenovirus.

Viral vector vaccines have been studied in human clinical trials before, including for Ebola. Johnson & Johnson’s Covid-19 shot borrows from its Ebola vaccine, which became the first adenovirus vaccine approved for human use in July 2020.

According to the Centers for Disease Control and Prevention, adenoviruses are being explored for vaccines because they can induce a robust immune response. Last year, Michael Diamond, MD, PhD, a viral immunologist at the Washington University School of Medicine, told me that adenovirus vaccines are good at generating a high titer of antibodies because viral vectors look like real viruses to the immune system. That’s why Johnson & Johnson’s vaccine only requires one dose.

One downside of using adenoviruses for vaccines is that they might not work for some people. Adenoviruses are incredibly common — estimated to be responsible for between 2% and 5% of all respiratory infections, according to the World Health Organization — so some people have preexisting immunity to them. Those who have been previously infected with an adenovirus may have antibodies against it. Upon encountering an adenovirus viral vector, their antibodies would attack it, preventing the vaccine from getting to their cells.

There are lots of adenoviruses though, and some are less common than others, making them a better option for a vaccine. “You have to try to find the right adenovirus that is going to work for the most people in a given area,” Diamond said.



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Emily Mullin

Emily Mullin

Former staff writer at Medium, where I covered biotech, genetics, and Covid-19 for OneZero, Future Human, Elemental, and the Coronavirus Blog.