Pfizer and Moderna have shared that their vaccines are more than 90 percent effective in protecting the body from a COVID-19 infection. But what does that mean, exactly?


The Food and Drug Administration (FDA) has already authorized two COVID-19 vaccines in the U.S. for use by the general public. Vaccine candidates from both Pfizer and Moderna have shown promising results in large-scale clinical trials, and health systems across the country are now rolling out these vaccines to the masses.

FDA Authorization of a COVID-19 Vaccine Is Imminent

It's all exciting news — especially after dragging through about a year of #pandemiclife — but it's only natural to have questions about the efficacy of the COVID-19 vaccine and what, exactly, this means for you.

How does the COVID-19 vaccine work?

There are two major vaccines getting attention in the U.S. right now: One is made by Pfizer, and the other by Moderna. Both companies are using a new type of vaccine called messenger RNA (mRNA).

These mRNA vaccines work by encoding a part of the spike protein that's found on the surface of SARS-CoV-2, the virus that causes COVID-19, according to the Centers for Disease Control and Prevention. Instead of putting an inactive virus in your body (as done with the flu vaccine), mRNA vaccines use pieces of that encoded protein from SARs-CoV-2 to prompt an immune response from your body and develop antibodies, explains infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security.

Your body eventually eliminates the protein and the mRNA, but the antibodies have staying power. The CDC reports that more data is needed to confirm just how long antibodies built from either vaccine will last. (Related: What Does a Positive Coronavirus Antibody Test Result Really Mean?)

Another vaccine coming down the pipeline is from Johnson & Johnson. The company recently announced its application to the FDA for emergency use authorization of its COVID vaccine, which works a little differently than the vaccines created by Pfizer and Moderna. For one thing, it's not an mRNA vaccine. Rather, the Johnson & Johnson COVID-19 vaccine is an adenovector vaccine, which means it uses an inactivated virus (adenovirus, which causes the common cold) as a carrier to deliver proteins (in this case, the spike protein on the surface of SARS-CoV-2) that your body can recognize as a threat and create antibodies against. (More here: Everything You Need to Know About Johnson & Johnson's COVID-19 Vaccine)

How effective is the COVID-19 vaccine?

Pfizer shared in early November that its vaccine is "more than 90 percent effective" at protecting the body from COVID-19 infection. Moderna has also revealed that its vaccine is specifically 94.5 percent effective at protecting people from COVID-19.

For context, there hasn't been an mRNA vaccine approved by the FDA before. "There are no licensed mRNA vaccines to date as this is a new vaccine technology," says Jill Weatherhead, M.D., assistant professor of tropical medicine and infectious diseases at Baylor College of Medicine. As a result, there is no available data, on the efficacy or otherwise, adds Dr. Weatherhead.

That said, these vaccines and the technology behind them have "been rigorously tested," Sarah Kreps, Ph.D., a professor in the department of government and an adjunct professor of law at Cornell University, who recently published a scientific paper on the factors that could influence U.S. adults' willingness to get the COVID-19 vaccine, tells Shape.

In fact, the CDC reports that researchers have been studying mRNA vaccines for "decades" in early-stage clinical trials for influenza, Zika, rabies, and cytomegalovirus (a type of herpesvirus). Those vaccines haven't made it past the initial stages for a number of reasons, including "unintended inflammatory outcomes" and "modest immune responses," according to the CDC. However, recent technological advancements "have mitigated these challenges and improved their stability, safety, and effectiveness," thus paving the way for COVID-19 vaccines, according to the agency. (Related: Can the Flu Shot Protect You from Coronavirus?)

As for Johnson & Johnson's adenovector vaccine, the company said in a press release that its large-scale clinical trial of nearly 44,000 people found that, overall, its COVID-19 vaccine was 85 percent effective in preventing severe COVID-19, with "complete protection against COVID-related hospitalization and death" 28 days after vaccination.

Unlike mRNA vaccines, adenovector vaccines such as Johnson & Johnson's aren't a novel concept. Oxford and AstraZeneca's COVID-19 vaccine — which was approved for use in the EU and the UK in January (the FDA is currently waiting on data from AstraZeneca's clinical trial before considering U.S. authorization, the New York Times reports) — uses similar adenovirus technology. Johnson & Johnson also used this technology to create its Ebola vaccine, which has been shown to be safe and effective in producing an immune response in the body.

What does this mean for you?

Saying that a vaccine is 90 percent (or more) effective sounds great. But does this mean the vaccines prevent COVID-19 or protect you from severe illness if infected — or both? It's a little confusing.

"[Moderna's and Pfizer's] trials were really designed to demonstrate efficacy against symptomatic disease, whatever those symptoms may be," says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. Basically, the high percentages of effectiveness suggest that you can expect to not have symptoms of COVID-19 after you're fully vaccinated (both Pfizer's and Moderna's vaccines require two doses — three weeks between shots for Pfizer, four weeks between shots for Moderna), explains Dr. Russo. And, if you do still develop a COVID-19 infection after vaccination, you likely won't experience a severe form of the virus, he adds. (Related: Can the Coronavirus Cause Diarrhea?)

While the vaccines appear to be "highly effective" in protecting the body from COVID-19, "we're now trying to figure out if they also prevent asymptomatic spread," says Dr. Adalja. Meaning, the data currently shows that the vaccines can greatly reduce the odds you'll develop symptoms of COVID-19 (or, at least, severe symptoms) if you come into contact with the virus. But the research doesn't currently show whether you can still contract COVID-19, not realize you have the virus, and pass it on to others post-vaccination.

With that in mind, it's "unclear at this point" whether the vaccine will stop people from spreading the virus, says Lewis Nelson, M.D., professor and chair of emergency medicine at Rutgers New Jersey Medical School and chief of service in the emergency department at University Hospital.

Bottom line: "Can this vaccine result in the elimination of the virus entirely, or protect us from symptomatic illness? We don't know," says Dr. Russo.

Also, the vaccines haven't been studied in large numbers of children, nor in pregnant or breastfeeding women, making it hard for doctors to recommend a COVID-19 vaccine to those populations at the moment. But that's changing, as "Pfizer and Moderna are enrolling children age 12 and older," says Dr. Weatherhead. While "efficacy data in children remains unknown," "there is no reason to think [the effect] will be significantly different than what the [current] studies show," adds Dr. Nelson.

Overall, experts urge people to be patient and to get vaccinated when they can. "These vaccines are going to be part of the solution to the pandemic," says Dr. Adalja. "But it will take some time for them to roll out and to see all the benefits they offer."

The information in this story is accurate as of press time. As updates about coronavirus COVID-19 continue to evolve, it’s possible that some information and recommendations in this story have changed since initial publication. We encourage you to check in regularly with resources such as the CDC, the WHO, and your local public health department for the most up-to-date data and recommendations.