The Johnson & Johnson (J&J) vaccine is a single-dose vaccine that can help prevent COVID-19. It’s the third COVID-19 vaccine authorized by the Food and Drug Administration (FDA) for emergency use in the United States.

Large-scale clinical trials in several locations across the globe found that a single dose of the J&J vaccine was effective at preventing COVID-19. They found that the vaccine can also protect against several viral variants.

Below, we’ll break down everything we know so far about the efficacy of the J&J COVID-19 vaccine. Continue reading to learn more.

The efficacy of the J&J vaccine was tested in a clinical trial involving more than 40,000 people. The FDA reviewed this data, along with manufacturing information provided by Johnson & Johnson, when deciding to grant emergency use authorization.

The results from this clinical trial found that the J&J vaccine could protect against mild to moderate and severe to critical cases of COVID-19.

Let’s take a closer look.

Mild to moderate COVID-19

In most people, COVID-19 is a mild or moderate illness. Many individuals can recover from mild to moderate COVID-19 at home.

J&J vaccine’s efficacy for mild to moderate COVID-19 14 days after vaccination was:

  • Overall: 66.3 percent
  • United States: 74.4 percent
  • Brazil: 66.2 percent
  • South Africa: 52 percent

Severe to critical COVID-19

The World Health Organization (WHO) estimates that 20 percent of people with COVID-19 experience a severe to critical illness that often requires hospitalization.

J&J vaccine efficacy for severe to critical COVID-19 14 days after vaccination was:

  • Overall: 76.3 percent
  • United States: 78 percent
  • Brazil: 81.9 percent
  • South Africa: 73.1 percent

Additionally, individuals receiving the J&J vaccine were less likely to be hospitalized than those who didn’t receive the vaccine. In fact, the J&J vaccine was 93 percent effective at preventing hospitalization for COVID-19 14 days after vaccination and 100 percent effective 28 days after.

How long the J&J and Moderna vaccines will protect against COVID-19 is currently unknown. Research suggests that the Pfizer vaccine will offer protection for 6 months, but studies are ongoing.

Earlier clinical trials of the J&J vaccine found that a single dose of the vaccine produced a strong immune response in most people. Measurements taken 71 days later found that levels of antibodies remained stable or had increased.

Overall, more research is needed to determine how long the J&J vaccine will protect against COVID-19. It’s possible, but not certain, that a booster may be needed in the future.

You may be wondering how the efficacy of the J&J vaccine stacks up against the other two COVID-19 vaccines authorized for use in the United States. These are the mRNA vaccines produced by Pfizer and Moderna.

Data from clinical trials of the Pfizer vaccine found an efficacy of 95 percent 7 days after the second dose. Clinical trial data of the Moderna vaccine reported an efficacy of 94.1 percent 14 days after the second dose.

These numbers may seem much higher than those of the J&J vaccine. However, it’s important to note that it’s not possible to make direct comparisons between the three COVID-19 vaccines. This is because the clinical trials:

  • used different protocols
  • included various pools of people across many geographic locations
  • were carried out at different points in the pandemic

The FDA notes that the only way to directly compare the efficacy of the three vaccines is to conduct a head-to-head clinical trial. This has not yet been done.

The CDC currently doesn’t recommend the use of one vaccine over another.

Preliminary data from the J&J vaccine clinical trial indicates that the J&J vaccine can protect against asymptomatic COVID-19. Asymptomatic means that a person doesn’t have symptoms but may still be able to spread the SARS-CoV-2 virus to others.

In the J&J vaccine clinical trial, 2,650 trial participants were tested using a COVID-19 antigen test 71 days after vaccination. This test identified 18 asymptomatic SARS-CoV-2 infections in the vaccinated group and 50 in the unvaccinated group.

Based on this data, researchers estimated that the efficacy of the J&J vaccine for preventing asymptomatic COVID-19 was at least 66 percent. This is comparable to the vaccine’s overall efficacy for preventing mild to moderate COVID-19.

The Pfizer and Moderna vaccines can also prevent asymptomatic COVID-19. Current research indicates that the vaccines can significantly reduce the rate of asymptomatic COVID-19.

It’s completely normal for viruses to mutate. When this occurs, new variants can emerge.

So far, several different variants of the novel coronavirus have been identified. Some of these have caused concern amongst public health officials. This is because the variants may be more contagious or escape the immune response.

The J&J vaccine clinical trial can give us some indication of how effective the vaccine is against these variants. This is because the trial was carried out at a time when viral variants were very common in some locations.

At the time of the trial, 95 percent of sequenced COVID-19 cases in South Africa were caused by the B.1.351 variant. The efficacy of the J&J vaccine at this location was 52 percent for mild to moderate COVID-19 and 73.1 percent for severe to critical COVID-19.

Similarly, in Brazil, 69 percent of the sequenced COVID-19 cases were caused by the P.2 lineage. The efficacy of the J&J vaccine in Brazil was 66.2 percent for mild to moderate COVID-19 and 81.9 percent for severe to critical COVID-19.

Compared with the two mRNA vaccines, the clinical trial of the J&J vaccine included more people of racial or ethnic backgrounds that may be at a higher risk of COVID-19. This is important, as it helps give researchers a better idea of the vaccine’s efficacy across many different racial and ethnic groups.

The table summarizes the demographics of the three vaccine trials.

Johnson & JohnsonPfizer-BioNTechModerna
Hispanic or Latino45.1%26.2%20%
Black or African American17.2%10%9.7%
Native American8.3%less than 1%less than 1%
Asian or Asian American3.5%4%4.7%
Pacific Islander or Native Hawaiian0.3%less than 1%less than 1%
Multiple races5.4 percentNot reportedNot reported

People who experienced a severe or immediate allergic reaction to any of the ingredients in the J&J vaccine should not receive the J&J vaccine. The information sheet for recipients and caregivers includes a list of the ingredients.

The CDC notes that female adults under the age of 50 should be aware of the risk of TTS following J&J vaccination. If TTS is a concern, the other COVID-19 vaccines aren’t associated with this very rare side effect.

The J&J vaccine is the third COVID-19 vaccine authorized by the FDA for emergency use in the United States. It consists of a single dose that’s been found to be both safe and effective at preventing COVID-19.

Clinical trials found that the J&J vaccine can prevent many types of COVID-19. Its overall efficacy was 66.3 percent for mild to moderate COVID-19 and 76.3 percent for severe to critical COVID-19. Additionally, it provides 100 percent protection from hospitalization for COVID-19 28 days after vaccination.

The J&J vaccine can also protect against asymptomatic COVID-19, as well as some viral variants. It’s currently unknown how long the vaccine protection will last. A booster shot may be needed in the future.

The efficacy of the three authorized COVID-19 vaccines can’t be compared directly, as the trials took place at different locations and at different times in the pandemic. Currently, no one vaccine is recommended over the others.