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.

What exactly do these numbers mean?

Vaccine efficacy is a measurement of the percent of reduction of illness among people who’ve been vaccinated.

According to the Centers for Disease Control and Prevention (CDC), the calculation for vaccine efficacy is: risk among unvaccinated group – risk among vaccinated group/ risk among unvaccinated group

Among people who received the J&J vaccine in the clinical trial, there was a 66.3 percent reduction in the number of cases of mild to moderate COVID-19, compared with those who did not receive it.

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.

The advantage of a single dose

As we discussed earlier, the early clinical trials of the J&J vaccine found that one dose produced a robust and stable immune response in most people. That’s why larger-scale clinical trials proceeded with a single-dose vaccine regimen.

The Pfizer and Moderna vaccines produced a lower immune response after one dose. As such, a second dose was added to help boost the immune response. Some real-world data indicate that these vaccines offer good protection after one dose, but more research is needed.

Clinical trials are also underway to assess the efficacy of a two-dose J&J vaccine.

Another advantage of a single-dose COVID-19 vaccine is that it can help speed up the rate of vaccination. When this happens, it can greatly help to reduce cases of COVID-19, thereby reducing hospitalizations and deaths.

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.

Why was the Johnson & Johnson vaccine paused for a while?

On April 13, 2021, the FDA and the CDC recommended a pause on J&J vaccinations. This pause was so instances of rare but serious blood clots could be further investigated.

These blood clots were found in the large blood vessels of the brain or other parts of the body and were associated with a low platelet count. The medical term for this condition is thrombosis with thrombocytopenia syndrome (TTS).

Most instances of TTS occurred in female adults between the ages of 18 and 48 years. Symptoms appeared about a week or two after receiving the J&J vaccine. In some cases, TTS resulted in death.

On April 23, 2021, the FDA and CDC recommended resuming J&J vaccinations. After a thorough review, it was determined that the benefits of the vaccine outweighed its potential risks.

Overall, the J&J vaccine is both safe and effective. The CDC estimates that TTS happens in 7 out of 1 million female adults between the ages of 18 and 49 that receive the J&J vaccine. Male and female adults ages 50 and over are at an even lower risk.

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.