COVID-19: Vaccine Safety and Efficacy

Important information

As of April 13, 2021, Wisconsin providers have paused giving the Johnson & Johnson (Janssen) COVID-19 vaccine. We have paused giving it because a very small number of people who got it had a very rare side effect – a blood clot.

I received the Johnson & Johnson (Janssen) vaccine, what do I need to do?

  • Understand that these severe events are incredibly rare.
  • Monitor for the following symptoms up to three weeks after you vaccination: severe headache, abdominal pain, leg pain, or shortness of breath. Symptoms associated with these severe events have occurred six to 13 days after receiving the vaccine.
  • If you develop any of these symptoms, contact your health care provider.
  • Use the v-safe program to report your symptoms.

 A vaccinated person has a far lower risk of getting severely sick, being hospitalized, or dying from COVID-19, no matter which vaccine they get.

Available COVID-19 vaccines

There are currently two COVID-19 vaccines available in the United States: the Pfizer vaccine and the Moderna vaccine. As of April 13, 2021, Wisconsin providers have paused giving the Johnson & Johnson (Janssen) COVID-19 vaccine. We have paused giving it because a very small number of people who got it had a very rare side effect – a blood clot. Other COVID-19 vaccines are in development. You can find the other COVID-19 vaccines that are in clinical trial phases II or III through the COVID-19 vaccine tracker. All the available vaccines are very safe and great at preventing hospitalization and death from COVID-19.

Pfizer

Authorized for individuals age 16 years and older.

Type: mRNA

Dose: 2 doses 21 days apart

Efficacy: Decreased risk of moderate to severe COVID-19 disease by 95%. Reduced the risk of hospitalization by 100%.

Source: Centers for Disease Control and Prevention (CDC)


Moderna

Authorized for individuals age 18 years and older.

Type: mRNA

Dose: 2 doses 28 days apart

Efficacy: Decreased risk of moderate to severe COVID-19 disease by 94.1% efficacy. Reduced the risk of hospitalization by 89%

Source: Centers for Disease Control and Prevention (CDC)


Janssen (Johnson & Johnson): Currently Paused

Authorized for individuals age 18 years older.

Type: viral vector

Dose: 1 Dose (it takes 28 days after vaccination for your body to build immunity)

Efficacy: Decreased risk of moderate to severe COVID-19 disease by 72% in the US and 64% in South Africa, where a highly contagious variant is causing the most cases. The vaccine also reduced risk of severe disease by 86% across all regions studied and reduced the risk of hospitalization and death by 100%.

Source: early data released by Johnson & Johnson

Vaccine efficacy and effectiveness

In clinical trials, researchers measure how well a vaccine prevents illness in a controlled setting. This is called efficacy. The efficacy of COVID-19 vaccines in clinical trials are impressive, showing the vaccine works at preventing symptomatic infection (feeling sick and testing positive for COVID-19).

After the FDA approves a vaccine, it continues to be studied to determine how well it works in the real world, outside of clinical trials. This is called effectiveness. The effectiveness of COVID-19 vaccines is continually studied as the vaccine is rolled out.

Efficacy example  95% efficacy does not mean that 95 out of every 100 people vaccinated will be protected from COVID-19. It does mean that the vaccine reduces the amount of disease by 95% for vaccinated people, compared to people who haven't gotten vaccinated.

Different types of vaccines

All vaccines help enhance our body’s own immune systems, so that when we are exposed to a virus, our immune systems are ready to fight it and can keep us healthy.

mRNA vaccines

mRNA vaccines give our cells instructions on how to make a protein that triggers an immune response. Our immune response produces antibodies that protect us from getting infected. mRNA vaccines do not put virus into our bodies and cannot give someone COVID19. Our cells break down and get rid of the mRNA as soon as it is finished using the instructions. The vaccine does not affect or interact with our DNA in any way.

  • Pfizer
  • Moderna

Viral vector vaccines

Viral vector vaccines use a safe, modified version of a different virus (the vector) to deliver important instructions to our cells. For COVID-19 viral vector vaccines, the vector will enter a cell in our body and will make a harmless piece of the virus (like the spike protein), so our immune system can learn how to fight it. At the end of the process, our bodies have learned how to protect us against future infection with the virus that causes COVID-19.

  • Janssen (Johnson & Johnson)
  • Oxford/AstraZeneca (not available in the United States)

Protein-based vaccines

Protein-based vaccines show the body a protein from the virus (like the COVID-19 spike protein), so our immune systems learn to recognize and fight it. Learn more about the different types of COVID-19 vaccines from the CDC.

  • Novavax (not available in the US)
  • Sanofi (not available in the US)

Developing COVID-19 vaccines

Developing new vaccines is a thorough and careful process. Safety is the top priority during all phases of vaccine development, approval, and use.

 Clinical trials

Before the U.S. Food and Drug Administration (FDA) can approve a vaccine, it has to be tested through clinical trials to make sure it is safe and see how well it prevents illness.

  • COVID-19 vaccines are approved for use under an Emergency Use Authorization (EUA) and were tested in large clinical trials to make sure they met well established safety standards and are continuing to be assessed and evaluated for safety with the inclusion of new populations such as children.
    • The participants of the trials included people of different ages, races, and ethnicity, as well as those with different medical conditions to make sure it works across the board.
    • Researchers enrolled far more clinical trial participants than is usually required, so they could notice and address safety concerns more easily and quickly.
  • The Food and Drug Administration (FDA), the Centers of Disease Control (CDC), and independent advisors review all vaccine safety and efficacy data before any vaccine is authorized for use.
  • CDC and FDA continue to monitor vaccine safety and effectiveness after they are approved or authorized. Medical experts are still learning whether COVID-19 vaccines are safe for children and pregnant people and how well vaccines prevent you from spreading the virus that causes COVID-19 to others, even if you do not get sick.

Learn more about medical experts ensure vaccines work both in clinical trials and the real world by visiting the CDC’s website.

 Emergency Use Authorization (EUA)

During a public health emergency, such as the COVID-19 pandemic, FDA can authorize safe and effective medical devices, treatments and vaccines more quickly using Emergency Use Authorization (EUA). EUAs help the necessary medical products get authorized more quickly while still being based on the best available science.

COVID-19 vaccines are going through all the same steps in the trial phases that all vaccines go through.

The EUA program has existed since 2004 and has helped public health officials and physicians use new products to detect, prevent, or treat a disease during emergencies. For example, the FDA issued EUAs for respirators, medications, and tests during the 2009 swine flu pandemic, 2014 Ebola epidemic, and now in the COVID-19 pandemic.

EUAs end when the Secretary of Health and Human Services decides the emergency has ended, or if the medical product receives typical approved.

Authorization vs. Approval
  • Vaccines authorized under an Emergency Use Authorization (EUA) can be used in the public, but might not have fully completed their phase 3 trials. EUAs end when the emergency has ended, or if the medical product is fully approved or licensed.
  • A COVID-19 vaccine can still get a Biologics License (full FDA approval) after is has gotten an EUA. The FDA and Advisory Committee on Immunization Practices (ACIP) will review the data again to approve and recommend the vaccine.

 

Learn more about EUAs by visiting FDA's website.

 Continuous safety monitoring

There are systems in place that track common side effects and rare adverse (or serious) reactions in people who receive any vaccine, including COVID-19 vaccines. CDC and FDA monitor these systems in real time to make sure safety issues are detected as quickly as possible.

V-safe is a new smartphone-based tool that makes it easy for people to report any health problems or serious reactions after they get their COVID-19 vaccines.

Learn more about v-safe by visiting the CDC’s website.


Frequently asked questions

See answers to frequently asked questions about COVID-19 vaccines, safety, and efficacy by opening the tabs below.

How do we know the COVID-19 vaccines are safe and work?

Vaccine approval is driven by science. The Food and Drug Administration (FDA), the Centers of Disease Control (CDC,) and independent advisors review all vaccine safety and efficacy data before any vaccine is approved and allowed to be used. COVID-19 vaccines are going through the same steps in the trial phases that all vaccines go through to get the full FDA vaccine license and approval.

During emergencies, like the COVID-19 pandemic, the FDA can issue an emergency use authorization (EUA) to let people get a vaccine before all the trials are complete. The FDA will only give a COVID-19 vaccine an EUA if the current phase III trial data shows the vaccine is safe. Like with all vaccines, after a COVID-19 vaccine is made available to the public, the FDA and CDC will continue to closely monitor the vaccine to help ensure any issues are immediately addressed.

COVID-19 vaccines were tested in large clinical trials to make sure they meet safety standards.

Visit CDC’s website for more information about the safety of COVID-19 vaccines.


Learn more about COVID-19 vaccines by checking out our publications.

 

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What kind of safety monitoring is being done to monitor for any adverse reactions?

There are a number of systems in place to monitor reactions to COVID-19 vaccines.

  • The Vaccine Adverse Event Reporting System (VAERS) is the national system that collects reports from healthcare professionals, vaccine manufacturers, and the public on adverse events that happen after vaccination. Reports of adverse events that are unexpected, appear to happen more often than expected, or have unusual patterns are followed up with specific studies. Providers are required to report any adverse events to this system.
  • V-safe, a smartphone-based after vaccination health checker program through the CDC, uses text messages and web surveys to check in with people who received a COVID-19 vaccine for the first 14 days after getting vaccinated. Text messages are also sent as reminders to receive the second vaccine dose if you received a vaccine that is part of a 2-dose series (such as Pfizer or Moderna).
  • Vaccine Safety Data Link is a network of nine integrated health care organizations across the United States that conduct active surveillance and research. The system is also used to help determine whether possible side effects identified through VAERS are actually related to vaccination.

Note: Vaccinators are instructed to monitor people for a short time (around 15-30 minutes) after getting vaccinated to quickly address any immediate reactions.

Are vaccines safe for pregnant women and nursing mothers?

CDC and the independent Advisory Committee on Immunization Practices (ACIP) have provided information to assist pregnant people with their decision to receive the COVID-19 vaccine. Learn more on CDC’s website.

How were COVID-19 vaccines developed so quickly?

No safety steps were skipped during the development of COVID-19 vaccines. A number of factors allowed safe and effective COVID-19 vaccines to be available quickly.

  • Earlier research on other coronaviruses (like SARS and MERS) jump started the COVID-19 vaccine development process. SARS was first detected in 2002, while MERS was first detected in 2012, and both have been researched ever since.
  • Through ground-breaking partnerships between leading medical experts, scientists were able to build on lessons learned from past pandemics (Zika, Ebola, H1N1) to make the COVID-19 vaccines.
  • Medical experts and other key players all dedicated their time, effort, and resources to developing a COVID-19 vaccines.
  • Issuing an Emergency Use Authorization (EUA) shortened the official process, but did not skip any safety steps. For example, the FDA invited more experts to review than usual. With all hands on deck, the review process was shortened from months to weeks. The EUA allowed faster public access to vaccines.

Learn more about the vaccine development process by checking out our publications.

 

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Infographic for the vaccine lifecycle
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How well do COVID-19 vaccines work?

Clinical trial data show all authorized COVID-19 vaccines are very good at preventing severe illness, hospitalization, and death. COVID-19 vaccines continue to be studied to determine how well they work in the real world, outside of clinical trials. The CDC will be sharing more of this data soon. 

 

How do COVID-19 mRNA vaccines work?

Vaccines work by triggering our immune system to recognize certain bacteria or viruses and build immunity to defend against them if they return.

Both the Pfizer and Moderna vaccines contain mRNA that instruct our body to make the coronavirus spike protein, which is harmless on its own. The spike protein is the same protein that is found on the surface of the SARS-CoV-2, the virus that causes COVID-19.

By copying the spike protein, our immune system is able to produce specific antibodies that can defend against SARS-CoV-2. This helps protect us from getting infected if the real virus enters our bodies because our immune system can now recognize the spike protein to defend itself from infection.

You can learn more about how COVID-19 mRNA vaccines work on CDC’s website.

Is the COVID-19 vaccine a live vaccine?  

No. The Pfizer and Moderna COVID-19 vaccines are made with mRNA technology. The Johnson & Johnson vaccine is a viral vector vaccine, which means that it contains just a part of the virus that causes COVID-19 inserted into the backbone of an adenovirus vector. This vector poses no threat of causing illness in humans.

What ingredients are in the COVID-19 vaccines?

Both the Pfizer and the Moderna vaccine contain the following ingredients:

  • mRNA (also known as messenger ribonucleic acid)
  • Lipids
  • Salts
  • Sugar

They do NOT contain fetal cells, blood products, the SARS-CoV-2 virus, mercury, egg, latex, pork products, preservatives, or microchips.

See full lists of ingredients for the Pfizer and Moderna vaccines.

Are COVID-19 vaccines safe for people with underlying medical conditions?

Yes. People with underlying conditions can get the COVID-19 vaccine as long as they are not allergic to any of the ingredients in the vaccines .When checking, it is important to look at the detailed list linked above. Vaccination is important for people with underlying medical conditions because they may be at an increased risk of severe illness from COVID-19.

Visit CDC’s website about vaccine considerations for people with underlying medical conditions to make an informed decision.

Is there risk of a severe allergic reaction with the COVID-19 vaccine?

Adverse reactions are possible, although they are very rare. Systems, like VAERS and v-safe, are in place to monitor for reactions to COVID-19 vaccines. This is why you may be asked to wait 15 minutes after receiving the COVID-19 vaccine so any reactions can immediately be addressed.

Will COVID-19 vaccines protect against any new strains of COVID-19?

Based on initial evidence all three authorized vaccines effectively reduce the risk of COVID-19 for all of the circulating variants.

Last Revised: April 16, 2021