Vaccine basics

COVID-19 Vaccines

COVID-19 Vaccines

As the COVID-19 pandemic continues to impact our lives and our community, we look forward to learning more about the COVID-19 vaccines that will soon be available.

Getting vaccinated against COVID-19 will be one of the best ways to protect yourself and others around you.

Below are the answers to frequently asked questions about COVID-19 vaccines. This information will continue to be updated as MCW receives new information from the CDC, DHS and other infectious disease experts.

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COVID-19 Vaccines What Experts Say About Safety

COVID-19 Vaccines: What Experts Say About Safety


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COVID-19 Vaccines Community Protection2

COVID-19 Vaccines: Community Protection


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COVID-19 Vaccines What to Expect

COVID-19 Vaccines: What to Expect


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Herd Immunity with Antoinette Spector, PT, DPT

Public and community health researcher Dr. Antoinette Spector shares her knowledge on herd immunity and community protection, and how the concepts apply to the COVID-19 pandemic.

Comparing Johnson & Johnson/Janssen COVID-19 Vaccine to Pfizer and Moderna

MCW associate professor Dr. Ben Weston shares his knowledge on the benefits, development technology, and the safety and efficacy of the Johnson & Johnson/Janssen COVID-19 vaccine.

Scientists Discuss why Coronavirus Variants are Concerning with COVID-19

Molecular biologist Dr. Nathan Ledeboer and infectious disease specialist Dr. Mary Beth Graham share their knowledge on coronavirus mutations, the impact they can have on our health and how we can prevent variants from spreading by wearing masks and social distancing.

What we know about the COVID-19 vaccine today

Vaccination developments

What COVID-19 vaccines are authorized for use in the U.S.?

The Pfizer-BioNTech vaccine was reviewed and approved for emergency use authorization (EUA) for those 16 and older on Dec. 11, 2020, and for ages 12-15 on May 10, 2021. The Moderna vaccine was approved for EUA for those 18 and older on Dec. 18, 2020, and the Johnson & Johnson/Janssen vaccine was approved for EUA for adults on Feb. 27, 2021.

Both Pfizer and Moderna have started applications to request the FDA's full approval for their COVID-19 vaccines, Pfizer for ages 16 and older and Moderna for ages 18 and older.

What does the FDA's Emergency Use Authorization mean?

When safety and quality criteria are met, Emergency Use Authorization (EUA) allows for the use of medical products in an emergency to diagnose, treat, or prevent serious or life-threatening diseases. The EUA process conducted by the FDA is extremely rigorous to ensure safety standards.

How are the COVID-19 vaccines that have been developed different from one another?

At their core, all vaccines are attempting to do the same thing. When a person gets sick from a virus, the body creates antibodies to help kill off the virus. The immune system also remembers how to make those antibodies so it will be unlikely you would get sick from the same virus a second time. All vaccines work by stimulating your immune system to remember how to make those antibodies quickly without having to become ill first.

While the Pfizer and Moderna vaccines use newer mRNA technology to help your immune system remember how to create antibodies for the virus, the Johnson & Johnson/Janssen vaccine uses a different technology that has been employed widely in vaccines in the past but has the same results.

Other differences include their doses; that they are not interchangeable (if you receive the Pfizer vaccine for your first dose, you must receive the Pfizer vaccine for your second dose); and that they require different storage.

For anyone interested in learning about the different technologies in more detail, we encourage you to visit HealthyMKE. The site contains a wealth of information, as well as links to additional resources from the CDC, U.S. Dept. of Health and Human Services and others.

Learn more on the COVID-19 Vaccine Tracker

Is receiving the COVID-19 vaccine safe?

The safety standards for vaccines are even more strict than other medical interventions as these are given to potentially millions of healthy people, and the COVID-19 vaccines authorized in the US have undergone the most rigorous testing of any vaccines in our history.

All three approved vaccines initally went through extensive study and review with tens of thousands of immunizations accomplished, and all data suggest that the vaccines are not only safe but highly effective. This process involves gathering data about the effectiveness and potential effects of the vaccines that are reviewed by multiple federal agencies responsible for public health and safety. The vaccines have also been tested among a broad range of ages, races, ethnicities and people with pre-existing comorbidities.

Since emergency use authorization, these vaccines have been administered to millions of individuals and continue to prove highly safe and effective. They are undergoing a high standard of safety and regulatory approvals based on results of research conducted. These safety approval boards include scientists and experts in infectious disease and vaccination that have no connection with the companies producing the vaccines. These individuals evaluate the safety based entirely on the data from the clinical trials.

The speed of the COVID-19 vaccine development up to the approval stage underscores the advancement in scientific research and is based on decades of experience with production of vaccines for hundreds of other diseases. It does not indicate rushing of the regulatory checks and approval processes.

Is there any risk involved with receiving the COVID-19 vaccine?

The risk of any vaccine causing serious harm, or death, is extremely small but possible. A vaccine, like any medicine, could cause serious problems, such as a severe allergic reaction. Allergic reactions have been quite rare. People with a history of severe allergic reactions (to medications or food) or anaphylaxis should consult with their health care provider prior to receiving COVID-19 vaccination. They will ask you about any past allergic reactions to identify any of the rare reasons for delaying or not getting the COVID-19 vaccine.

The COVID-19 vaccines are new, and some effects may not yet be known. Historically, long-term side effects from vaccines have been rare.

The side effect of blood clots that caused the FDA and CDC to recommend a temporary pause on the distribution of the Johnson & Johnson/Janssen vaccine has been exceedingly rare. Of the nearly 7 million doses of the vaccine administered when the pause was initiated, there were only six instances of abnormal blood clotting. As such, the FDA and CDC have determined the Johnson & Johnson vaccine is safe, effective and that its benefits far outweigh the low risks.

How effective are the COVID-19 vaccines?

Each of the currently authorized vaccines has been proven to be highly effective in protecting the recipient from COVID-19, and new data is showing them to be highly effective at reducing the spread of the virus as well.

The latest CDC data continue to show the vaccines to be 99.99% effective at preventing illness from COVID-19.

Is this a live virus vaccine? Could it make others I come in contact with sick?

No, the FDA-approved vaccines do not contain any live virus, nor do they in any way alter the recipient’s DNA or genetic makeup. They are simply teaching our immune systems how to produce antibodies for the virus without needing to get sick first. Thus, it is impossible to get COVID-19 from the vaccine. You will not be contagious or shed live virus.

If I receive the vaccine, does that mean I won't get COVID-19?

“Breakthrough cases” – contracting COVID-19 even after getting vaccinated – are quite rare, but garner attention when they occur.

While many of the vaccines are nearly 100% effective at preventing severe illness, hospitalization and death, in rare instances a fully vaccinated individual can still contract the virus. More often than not, these COVID-19 cases are either asymptomatic or very mild – comparable to a common cold. The latest CDC data on breakthrough cases continue to show the vaccines to be 99.99% effective.

The possibility for occasional breakthrough cases underscores the value of getting vaccinated. With any vaccine, there is potential for a very small group of people not to experience its full protection. We can protect that group by getting vaccinated ourselves to build community immunity.

Does the COVID-19 vaccine stop the transmission of the virus?

We know that the COVID-19 vaccines protect a person from getting moderately to severely ill with COVID-19. A small number of people may get COVID-19 infection with mild symptoms. There is new evidence that suggests that the chances that fully vaccinated people will transmit COVID-19 to others are far lower than if they had remained unvaccinated; however, more conclusive evidence is needed.

Will the vaccines be effective against the “new strains” or variants of the virus that have been found?

Viruses constantly change through mutation, and new strains or variants of a virus are expected to occur over time. Multiple variants of the virus that causes COVID-19 have been documented in the United States and globally during this pandemic, with these most recently discovered variants seeming to spread more easily and quickly than others, which may lead to more cases of COVID-19.

However, there is a high degree of confidence that the mutations of the virus so far do not significantly change the structure of its spike protein – which means that vaccines will still be effective. Going forward, the COVID-19 virus might accumulate sufficient mutations that we may require a customized booster shot every year – much like we do for the seasonal flu.

How would a virus mutation effect the efficacy of the current vaccine?
The vaccine developers are carefully monitoring any virus mutations to make sure the current vaccines are still effective, and, if needed, they will swiftly adjust the vaccine just like they do for the seasonal flu.
How many people will need to be vaccinated against COVID-19 to achieve herd/community immunity resulting in community protection?

Herd immunity (or community protection) is a term to describe the situation when most individuals have protection from the virus so that there are not enough susceptible people remaining in a community for the virus to spread. As a result, everyone within the community is protected, even if a small number of people do not have any protection themselves. The percentage of people who need to have protection in order to achieve herd immunity varies by disease. At this time, experts estimate that at least 80% of people within a community will need to have developed immunity to begin controlling the COVID-19 pandemic.

Where can I learn more specifically about the Pfizer and Moderna COVID-19 vaccines?

The FDA has created Emergency Use Authorization (EUA) fact sheets for the Pfizer-BioNTech COVID-19 Vaccine (with translations), the Moderna COVID-19 Vaccine (with translations) and the Johnson & Johnson/Janssen vaccine that contain information to help the public understand the risks and benefits of each specific vaccine.

Considerations for receiving the vaccine

Why is it important to consider receiving the COVID-19 vaccine?

The devastating impact of COVID-19 has resulted in hundreds of thousands of deaths in the United States, to date. These vaccines offer a safe and effective way to slow and potentially stop the toll of this public health crisis. The vaccines work with your immune system so that your body will be ready to fight the virus if you are exposed. It is important for each person to carefully weigh all available information when making the best-informed choice about receiving the vaccine.

Are there people who shouldn't get it? (age, immunocompromised, etc.)

The COVID-19 vaccine has not yet been approved for children under the age of 12, and only the Pfizer vaccine is approved for those ages 12-17. People who are pregnant or breastfeeding or those with severe allergies to vaccines should consult their health care provider before taking the vaccine.

When will a vaccine for children under the age of 12 be coming out?

It is going to take time before those younger than 12 can get a COVID-19 vaccine. Millions more Americans are now eligible for vaccination with news that the FDA expanded the emergency use authorization for the Pfizer vaccine for children ages 12-15. Clinical trials showed the Pfizer vaccine to have 100% efficacy in this age group, even higher than for older recipients. Parents interested in getting their 12 to 15-year-old children vaccinated can find locations administering the Pfizer vaccine at or Parental or guardian permission is required for anyone under age 18 seeking a vaccination.

Pfizer, Moderna and Johnson & Johnson are all currently conducting trials for their vaccines on children as young as 6 months. Moderna’s ongoing trial in children ages 12-17 continues to produce excellent results, potentially opening the door for expanded eligibility sometime this summer.

Can those who are pregnant or trying to become pregnant get the COVID-19 vaccine?

Those who are pregnant or those trying to become pregnant and those who are breastfeeding were not included in vaccine trials and the risk to the baby is unknown. Discussing the risks and benefits of the vaccine should be considered with your health care provider.

Does the COVID-19 vaccine interact with other medications you are on?
Approved vaccines should not interact with other medications, but if you have concerns, discuss your medications and the vaccine with your primary care doctor.
Is there a list of potentially allergy-inducing ingredients that people can check before getting a vaccine to avoid a serious reaction?

The COVID-19 vaccines do not contain any dairy products, latex, or artificial preservatives. For a full list of ingredients in the Pfizer COVID-19 Vaccine, view the Pfizer Fact Sheet for Recipients and Caregivers (additional translations available from the CDC). For a full list of ingredients in the Moderna COVID-19 vaccine, view the Moderna Fact Sheet for Recipients and Caregivers (additional translations available from the CDC). For a full list of ingredients in the Johnson & Johnson/Janssen vaccine, view the Janssen Fact Sheet for Recipients and Caregivers.

Should I still get the vaccine if I was COVID-19 positive in the past?

Yes. According to the CDC website, there is not enough information currently available to say if or for how long after infection someone is protected from getting COVID-19 again; this is called natural immunity. Early evidence suggests natural immunity from COVID-19 may not last very long, but more studies are needed to better understand this.

How long after having COVID-19 should people wait before being vaccinated?

You may receive a COVID-19 vaccine if you have already had COVID-19 infection, are not acutely ill and have met the CDC criteria for discontinuing isolation: At least 10 days have passed since symptom onset AND at least 24 hours have passed since resolution of the fever without use of fever-reducing medications AND other symptoms have improved. If you have had prior COVID-19 infection, you may delay vaccination for 90 days after the infection, since reinfection is not common during this time frame.

If I have a cold or mild fever, should I get vaccine?
If an individual gets vaccinated when they have a cold or mild fever, the vaccine will still be safe and effective. However, during this pandemic, it is important that an individual with a cold or mild fever strongly consider being tested for COVID-19 and stay home until they receive negative results and their symptoms go away. A reason for not vaccinating someone with a cold or mild fever is because that person may have COVID-19 and spread it to people they come in contact with during the vaccination process. In addition, assessing symptoms following the vaccine when already having symptoms from a cold could be confusing.

What to expect with the vaccine

What can you expect from the COVID-19 vaccine?

Reported responses to the COVID-19 vaccines are similar to other vaccines like the flu shot, and may include soreness, redness and swelling at the site of the injection; and fatigue, nausea, muscle pain, chills, joint pain and headache. These are all signs that your immune system is working. These minor symptoms develop within one to two days from the shots and last generally one day. Some people are lucky enough to develop good protection from COVID-19 while having very few side effects; having no side effects does not mean the vaccine didn’t work.

The CDC indicates the benefits of the vaccine outweigh the risks of getting ill from COVID-19. If you experience other symptoms after receiving the vaccine, please consult your health care provider and follow their recommendations for self-isolation and care.

How are the COVID-19 vaccines administered? Why are some two shots and others one?

Each vaccine is made and tested differently, and the recommended dosing schedule is based upon testing results and what was determined to be most effective for that particular vaccine.

Both the FDA-authorized Pfizer and Moderna vaccines require two shots. The first shot (initial dose) starts building resistance and the second shot (booster), administered three (Pfizer) or four (Moderna) weeks later, is needed to provide the most protection the vaccine has to offer and increase the likelihood that you will develop antibodies and build immunity.

The FDA-authorized Johnson & Johnson/Janssen vaccine only requires one dose. It’s important to remember that two doses do not imply twice as much protection as one dose, nor does the convenience of one dose imply a more efficient or effective vaccine than one requiring two doses.

How long past the second injection date can someone receive their second dose for the Pfizer and Moderna vaccines?

We do not know the answer to this question yet. One shot gives about 50% protection. Two shots give 95% protection. The CDC recommends that the second shot be administered as close as possible to the target date: 21 days for the Pfizer vaccine and 28 days for the Moderna vaccine. Manufacturers are talking about a 1- to 2-week window leeway for the second shot, but that guidance might change based on experience going forward.

After the first dose (the “priming dose”), your immune cells need a few weeks to build up antibodies as your body is introduced to the vaccine. The second dose is referred to as the “boosting dose” because your body needs to fine-tune the antibody and cellular responses to ensure even stronger protection. Experts recommend not getting the second dose more than four days early or 42 days late to create the strongest immune response. It is most important that both doses should come from the same manufacturer.

Will there be any sort of certificate or record we can use to prove we have had the vaccine?

You will receive a CDC vaccination card at the time of your vaccination that will indicate what COVID-19 vaccine you received, the date you received it and where you received it. Vaccine clinics also report all COVID-19 vaccine administrations to the Wisconsin Immunization Registry, and you can look up your record online. If you received the Pfizer or Modera vaccines, you need to return to the same location (within the specified time period) that you received the first dose to ensure you are receiving the correct second dose.

How do I know if the COVID-19 vaccine is working?

Unfortunately, you won't be able to tell if you are protected. Even after getting vaccinated you need to continue to limit trips outside of your home, practice physical distancing and wearing face coverings.

What to expect after vaccination

Do individuals need to quarantine after being vaccinated?

No. The COVID-19 vaccines do not contain virus or virus particles. Thus, it is impossible to get COVID-19 from the vaccine. You will not be contagious or shed live virus. Individuals should monitor for symptoms and follow appropriate guidance from their primary care provider.

How long does the COVID-19 vaccine take to become fully effective in my system or for me to develop immunity?

While there are slight differences in each vaccine, in general it takes two weeks for your body to fully learn to protect itself from the virus. For the Pfizer and Moderna vaccines, that would be two weeks after the second dose. These results were consistent across gender, age, race and ethnicity. The length of immunity following vaccination is not yet known for COVID-19. Given the novel nature of this virus and vaccine development, long-term data is not yet available to guide future vaccine protocols.

How long is the COVID-19 vaccine effective?

Data has emerged demonstrating that protection from both the Pfizer and the Moderna vaccines remains active in the body for at least six months. It’s important to note that the true length of protection can only be answered once more time has passed and is likely longer than six months. The duration and level of protection will continue to be evaluated as trials continue.

Will those who are vaccinated test positive for COVID-19 following vaccination?

You should not test positive for COVID-19 due to being vaccinated but will test positive for the antibodies that fight the virus.

Do I still need to quarantine and get tested if I have received the vaccine but then am exposed to someone with COVID-19?

The Centers for Disease Control and Prevention (CDC) recently updated their quarantine guidelines. Individuals who have been fully vaccinated against COVID-19 within the last three months do not need to quarantine if they have been in close contact with someone who has COVID-19, so long as the vaccinated person shows no symptoms. Wisconsin Department of Health Services (DHS) also has endorsed and supports adoption of the new quarantine guidance.

Will I have to get the COVID-19 vaccine every year?

At this time, we are anticipating that this vaccine will be given only using two shots (21 or 28 days from each other), but future booster shots might be needed.

Do I still need to wear a mask and practice social distancing after I am vaccinated?

For now, fully vaccinated people should continue to:

  • Take precautions in public like wearing a well-fitted mask and physical distancing
  • Wear masks, practice physical distancing, and adhere to other prevention measures when visiting with unvaccinated people who are at increased risk for severe COVID-19 disease or who have an unvaccinated household member who is at increased risk for severe COVID-19 disease
  • Wear masks, maintain physical distance, and practice other prevention measures when visiting with unvaccinated people from multiple households
  • Avoid medium- and large-sized in-person gatherings
  • Get tested if experiencing COVID-19 symptoms
  • Follow guidance issued by individual employers
  • Follow CDC and health department travel requirements and recommendations
What can I do once I'm vaccinated? Can I travel?

Per CDC interim guidance, fully vaccinated people can:

  • Visit with other fully vaccinated people indoors without wearing masks or physical distancing
  • Visit with unvaccinated people from a single household who are at low risk for severe COVID-19 disease indoors without wearing masks or physical distancing
  • Refrain from quarantine and testing following a known exposure if asymptomatic

The CDC also issued new guidelines for people fully vaccinated against COVID-19 and travel within the U.S.

  • Individuals who are fully vaccinated do not need to get tested before or after travel, unless they are symptomatic, or their destination requires it.
  • Fully vaccinated travelers do not need to self-quarantine after travel unless there are other state or local recommendations or requirements. Wisconsin does not require individuals who are fully vaccinated to quarantine after a potential exposure so long as they do not have any symptoms. Travelers should still wear a mask, maintain social distancing, and practice good hand hygiene.

How can I learn more?

Carefully review all materials provided to you when you are notified that you are eligible to receive the COVID-19 vaccine BEFORE you schedule your vaccination. You can also visit the following online resources:

What we know about COVID-19 vaccine distribution – general

Who is currently eligible to be receive the COVID-19 vaccine?

Everyone in Wisconsin ages 12 and older is eligible to get the COVID-19 vaccine at no cost.

Note that the only vaccine currently approved for 12- to 17-year-olds is the Pfizer vaccine. These individuals should verify appointment sites are administering Pfizer while scheduling. Clinical trials showed the Pfizer vaccine to have 100% efficacy in this age group, even higher than for older recipients. Parents interested in getting their 12 to 15-year-old children vaccinated can find locations administering the Pfizer vaccine at or Parental or guardian permission is required for anyone under age 18 seeking a vaccination.

Where can eligible groups get the COVID-19 vaccine?

The federal government has launched to help all more easily find vaccine locations. Users can even search by vaccine brand to be able to find locations where the Pfizer vaccine is being offered to those age 12 and over.

What is the cost of the COVID-19 vaccine?

The federal government is providing the vaccine free of charge to people living in the United States. While a health care provider may bill your insurance company, Medicaid or Medicare for an administration fee, the Froedtert & the Medical College of Wisconsin health network will administer all vaccines at no cost to recipients.

What can we do to build trust among community members of color so they are more comfortable getting the vaccine?
It is critical that we work together to build trust in communities of color around the COVID-19 vaccine. Distrust of the healthcare system is, in part, due to a history of racism and lived experiences. People have lots of questions and deserve to have them answered. In order for communities of color to be receptive to messages about vaccine safety, one important avenue is to share them through popular opinion leaders within their community.

Vaccine Basics

Through vaccination, people can develop immunity without suffering from the actual diseases that vaccines prevent. Learn the basics of vaccines and why they are so important to the health of our community.

Key Terms

Antibody: A protein found in the blood that is produced in response to foreign substances (e.g. bacteria or viruses) invading the body. Antibodies protect the body from disease by binding to these organisms and destroying them.

Bacteria: Single-celled organisms that live in and around us with a distinct structure from other microbes. Bacteria can be helpful, but can also cause illnesses such as strep throat, ear infections, and pneumonia.

Germs: Living organisms, like bacteria, fungi, or viruses, which can cause infections or disease.

Herd Immunity: When a large percentage of the population becomes immune to a disease through vaccination and/ or recovery from infection. If enough people are immune, it limits spread and protects the entire community.

Immunity: Protection from an infectious disease. If you are immune to a disease, you can be exposed to it without becoming infected.

Immunization: A process by which a person becomes protected against a disease through vaccination. This term is often used interchangeably with vaccination or inoculation.

Outbreak: More cases of a disease at a specific time or place than expected. If the disease is new, rare, or has serious public health implications, an outbreak may be declared with as few as one or two cases.

Vaccination: The act of introducing a vaccine into the body to produce immunity to a specific disease.

Vaccine: A product that stimulates a person’s immune system to produce immunity to a specific disease, protecting the person from that disease. Vaccines are usually administered through needle injections but can also be administered by mouth or sprayed into the nose.

Virus: A tiny organism that multiplies within cells and causes disease such as chickenpox, measles, mumps, rubella, pertussis and hepatitis. Viruses are not affected by antibiotics, the drugs used to kill bacteria.

Frequently Asked Questions

How do vaccines prevent disease?

Vaccines contain the virus or bacteria (or parts of virus or bacteria) that cause disease. For example, measles vaccine contains measles virus. The virus in the measles vaccine is weakened to the point that it doesn’t cause disease. Some vaccines have a killed version of the germ, such as the vaccine against Hepatitis A. Others have only a piece of the germ, such as with the meningococcal vaccine. All of these vaccines are strong enough to make the immune system produce antibodies that lead to immunity. In other words, a vaccine is a safer substitute for a person’s first exposure to a disease. The person gets protection without having to get sick. Through vaccination, people can develop immunity without suffering from the actual diseases that vaccines prevent.

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Why immunize?

Thanks to vaccines, some diseases are almost gone from the U.S. But experts still recommend that we immunize our children, ourselves as adults, and the elderly. It’s true, some diseases (like polio and diphtheria) are becoming very rare in the U.S. Of course, they are becoming rare largely because we have been vaccinating against them.

It’s much like bailing out a boat with a slow leak. When we started bailing, the boat was filled with water. But we have been bailing fast and hard, and now it is almost dry. We could say, “Good. The boat is dry now, so we can throw away the bucket and relax.” But the leak hasn’t stopped. Before long we’d notice a little water seeping in, and soon it might be back up to the same level as when we started.

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Why must we continue to immunize?

Unless we can “stop the leak” (eliminate the disease), it is important to keep immunizing. Even if there are only a few cases of disease today, if we take away the protection given by vaccination, more and more people will become infected and will spread disease to others. Soon we will undo the progress we have made over the years.

We don’t vaccinate just to protect our children. We also vaccinate to protect our grandchildren and their grandchildren. With one disease, smallpox, we “stopped the leak” in the boat by eradicating the disease. Our children don’t have to get smallpox shots anymore because the disease no longer exists. Smallpox is now only a memory, and if we keep vaccinating against other diseases, the same will someday be true for them too. Vaccinations are one of the best ways to put an end to the serious effects of certain diseases.

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What if we stop immunizing?

Diseases that are almost unknown would stage a comeback. Before long we would see epidemics of diseases that are nearly under control today. More children would get sick and more would die.

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How do we know vaccines are safe?

The United States has the safest, most effective vaccine supply in its history. The nation’s longstanding vaccine safety system ensures that vaccines are as safe as possible. As new information and science become available, the system is updated and improved.

As part of these vaccine safety efforts, scientists ensure the safety of vaccines by conducting different types of studies:

  • Clinical trials are done before a vaccine is made available. Vaccine manufacturers conduct these studies as part of the development, testing, and approval process. The Food and Drug Administration (FDA) uses the information from clinical trials to decide if a vaccine is safe, effective, and ready to be licensed for use.
  • Post-licensure safety studies are conducted after a vaccine is approved and in use. These studies are part of continuous vaccine safety monitoring and often look at populations that might be underrepresented in clinical trials. Importantly, these studies can look for rare adverse events.

The United States’ long-standing vaccine safety program closely and constantly monitors the safety of vaccines.

A critical part of the program, CDC’s Immunization Safety Office identifies possible vaccine side effects and conducts studies to determine whether health problems are caused by vaccines.

Data show that the current U.S. vaccine supply is the safest in history.

What are the differences between active and passive immunity?

Immunity to a disease is achieved through the presence of antibodies to that disease in a person’s system. Antibodies are proteins produced by the body to neutralize or destroy toxins or disease-causing organisms. Antibodies are disease-specific. For example, measles antibody will protect a person who is exposed to measles but will have no effect if he or she is exposed to mumps.

Active immunity results when exposure to a disease organism triggers the immune system to produce antibodies to that disease. Exposure to the disease organism can occur through infection with the actual disease (resulting in natural immunity), or introduction of a killed or weakened form of the disease organism through vaccination (vaccine-induced immunity). Either way, if an immune person comes into contact with that disease in the future, their immune system will recognize it and immediately produce the antibodies needed to fight it.

Active immunity is long-lasting, and sometimes lifelong.

Passive immunity is provided when a person is given antibodies to a disease rather than producing them through his or her own immune system.

A newborn baby acquires passive immunity from its mother with antibodies that cross the placenta. A person can also get passive immunity through antibody-containing blood products such as immune globulin, which may be given when immediate protection from a specific disease is needed. This is the major advantage to passive immunity; protection is immediate, whereas active immunity takes time (usually several weeks) to develop.

However, passive immunity lasts only for a few weeks or months. Only active immunity is long-lasting.

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