A Look at Each Vaccine: COVID-19 Vaccine

Illustration of COVID-19 disease vs. vaccine SARS-CoV-2, a bat coronavirus, made its debut in the human population in November 2019. By January 2020, the virus had been isolated and its genetic sequence defined. With that information in hand, it was now possible to make a vaccine to prevent it. By the end of 2020, vaccines targeting the coronavirus spike protein became available in the U.S. and other countries. The spike protein is responsible for attaching the virus to cells. The speed with which COVID-19 vaccines were made was due to the dedication of massive financial and human resources, including clinical trials involving tens of thousands of participants. The results of these studies were carefully reviewed to ensure their safety. In the U.S., this review was completed by independent experts advising both the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC).

View a list of questions submitted by readers on our companion webpage at COVIDVaccineAnswers.org.

Click here to view a full-size image of the infographic [PDF, 239KB].

The disease

What is SARS-CoV-2?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a virus that causes coronavirus disease 2019 (COVID-19). SARS-CoV-2 was first identified in China in late 2019. This new virus developed when a bat coronavirus infected another animal, which has yet to be identified, and changed in a way that enabled it to gain two important capabilities. First, it could infect people. Second, it could easily spread from one person to another. These new capabilities set the stage for a global epidemic, called a pandemic. In March 2020, the World Health Organization determined that the spread was worldwide and declared a COVID-19 pandemic.

SARS-CoV-2 infections lead to a wide range of disease experiences and symptoms. Some people never have symptoms or know they were infected. Most people have mild symptoms, similar to those of a cold. But a significant number of infected people develop severe symptoms that can result in weeks of illness, hospitalization or death.

Common symptoms of COVID-19 include:

  • Fever
  • Dry cough
  • Fatigue

Other symptoms can include:

  • Loss of taste or smell
  • Nasal congestion
  • Conjunctivitis (“pink eye”)
  • Sore throat
  • Headache
  • Muscle or joint pain
  • Nausea or vomiting
  • Diarrhea
  • Loss of appetite
  • Rash or discoloration of fingers or toes

Symptoms of severe disease can include:

  • Shortness of breath or difficulty breathing
  • Confusion
  • Chest pain
  • High fever
  • Blood clots

Some groups of people, including those over 65 years of age, pregnant women, and individuals with high blood pressure, chronic heart and lung disorders, diabetes and obesity are at a greater risk of developing severe disease. However, all people, regardless of age or underlying conditions, can experience severe COVID-19 disease.

Some children and adults experience a delayed onset of symptoms called multisystem inflammatory syndrome (MIS-C or MIS-A). This syndrome can occur in anyone who was infected with COVID-19, including people who had minimal or no symptoms during infection. The condition develops a few weeks after infection (up to 6 weeks) at a time when individuals no longer test positive for SARS-CoV-2. Symptoms may include fever, abdominal pain, vomiting, diarrhea, rash, pink eye, fatigue, neck pain, or headache. While it is not known why this syndrome develops, it is important to realize that SARS-CoV-2 can infect various types of cells, which means that individuals who develop MIS-C or MIS-A may experience symptoms related to the heart, blood, lungs, kidneys, eyes, stomach, intestines, or skin.

Additionally, some people are experiencing side effects for long periods after infection (months). Symptoms of “long COVID” can include fatigue, difficulty thinking or concentrating (so-called “brain fog”), headache, loss of taste or smell, dizziness, heart palpitations, chest pain, shortness of breath, cough, joint or muscle pain, anxiety, depression, or fever. Symptoms may appear or worsen after physical or mental activity. We are still learning why this happens and how long the effects may linger.

How do you catch SARS-CoV-2?

Coronaviruses are contained in saliva and spread by talking, coughing, sneezing or singing. These respiratory secretions are usually “large droplets” that do not stay in the air for long periods of time. However, smaller virus-containing droplets, sometimes referred to as “aerosolized droplets,” can remain in the air for long periods of time and are, therefore, considered more contagious. Virus-containing respiratory droplets can enter a person’s eyes, nose or mouth. Once in the body, the virus can infect cells that line the nose, throat, lungs and intestines.

How can you avoid catching SARS-CoV-2?

We can protect ourselves from catching COVID-19 in a variety of ways:

  • Handwashing — Germs can be found on many surfaces, so frequent handwashing is an important way to decrease the chance of accidentally infecting ourselves when we touch our faces with unclean hands.
  • Physical distancing — Respiratory droplets can travel some distance after leaving our noses and mouths, especially if they are from a cough or sneeze. Therefore, keeping a distance of at least 6 feet between individuals can reduce the risk of coming into contact with viruses traveling in respiratory droplets.
  • Masks — Masks decrease the spread of respiratory droplets. While they are most helpful in protecting others, they also can decrease the risk of infection for the person wearing them.
  • Vaccination — Humankind has never eliminated an infection solely through the use of public health measures, such as those described above, therefore, COVID-19 vaccines are important for stopping the spread of COVID-19.

The vaccine

How is the COVID-19 vaccine made?

The COVID-19 vaccines currently available in the U.S. include two mRNA vaccines and one adenovirus-vector vaccine:

  • mRNA vaccines — These types of vaccines provide the code, or blueprint, for making viral proteins. COVID-19 mRNA vaccines provide the blueprint for the SARS-CoV-2 spike protein. After getting the shot, the vaccinated person’s dendritic cells near the injection site make the SARS-CoV-2 spike protein. The immune system recognizes the protein as foreign and responds to remove it. In the process, the body creates a memory immune response, which will quickly activate in the future if the person is exposed to COVID-19 in the community. Watch this animation to see how the COVID-19 mRNA vaccine works. Two COVID-19 mRNA vaccines are currently available.
    • Pfizer’s vaccine – Two doses administered as a shot intramuscularly and separated by 21 days. 
    • Moderna’s vaccine – Two doses administered as a shot intramuscularly and separated by 28 days.
    • Some individuals are recommended to get a third dose of mRNA vaccine. Talk to your healthcare provider if you are uncertain about whether you need this additional dose.
  • Adenovirus vaccine — These types of vaccines use an adenovirus as a delivery vehicle for the gene that codes for the SARS-CoV-2 spike protein. The adenovirus has not only been altered to include the spike protein gene, but it has also been changed so that it cannot replicate in people. Once in the cell, the adenovirus DNA, including the coronavirus spike protein, enters the nucleus, where it is used to make mRNA. The mRNA is then released to the cytoplasm where it serves as a blueprint to produce proteins, including the SARS-CoV-2 spike protein. The immune system recognizes the protein as foreign and responds to remove it. In the process, the body creates a memory immune response, which will quickly activate in the future if the person is exposed to COVID-19 in the community. Watch this animation to see how the COVID-19 viral vector vaccine works.
    • One COVID-19 viral vector vaccine is available in the U.S., but others are available in other countries. In the U.S., the Johnson & Johnson (J&J)/Janssen vaccine is this type of vaccine; it is administered as two doses given intramuscularly and separated by at least two months.

Does the COVID-19 vaccine have side effects?

After receiving the COVID-19 vaccine, some people may feel tired and some will experience headache and muscle aches as well as injection site pain. Some will also experience fever.

For the mRNA vaccines (Pfizer and Moderna), the side effects are more common after the second dose and more likely in younger, rather than older, vaccine recipients. For both types of vaccines, the side effects can occur up to a week after vaccination but are most likely to occur one or two days after vaccination and last for a day or two. These types of side effects are the result of your immune system responding to the vaccine.

A small number of young men have experienced a temporary inflammation of the heart, called myocarditis, within 4 days of receipt of the second dose of the mRNA vaccine. Those affected can feel chest pain or shortness of breath and should seek medical care to ensure proper diagnosis. The condition most commonly goes away in a few days and does not appear to cause permanent damage to the heart, but it can be scary. Infection with SARS-CoV-2 can more frequently cause myocarditis, either during primary infection or later if individuals develop multisystem inflammatory syndrome (MIS-C or MIS-A); therefore, vaccination continues to be recommended. This condition is currently estimated to occur in about 1 of 50,000 people (predominantly males) in their teens and 20s after receiving the second dose of the mRNA vaccine.

On occasion, the adenovirus-based viral vector vaccine (Johnson & Johnson (J&J)/Janssen) can cause two concerning side effects:

  • Thrombosis with thrombocytopenia syndrome (TTS) — a very rare blood-clotting condition that also causes low platelet counts. The condition occurs in about 1 per 500,000 people who get the J&J/Janssen vaccine.
  • Guillain-Barre syndrome (GBS) — this neurologic condition is characterized by muscle weakness. The condition can occur following a viral infection but has also been associated with receipt of the J&J/Janssen vaccine in about 1 per 120,000 people. 

Find out more about these side effects on the COVIDVaccineAnswers.org page.

Who should get the COVID-19 vaccine?

Almost everyone can get the COVID-19 vaccines, but a few people should not get the vaccine and some others should first consult with their healthcare provider.

Those who should NOT get the COVID-19 vaccine include:

  • Those under 5 years of age. Those 5- to 18-years of age can get the Pfizer, but not the Moderna or J&J/Janssen vaccines. Those 5 to 11 years of age should get the lower dose version (orange cap), which contains one-third the amount of active ingredient compared with the version (purple cap) given to those 12 years of age and older.
  • If someone currently has symptoms of COVID-19 or is isolating per Centers for Disease Control and Prevention (CDC) protocols, they should wait until they recover.
  • If a person recently had COVID-19 and was treated with monoclonal antibodies or convalescent plasma, they should wait 90 days before getting vaccinated.
  • In most cases if someone is exposed to COVID-19, they should wait until the quarantine period is over. Exceptions to this are people living in group settings (e.g., nursing homes, correctional facilities, or homeless shelters).

Those who cannot get the mRNA vaccine, but may be able to get the adenovirus vaccine:

  • Anyone with a previous severe or immediate allergic reaction (i.e., one that causes anaphylaxis or requires medical intervention) to a COVID-19 mRNA vaccine dose or an mRNA vaccine component.

Those who cannot get the adenovirus vaccine, but may be able to get the mRNA vaccine:

  • Anyone with a previous severe or immediate allergic reaction (i.e., one that causes anaphylaxis or requires medical intervention) to the COVID-19 adenovirus vaccine or one of its components
  • Anyone with a known polysorbate allergy

Those who may get the vaccine but should be aware of the potential risks and benefits (through consultation with a healthcare provider, as necessary) or who should follow special procedures include:

  • Individuals with a history of severe or immediate allergic reaction to any vaccine or injectable medication – remain at location where vaccine is given for 30 minutes after receiving the vaccine. Speak with your doctor before vaccination if you have questions or concerns.
  • People who have a severe or immediate allergic reaction to one of the types of vaccines and for whom the cause of the reaction is unknown (i.e., which component caused the reaction) – consult an allergist or immunologist to determine whether the individual can get the other version. If they proceed, they should be vaccinated at a location with medical facilities and staff prepared to respond to medical emergencies and remain for 30 minutes of observation.
  • Women who are pregnant or breastfeeding – Pregnant women who opt to get the vaccine and develop a fever, should take acetaminophen as fever during pregnancy can be harmful to the developing baby.
  • People with immune-compromising conditions – consult healthcare provider for guidance
  • Those taking blood thinners (anticoagulants) – Given that the vaccine is administered intramuscularly, there is some chance for bleeding. Individuals should consult their healthcare provider with questions and alert clinic staff that they are taking blood thinners.
  • People who recently had COVID-19 and were treated with antibody-based therapies (e.g., monoclonal antibodies or convalescent plasma) should wait until 90 days after treatment to be vaccinated.
  • People with a known COVID-19 exposure should wait until their quarantine is over before getting vaccinated (unless they live in a group setting, such as a nursing home, correctional facility, or homeless shelter, in which case they can be vaccinated during the quarantine period).

All individuals should stay at the vaccination site for 15-30 minutes of observation after getting vaccinated.

Other questions you might have

Find answers to dozens of questions related to COVID-19 vaccines on our dedicated page, COVIDVaccineAnswers.org.

If I already had COVID-19, do I still need to get vaccinated?

Currently, the CDC recommends that even those who have had COVID-19 get the COVID-19 vaccine. Although some data were suggestive that previously infected individuals may not need the vaccine, that changed with the spread of the delta variant. If someone with previous infection gets the vaccine, it will not harm them and will likely boost their immunity.

If I get the COVID-19 vaccine, do I still need to get the flu shot?

Yes. While COVID-19 and the flu are both caused by respiratory viruses, the viruses are different. So, separate vaccines are needed to protect you from both viruses.

Relative risks and benefits

Do the benefits of COVID-19 vaccination outweigh its theoretical risks?

COVID-19 can cause severe disease or even death. Some people may develop a multisystem inflammatory syndrome (MIS-C or MIS-A) several weeks after infection or long-lasting effects (“long COVID”), resulting in symptoms that last for weeks or months, even when the virus can no longer be detected in the body.

On the other hand, the COVID-19 vaccine has not been shown to cause serious side effects in most people. Even though a small number of people, particularly young males, might experience short-live inflammation of the heart following receipt of the mRNA vaccine, this condition resolves within a few days. Likewise, a small number of people, particularly females younger than 50 years of age, might develop TTS following receipt of the adenovirus vaccine; however, this condition can be treated, particularly if identified early. Similarly, although GBS may occur in a small number of people, particularly men 50 to 64 years of age, this condition typically resolves, but may take several months or a few years. A small number of people may experience permanent muscle weakness. Finally, it is important to realize that these (myocarditis, GBS) or similar conditions (blood clots) can also result from infection with COVID-19 and likely occur more frequently. As such, while vaccinated individuals should monitor for any of these rare side effects, the benefits of the COVID-19 vaccine still outweigh the risks.

Disease risks

  • Pneumonia (infection of the lungs)
  • Respiratory failure
  • Acute respiratory distress syndrome (ARDS)
  • Sepsis (bloodstream infection) and septic shock
  • Multi-organ failure
  • Children and adults can develop a rare, yet severe, inflammatory disorder weeks after infection, called multisystem inflammatory syndrome (MIS-C or MIS-A)
  • Long-term symptoms (“long COVID”)
  • Disease can be fatal

Vaccine risks

  • Fatigue
  • Headache
  • Muscle aches
  • Swelling of the lymph nodes under the arm in which the vaccine was given
  • Myocarditis following mRNA vaccination (1 per 50,000 people)
  • TTS following J&J/Janssen vaccination (1-2 per 1 million people)
  • GBS following J&J/Janssen vaccination (7-8 per 1 million people)
  • Anaphylaxis (3-5 per 1 million people)


Reviewed by Paul A. Offit, MD on November 10, 2021

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.