A Look at Each Vaccine: COVID-19 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, two vaccines were available in the United States, both based on a strategy that employed messenger RNA (mRNA), which serves as a blueprint to direct cells to make the coronavirus spike protein. This protein is responsible for attaching the virus to cells. The speed with which this vaccine was made owed largely to the $24 billion allotted by the federal government to mass produce the vaccine at the same time that large clinical trials involving tens of thousands of participants were performed to prove that the vaccine was safe and effective. Once completed, the results of these studies were carefully reviewed by independent experts advising both the FDA and CDC.

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

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 and those 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.

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 will play an important role in stopping the spread of COVID-19.

The vaccine

How is the COVID-19 vaccine made?

The COVID-19 vaccines currently available are mRNA vaccines. mRNA 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 muscle 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.

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.

The two vaccines are not interchangeable.

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. The side effects are more common after the second dose and more likely in younger, rather than older, vaccine recipients. 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.

Who should get the COVID-19 vaccine?

Almost everyone can get the COVID-19 mRNA 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 vaccine include:

  • People with severe allergies to a vaccine ingredient, another vaccine, or any injectable medications. “Severe” denotes allergies that are severe enough that they cause anaphylaxis, use of an “epi pen” or require medical treatment.
  • Those under 16 cannot get either of the mRNA vaccines because studies have not yet been completed in children younger than 16 years of age. Those 16- to 18-years of age can get the Pfizer, but not the Moderna, vaccine.

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. Likewise, if a person recently had COVID-19 and was treated with monoclonal antibodies or convalescent plasma, they should wait 90 days before getting vaccinated. Finally, 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 may get the vaccine but should be aware of the potential risks and benefits (through consultation with a healthcare provider, as necessary) include:

  • 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
  • Those taking blood thinners (anticoagulants). Given that the vaccine is administered intramuscularly, there is some chance for bleeding.

Finally, people should stay at the vaccination site for 15 minutes of observation after getting vaccinated. However, those with severe allergies to anything other than vaccine ingredients, any vaccine, or injectable medications can get vaccinated, but should stay at the vaccination site for 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?

Yes. People who have had COVID-19 are still recommended to be vaccinated, particularly because we do not know how long-lasting the immune response to SARS-CoV-2 is after infection and because the vaccine was found to be safe when given to people previously infected with SARS-CoV-2. Clinical trials have shown that the vaccine strengthens immunity created in response to the infection.

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 long-lasting effects following COVID-19 infection, 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 any serious side effects. Therefore, the benefits of the COVID-19 vaccine clearly 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 can develop a rare, yet severe, inflammatory disorder weeks after infection
  • Disease can be fatal

Vaccine risks

  • Fatigue
  • Headache
  • Muscle aches


Reviewed by Paul A. Offit, MD on December 21, 2020

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.