SARS-CoV-2, a bat coronavirus, made its debut in the human population in 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). Importantly, COVID-19 vaccine development was not done by skipping steps, but rather by conducting several aspects of development in parallel rather than sequentially. You can think of COVID-19 vaccine development steps being completed like several cars traveling in separate lanes of a multilane highway, compared with the more traditional vaccine development process in which steps are completed like cars of a train, one after the other. This change of process, coupled with the financial resources provided through Operation Warp Speed, enabled this extraordinary scientific achievement, likely saving millions of lives.

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. Since that time, the virus has continued to change, creating “variants,” or versions, of the original virus that have genetic differences. The most successful variants of a virus tend to be those that either improve the virus’s ability to spread or allow the virus to evade the immune response generated by vaccination.

Watch as Dr. Offit discusses the origins of SARS-CoV-2 virus and why the so-called “lab leak theory” is unlikely.

What are the symptoms of COVID-19?

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 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). Adults might suffer a similar problem called 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 or years). Symptoms of “long COVID” can include fatigue, difficulty thinking or concentrating (so-called “brain fog”), headache, change in or loss of taste or smell, dizziness, heart palpitations, chest pain, shortness of breath, cough, joint or muscle pain, anxiety, depression, sleep problems, feelings like “pins and needles,” diarrhea or stomach pain, rash, changes in menstrual cycle or fever. Symptoms may appear or worsen after physical or mental activity. Some people, particularly those who experienced severe disease, may develop new chronic conditions, like diabetes, heart conditions, or neurological conditions. We are still learning why this happens and how long the effects may linger, but currently, it seems that some people may be more likely to experience lingering symptoms, including those who:

• Had more severe disease (hospitalization or intensive care)
• Developed multisystem inflammatory syndrome (MIS-C or MIS-A)
• Had pre-existing health conditions
• Were unvaccinated

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 with public health measures, such as those described above; therefore, COVID-19 vaccines are important for stopping the spread of the virus. Even after vaccination, some people will be infected with the virus that causes COVID-19; however, they are less likely to become severely ill when compared with non-immune individuals. Likewise, those who have been both vaccinated and infected appear to have the most comprehensive immunologic memory, so even people who were infected can benefit from vaccination.

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 recombinant 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 – One to three doses administered as a shot intramuscularly.
  • Moderna’s vaccine – One to three doses administered as a shot intramuscularly.
  • Many individuals are recommended to get additional doses of mRNA vaccine. Talk to your healthcare provider if you are uncertain about how many doses you are recommended to receive. You can also find more information on this page of our website in the answer to “Who should get a booster dose of COVID-19 vaccine?”
• Recombinant vaccine – This vaccine is made by producing the SARS-CoV-2 spike protein in an insect-cell system in the laboratory, purifying it, and adding an adjuvant, called Matrix-M, to enhance the immune response to the single protein. This method is similar to that used for other routinely recommended vaccines, like shingles, HPV, hepatitis B, and one type of influenza vaccine (Flublok®).
  • One recombinant COVID-19 vaccine is available in the U.S. The Novavax vaccine is given as two doses administered as a shot intramuscularly.
• Adenovirus vaccines are no longer available in the U.S due to some serious side effects that they cause and the availability of alternative options. However, this type of COVID-19 vaccine is available in some other countries. 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.

For information about ingredients in COVID-19 vaccines, check out the “COVID-19 Vaccines: What’s in the Vial?” infographic or accompanying article.

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) and the recombinant vaccine (Novavax), the side effects are more common after the second dose and more likely in younger, rather than older, vaccine recipients. For all three 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. Side effects in young children were similar to those in adults. These types of side effects are the result of the immune system responding to the vaccine.

A small number of people, particularly 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. This effect has not been detected in younger children, who receive lower doses of the vaccine. However, parents should continue to monitor their children after receipt of the vaccine until we know that this rare side effect does not also occur in that age group. Myocarditis is also likely a rare side effect of the Novavax vaccine.

Adenovirus-based viral vector vaccines, no longer available in the U.S., can cause two concerning side effects, which is why their use has been limited:

• 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 6 months of age. Those 6 months to 18 years of age can get the mRNA vaccines. The recombinant vaccine (Novavax) can only be used for those 12 years of age and older. Dosing for children differs based on age, brand of vaccine and, in some cases, pre-existing health conditions. Talk with your child’s healthcare provider if you are unsure about the recommendations for your child. 
• People with COVID-19 should wait until they recover. These individuals may opt to delay vaccination for about three months after infection to improve their immune response to the vaccine.

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

• Anyone with a previous severe or immediate allergic reaction (i.e., one that causes anaphylaxis, includes swelling of the airway (i.e., tongue, uvula, or larynx), or leads to a diffuse rash that also involves respiratory surfaces (e.g., Stevens-Johnson Syndrome)) to a COVID-19 mRNA vaccine dose or an mRNA vaccine component.
• Anyone with a known allergy to polyethylene glycol.

Those who cannot get the recombinant (protein-based) 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 includes swelling of the airway (i.e., tongue, uvula, or larynx), or leads to a diffuse rash that also involves respiratory surfaces (e.g., Stevens-Johnson Syndrome)) to the COVID-19 recombinant vaccine or one of its components.
• Anyone with a known polysorbate allergy.
• Those younger than 12 years of age.

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 a non-severe, immediate (within 4 hours) allergic reaction to a previous dose of COVID-19 vaccine – 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.
• People who are pregnant– Pregnant people 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 who are moderately or severely ill (regardless of whether they have a fever) may delay vaccination until they feel better.
• People with a history of MIS-C or MIS-A should delay vaccination until at least 90 days after diagnosis and they experience a return of normal cardiac function and are considered clinically recovered.
• People who experienced myocarditis or pericarditis within three weeks after receipt of mRNA vaccine are typically advised not to get additional doses of any COVID-19 vaccine. In some instances, individuals and their healthcare providers may decide to proceed with an additional dose based on the risk-benefit assessment. Note: This does not apply to people with history of myocarditis or pericarditis unrelated to COVID-19 vaccination (including from COVID-19 infection prior to vaccination or more than three weeks after COVID-19 vaccination). Nor does it apply to people with a history of heart disease.
• People treated with convalescent plasma should not receive measles- or varicella-containing vaccines until at least seven months after receipt of the plasma.
• People with a known COVID-19 exposure can get vaccinated if they do not have symptoms.
• People with a current infection can get vaccinated after they recover; however, they may prefer to wait until at least three months after the start of symptoms in order to develop a better immune response to the vaccine.

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?

Yes. The CDC recommends that even those who have had COVID-19 get the COVID-19 vaccine. Studies have found that people who were both vaccinated and infected are among the best protected because the vaccine adds to the individual’s breadth of protection.

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-lived inflammation of the heart (myocarditis) following receipt of the mRNA vaccine (and likely after the recombinant vaccine), this condition resolves within a few days and is less severe than the heart inflammation associated with COVID-19 infection. 

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) and likely after vaccination with the recombinant vaccine.
• Anaphylaxis (3-5 per 1 million people)

References

• Coronavirus disease (COVID-19) – World Health Organization
• COVID-19 Vaccines – Centers for Disease Control and Prevention

Reviewed by Paul A. Offit, MD on May 15, 2024