COVID-19 Vaccine FAQs

Medically Reviewed By William C. Lloyd III, MD, FACS

What is the COVID-19 vaccine?

The COVID-19 vaccine is a special type of drug designed to stimulate your immune system to recognize, attack and neutralize the SARS-CoV-2 virus, which causes COVID-19.

Why are there different types of COVID-19 vaccines?

As part of the national effort to produce a vaccine capable of preventing COVID-19, vaccine developers tested many vaccine types to identify the most effective ones. The vaccine technology was developed decades ago, so research and development of vaccines for other viral diseases was in progress and easily shifted to SARS-CoV-2 and COVID-19. Currently, three types of COVID-19 vaccines are either in U.S. clinical trials or already authorized for emergency use as part of the vaccination program. These include:

  • Protein subunit vaccines, which use a distinctive fragment—the spike, or S protein—of SARS-CoV-2 to trigger an immune response. Two vaccine developers using this technology include Novavax and Sanofi (in partnership with GlaxoSmithKline).
  • Viral vector vaccines, which are harmless viruses carrying the S protein gene. Viral vector vaccines mimic SARS-CoV-2 infection by producing the S protein, triggering the immune system to make antibodies against it. Two vaccine developers using this technology include Janssen Pharmaceuticals (a Johnson & Johnson company) and AstraZeneca-University of Oxford in the United Kingdom.
  • Nucleic acid vaccines, which give your body a message, or recipe to make the S protein. Your immune system reacts to the S protein and creates antibodies to it. Nucleic acid is DNA or messenger RNA (mRNA). The first two authorized vaccines—Pfizer-BioNTech and Moderna—are mRNA vaccines.

A fourth type of vaccine in production in other countries is an inactivated form of SARS-CoV-2. Flu and measles vaccines are examples of inactivated vaccines. There are no inactivated SARS-CoV-2 vaccines in U.S. clinical trials.

What is the Pfizer-BioNTech COVID-19 vaccine?

The Pfizer-BioNTech vaccine, BNT162b2, is an mRNA vaccine. Although the technology behind mRNA vaccines began more than two decade ago, the vaccine to fight COVID-19 is the first one of its kind approved by the U.S. Food and Drug Administration (FDA).

The mRNA, which carries the information for making a specific SARS-CoV-2 protein (the spike protein), is formulated with fats (lipids) to help get it inside cells. After the vaccine is injected into the arm, the body’s cells take up the mRNA and make the viral protein. The protein is foreign to the body, so it triggers an immune response. The response to the S protein also creates a “memory” of how to fight SARS-CoV-2 if you are exposed to it in the future. Two doses of the vaccine, spaced 21 days apart are the most protective. The first dose “primes” the immune system; the second dose strengthens it.

In Pfizer-BioNTech’s 2020 clinical trial, the vaccine was 95% effective in preventing COVID-19, with minimal side effects. From COVID-19 case counts tabulated after large-scale vaccination was underway, vaccine effectiveness against infection was 90%. Vaccination is slightly less protective against the COVID-19 Delta variant, a mutated form of SARS-CoV-2. Recent studies estimate 88% efficacy against hospitalization from Delta.

How long the vaccine remains protective is not known exactly. Levels of vaccine-induced antibodies begin to decline several months after vaccination, although that is not unusual for a vaccine. Vaccination also stimulates cellular immunity, a type of immunity that can last many years. Still, the FDA authorized a booster dose of the Pfizer vaccine for certain at-risk groups, to be administered at least six months after the primary vaccination (two-dose series).

The COVID-19 vaccine may not be as effective in immunocompromised people compared to the general public. The Centers for Disease Control and Prevention (CDC) advises moderately to severely immunocompromised people receive an additional dose of the Pfizer-BioNTech vaccine. The additional dose may improve protection against COVID-19.

The vaccine gained full approval for people aged 16 and older from the FDA. Individuals aged 12 to 15 years may receive the Pfizer-BioNTech COVID-19 vaccine under the FDA’s current emergency use authorization (EUA). Pfizer and BioNTech announced a good safety and efficacy profile from a trial involving younger children, ages 5 to 11.

There are health risks associated with the Pfizer-BioNTech vaccine. There is a remote chance of a severe allergic reaction from the vaccine. People allergic to any of the vaccine’s ingredients—including lipids like polyethylene glycol, various potassium and sodium salts, and sucrose—should not get the Pfizer-BioNTech vaccine. There is also a rare chance of developing myocarditis—inflammation of the heart—following vaccination. It is often mild and resolves with rest and medication, although some people have been hospitalized for the complication.

What is the Moderna COVID-19 vaccine?

The Moderna COVID-19 vaccine is also an mRNA vaccine (“mRNA-1273”) packaged in lipids (fat molecules) to help it get inside the body’s cells. The mRNA carries the information to make the SARS-CoV-2 spike protein. After vaccination, the body’s cells make the S protein to trigger the immune system. Within weeks, your immune system creates a “memory” of the spike protein. If you are exposed to the actual virus while you are immune, your body already knows the virus is foreign and attacks it before it can multiply and make you sick. Two doses of the vaccine are required, spaced 28 days apart.

The 2020 Moderna COVID-19 vaccine clinical trial showed the vaccine was nearly 95% effective in preventing COVID-19. Moderna's vaccine received an emergency use authorization (EUA) in December 2020. Like the Pfizer vaccine, the Moderna vaccine was about 90% effective in preventing COVID-19 in “real-world” conditions, but is less effective against the COVID-19 Delta variant. Recent studies estimate the Moderna vaccine is 93% effective against COVID-19 hospitalization caused by the Delta variant, and does not see the same reduction in efficacy as the Pfizer vaccine.

Individuals 18 years and older can get the Moderna COVID-19 vaccine under the FDA’s current EUA (emergency use authorization). The company has applied for full FDA approval. The FDA has also authorized a booster dose (half dose) and an additional dose of the Moderna vaccine for immunocompromised people who have already received two doses of the Moderna vaccine.

Like the Pfizer COVID-19 vaccine, people who have received the Moderna vaccine have developed severe allergic reactions. It remains a remote possibility. People who have had a severe allergic reaction to any of the vaccine’s ingredients—including lipids like polyethylene glycol as well as tromethamine, acetic acid, sodium acetate trihydrate, and sucrose—should not receive the Moderna vaccine. There have also been cases of myocarditis following vaccine administration. The risk of myocarditis is very low.

What is the Janssen (Johnson & Johnson) COVID-19 vaccine?

Janssen Research and Development, part of Johnson & Johnson (J&J) was one of the first pharmaceutical companies to partner with the U.S. effort to develop a COVID-19 vaccine. The J&J vaccine, Ad26 SARS-CoV-2, is a viral vector type of vaccine. The vector is based on a common cold virus called an adenovirus. The vector cannot multiply inside cells; it only serves to transport the SARS-CoV-2 spike protein instructions into the body. Upon vaccination, cells “read” the instructions and produce the coronavirus spike protein, which triggers the same type of immune response as other vaccines. (Janssen successfully used the same adenovirus technology in their Ebola vaccine currently in use in the Democratic Republic of Congo.)

From clinical trials, J&J/Janssen determined a single dose of the vaccine was safe and highly effective: 66% effective in preventing symptomatic COVID-19 and 85% effective in preventing hospitalization. The FDA authorized the J&J vaccine for emergency use in late February 2021 for individuals 18 and older.

The J&J vaccine also protects people from the Delta variant, but immunity may not be as strong compared to the original SARS-CoV-2 virus. Recent studies comparing the J&J, Moderna and Pfizer vaccines estimate the J&J vaccine is 71% effective against hospitalization caused by the Delta variant. However, a larger study estimated 81% vaccine effectiveness.

J&J announced that immunity from their vaccine is strong and long-lasting, at least 5 to 6 months post-vaccination. However, the FDA authorized a booster dose because it increases protection against disease.

Side effects of the J&J vaccine are similar to other vaccines: Injection site pain and swelling, as well as fever, chills and aches. Serious health effects have also been reported. Vaccine adverse event reporting revealed a connection between blood clots and the J&J vaccine. Specifically, some people developed rare types of blood clots combined with low platelet count 1 to 2 weeks after getting the vaccine.

Officials temporarily paused J&J vaccinations while investigating the cases, most of which occurred in females between the ages of 18 and 49. The adverse event is extremely rare, but some women died. An expert panel reviewed all known cases and decided the J&J vaccine is still very safe and that the benefits of the J&J vaccine outweigh the risks. The pause was lifted, and J&J vaccine fact sheet includes a warning about the remote chance of developing thrombosis with thrombocytopenia syndrome (TTS).

Who is eligible to receive the COVID-19 vaccine?

The Pfizer-BioNTech COVID-19 vaccine has emergency use authorization (EUA) for ages 5 to 15 and full FDA approval for ages 16 and older. The Moderna and J&J vaccines have an EUA for people ages 18 and older.

Vaccination is free to any eligible person living in the United States.

Can I be forced to get to the COVID-19 vaccine?

Maybe. Some employers are requiring it of their employees, and some schools are requiring students be vaccinated to attend. Each state, municipality or corporation can set rules regarding vaccine mandates for certain categories of citizens and workers. Employees (and students) may file for an exemption based on medical or religious reasons.

The Biden Administration is mandating all Federal workers be vaccinated against COVID-19, under an executive order signed by President Biden on September 9, 2021. The vaccine mandate is subject to exceptions required by law.

When will a vaccine be available for babies and very young children?

Vaccine developers are currently conducting clinical trials in younger participants, ages 6 months to 5 years.

Why should I get vaccinated when I’m eligible?

COVID-19 is a public health and life-threatening emergency, and there is only one FDA-approved treatment for COVID-19. (The FDA has authorized, for emergency use, some monoclonal antibody treatments.) Vaccination is a more effective and safer way to stop the spread of COVID-19 and save lives.

Reasons to get vaccinated include:

  • Being fully vaccinated protects you from developing a severe case of COVID-19 or dying of COVID-19. Although breakthrough infections are possible, vaccination limits the length of time you are contagious (including Delta variant infections), which will help reduce the spread of the virus to others.
  • Being fully vaccinated protects those around you who cannot get vaccinated, including young children, and those who are vaccinated but have a weakened immune system.
  • The FDA approved the Pfizer-BioNTech COVID-19 vaccine, marketed as Comirnaty. The vaccine meets the FDA's high standards for safety, effectiveness, and manufacturing quality. People ages 12 to 15 can still receive the Pfizer-BioNTech vaccine under the vaccine's emergency use authorization (EUA), originally granted in December 2020. Full approval of the Moderna and Johnson & Johnson vaccines are expected.
  • The vaccines are very safe. Although all three vaccines can trigger serious adverse health events, these events are rare. After careful consideration of the risks and benefits, a panel of experts advised the CDC that the known risks do not outweigh the known and potential benefits of the vaccines.
  • Getting vaccinated gives you more freedom. Fully vaccinated people may travel publicly without worrying about COVID-19 testing requirements, unless your destination requires it. It is also low risk to spend in-person time with other fully vaccinated friends and family.

If I already had COVID-19, should I still get vaccinated?

Yes. Natural immunity offers some protection from reinfection, but studies show that vaccination of previously infected people increases the level of antibodies and protection against SARS-CoV-2. It is not known for sure how long natural or vaccine immunity lasts, but vaccination appears to strengthen any existing immunity from prior infection. Some studies show that antibodies produced after infection last at least six months, possibly longer; whereas, antibodies after vaccination may last at least eight months. If you've recovered from COVID-19 infection, getting vaccinated further reduces your risk of reinfection.

Where do I get the COVID-19 vaccine?

You can get the COVID-19 vaccine at most pharmacies, hospitals, and specially designated vaccination sites in your community. Your state’s public health department can provide information about where to get a COVID-19 vaccine. You may also select your state from the CDC's vaccine center Trusted Source Centers for Disease Control and Prevention (CDC) Governmental authority Go to source . Many places no longer require an appointment.

How much does the COVID-19 vaccine cost?

All COVID-19 vaccines and booster doses are free. The U.S. Department of Health and Human Services is partnering with many major pharmacies and health networks to provide the vaccine with no out-of-pocket costs. Healthcare providers outside of a partnership may assess a fee for administering the injection.

How is COVID-19 vaccination performed?

A healthcare provider injects the vaccine into the upper arm muscle. Someone may monitor you for up to 30 minutes after receiving the injection to make sure you don’t experience a serious reaction. The vast majority of people tolerate the vaccine without serious symptoms.

What are the known COVID-19 vaccine side effects?

Like many other vaccines, the COVID-19 vaccine will likely cause some side effects. These signs tell you your immune system is responding to the vaccine. It is rare to experience serious or life-threatening reactions to the COVID-19 vaccines.

The most common COVID-19 vaccine side effects include:

  • Fatigue
  • Fever
  • Muscle and joint aches
  • Pain, redness or swelling at the injection site

How will I feel after I get the COVID-19 vaccine?

The most common side effect is injection site pain for a few days. Some people report feel ill with flu-like symptoms within 24 hours of receiving the COVID-19 vaccine. Some people report that these symptoms occur more strongly after the second dose of the vaccine. This is normal, and you should feel better within 48 hours. Acetaminophen (Tylenol) can help relieve pain, but take it after your vaccination.

If you get the vaccine after recovering from COVID-19, you might experience more side effects after the first dose than the second dose (if you are getting a two-dose vaccine).

What are the potential risks and complications of the COVID-19 vaccine?

The Pfizer-BioNTech, Moderna, and J&J COVID-19 vaccines did not cause any serious, adverse events in the thousands of clinical trial participants. However, after millions of vaccinations, it became clear that some people experienced adverse reactions.


Severe allergic reactions, including anaphylaxis, have occurred in people after receiving the Pfizer-BioNTech or Moderna vaccines. In one analysis, anaphylaxis occurred at a rate of 1 in 100,000 doses of Pfizer vaccine administered, which is a rare event. Allergic reactions are possible with other types of vaccines as well. Anaphylaxis symptoms include:

  • Loss of consciousness
  • Swelling of the lips, tongue, or throat

If these symptoms occur after receiving a vaccine, call 911 for emergency medical attention.

The Janssen COVID-19 vaccine uses a different technology and ingredients than the other two authorized vaccines. Still, the J&J fact sheet states "there is a remote chance that the Janssen COVID-19 Vaccine could cause a severe allergic reaction." If a reaction occurred, it would likely occur within a few minutes to an hour after receiving the injection. The most common side effects are injection site pain, headache, fatigue, and muscle pain.

Blood clotting syndrome

A blood clot (thrombosis) in the brain, abdomen or legs, combined with a low platelet count, has occurred in some people 1 to 2 weeks after receiving the J&J vaccine. The complication, called thrombosis with thrombocytopenia syndrome (TTS) is very rare, but potentially fatal. Contact your healthcare provider for any of these signs and symptoms of a blood clot after receiving the J&J vaccine:

  • Changes in vision, decreased motor function, loss of sensation or other neurologic symptoms
  • Severe or persistent headache


Inflammation of the heart has occurred in some people following mRNA vaccine administration (Moderna and Pfizer-BioNTech). Based on known cases, it is more common in adolescent males following the second dose. Most developed a mild illness and completely recovered with medication and rest. Officials are investigating each case and watching for new cases in order to further define the possible risk factors for this adverse event. The CDC, FDA, and American Heart Association/American Stroke Association recommend vaccination (with either the mRNA vaccines or J&J vaccine) for everyone eligible, because the benefit of protection from COVID-19, which can cause heart problems, outweighs the risk of myocarditis.

Guillain-Barré syndrome (GBS)

There have been reports of Guillain-Barré syndrome, a neurological condition, in some people after receiving the J&J vaccine (about 100 preliminary reports out of 12.8 million J&J vaccine doses administered since July 12, 2021). GBS is a very rare complication of certain infections, but can also occur after surgery or vaccination. The CDC is investigating all GBS reports to learn more about potential risk factors.

You can register for and use v-safe Trusted Source Centers for Disease Control and Prevention (CDC) Governmental authority Go to source , a voluntary surveillance system, to report any side effects or complications. You may also use the Vaccine Adverse Event Reporting System, or VAERS. The CDC or FDA studies the data on an ongoing basis and follows up with people on certain events. Healthcare providers are required by law to report certain adverse events, including any of the events listed above.

Will I need a booster dose?

No one expects vaccination to deliver lifelong immunity to COVID-19, but some researchers predicted it would protect you for a year or two. That prediction has been called into question due to a rise in COVID-19 breakthrough cases—infections among vaccinated individuals. However, the rise in breakthrough cases coincided with spread of the Delta variant, a highly contagious and infectious form of the pandemic virus capable of blunting immunity.

The objective of the COVID-19 vaccine was to prevent serious illness, hospitalization and death, and all three vaccines can do that for several months post-vaccination, even with the Delta variant. Pfizer vaccine effectiveness against COVID-19 hospitalization wanes after four months, according to a recent study comparing Moderna, Pfizer and J&J vaccine effectiveness.

Waning immunity and the threat of the Delta variant intensified calls for a booster dose of vaccine. After reviewing all available evidence, the U.S. Food and Drug Administration (FDA) authorized a booster dose of the Pfizer-BioNTech, Moderna, and J&J vaccines. You may also "mix and match" vaccines, or receive a booster of a vaccine different from the primary vaccine. Eligibility and dosing schedules vary by vaccine.

How many people have to get vaccinated to reach herd immunity?

Herd immunity is when enough people are immune that more vulnerable groups in the herd are indirectly protected from the disease. A vaccine is a faster and much safer way to reach herd immunity than exposing people to a life-threatening infection. Experts estimate 75 to 90% of the population needs to be vaccinated before achieving herd immunity. For the United States, with a population of about 330 million, more than 200 million people would need to be vaccinated or infected to reach herd immunity.

Can I stop wearing a face mask and see my friends again after we’re all vaccinated?

Sometimes. It takes several days after vaccination for the body to begin manufacturing protective antibodies against SARS-CoV-2. With the Pfizer and Moderna vaccines, immunity requires two doses of the vaccine, spaced weeks apart. You are considered fully vaccinated two weeks after your second dose or after a single dose of the J&J COVID-19 vaccine. Until that time, you have a greater risk of becoming infected and of spreading the infection to others around you. The COVID-19 Delta variant is partially resistant to the vaccine, so it is important to be fully vaccinated.

It is considered low risk for fully vaccinated people to visit with other vaccinated people without wearing a mask. It is also low risk when you are with unvaccinated people with a low risk of developing severe COVID-19. But due to the more contagious Delta variant, the CDC advises fully vaccinated people to continue to wear a mask in indoor, public spaces. This is especially important for people who are more susceptible to COVID-19 than others, either because they have a weakened immune system or an underlying condition that increases their risk of developing severe COVID-19. You may choose to wear a mask, regardless of transmission level, if someone in your household is at risk.

What happens if I miss the second dose of COVID-19 vaccine?

If you received a COVID-19 vaccine that requires two doses, you should get the second dose as soon as possible. The first dose of vaccine primes the immune system, but the second shot gives it a boost and is necessary to reach maximum effectiveness at preventing mild to severe COVID-19.

It is important to be fully vaccinated because the COVID-19 Delta variant, which is more contagious than the prior most common variant (Alpha) in the U.S., is partially resistant to virus-specific antibodies. In other words, it takes stronger immunity to resist the Delta variant than the original SARS-CoV-2 virus.

Was this helpful?
  1. Different COVID-19 Vaccines. U.S. Centers for Disease Control and Prevention.
  2. There are Four Types of COVID-19 Vaccines: Here’s How They Work. Gavi, the Vaccine Alliance.
  3. What to Expect After Getting a COVID-19 Vaccine. U.S. Centers for Disease Control and Prevention.
  4. Pfizer-BioNTech COVID-19 Vaccine. U.S. Food and Drug Administration.
  5. COVID-19 Vaccine FAQ. UCI Health.
  6. Moderna CEO Says Vaccine Likely to Protect for “Couple of Years.” Reuters.
  7. COVID-19 Vaccines: Get the Facts. Mayo Clinic.
  8. Desai A, Majumder M. What is Herd Immunity? JAMA. 2020;324(20):2113.
  9. COVID-19 Vaccines and Allergic Reactions. U.S. Centers for Disease Control and Prevention.
  10. Lumley SF, O’Donnell D, Stoesser NE, et. al.; Oxford University Hospitals Staff Testing Group. Antibody status and incidence of SARS-CoV-2 infection in health care workers [published online December 23, 2020]. NEJM. 2020.
  11. Coronavirus Disease 2019 (COVID-19) Daily Research Briefs. American Family Physician.
  12. A side-by-side comparison of the Pfizer/BioNTech and Moderna vaccines. STAT.
  13. From the Frontlines: Understanding Herd Immunity. American Lung Association.
  14. Fauci says the US could start vaccinating 1 million people per day — double its current rate over the last few days, January 3, 2021. Business Insider.
  15. Statement on California Department of Public Health (CDPH) Report. Moderna.
  16. Why Do I Need the Second COVID-19 Vaccine Shot? GoodRx.
  17. Should We Change COVID Vaccine Doses to Reach More People? What the Data Say, January 14, 2021. Scientific American.
  18. Interim Clinical Considerations for Use of mRNA COVID-19 Vaccines Currently Authorized in the United States. U.S. Centers for Disease Control and Prevention.
  19. Johnson & Johnson COVID-19 Vaccine Authorized by U.S. FDA For Emergency Use - First Single-Shot Vaccine in Fight Against Global Pandemic. Johnson & Johnson.
  20. Myocarditis and Pericarditis Following mRNA COVID-19 Vaccination. U.S. Centers for Disease Control and Prevention.
  21. Vaccines and Related Biological Products Advisory Committee June 10, 2021 Meeting Announcement. U.S. Food & Drug Administration.
  22. Self WH, Tenforde MW, Rhoads JP, et al. Comparative Effectiveness of Moderna, Pfizer-BioNTech, and Janssen (Johnson & Johnson) Vaccines in Preventing COVID-19 Hospitalizations Among Adults Without Immunocompromising Conditions — United States, March–August 2021. MMWR Morb Mortal Wkly Rep 2021;70:1337–1343. DOI:
  23. Johnson & Johnson Announces Real-World Evidence and Phase 3 Data Confirming Strong and Long-Lasting Protection of Single-Shot COVID-19 Vaccine in the U.S. Johnson & Johnson.
  24. Polinksi JM, Weckstein AR, Batech M, et al. Effectiveness of the Single-Dose Ad26.COV2.S COVID Vaccine. medRxiv 2021.09.10.21263385; doi:

Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2021 Apr 16
View All Vaccines Articles
THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.