Covid-19 Vaccine FAQ


Covid-19 Vaccine FAQs


Updated January 7, 2021

Information you can use to answer patients’ most common questions about the vaccines.

What vaccines are available?
There are currently three vaccines in the U.S. against COVID-19. All vaccines are effective at preventing hospitalizations and death and are recommended by the CDC and AAFP. 
The Pfizer-BioNTech mRNA vaccine, Comirnaty, was given full approval by the FDA for individuals 16 years and older. It is still is under emergency use authorization (EUA) for individuals aged 5-15. The Moderna mRNA vaccine and Janssen (Johnson & Johnson) adenovirus vaccine were authorized for individuals 18 years and older.

In most situations, Pfizer-BioNTech or Moderna COVID-19 vaccines are preferred over the Janssen COVID-19 vaccine for primary and booster vaccination.

How do COVID-19 vaccines work?
Two COVID-19 vaccines use messenger RNA (mRNA), which is a set of instructions that tells a cell to make a specific protein. For SARS-CoV-2 (COVID-19), this is the spike protein that is found on the surface of the viral envelope. The mRNA used in the vaccines don’t enter the cell’s nucleus and has no interaction with a cell’s DNA. It is also not a full virus and cannot replicate itself. The mRNA is rapidly broken down by the cell once the instructions have been transmitted, so it doesn’t cause mutations or cellular defects, and has not been associated with infertility.

The other vaccine uses a modified adenovirus that contains DNA for the spike protein. The adenovirus is able to enter a cell and cause the spike protein to be made. Adenoviruses are a source of the common cold, but this particular virus can’t replicate so it won’t cause disease.

Once the spike protein is made, it is put on the surface of the cell, where it is seen by the immune cells and causes them to become activated and respond. The result is the production of neutralizing antibodies. If a person who is immunized becomes infected with the virus, the neutralizing antibodies will bind to the virus and prevent it from entering cells and causing disease.

What’s the difference between a booster shot and an additional dose?
An “additional dose” refers to people who are moderately to severely immunocompromised receiving an additional dose of an mRNA COVID-19 Vaccine (Pfizer-BioNTech or Moderna). This is because they may not have received adequate protection from their initial 2-dose vaccine series.

“booster dose” is a supplemental vaccine dose given to people when the immune response to a primary vaccine series was adequate but is likely to have decreased over time. Learn more about the COVID-19 vaccine boosters.

Can the vaccines cause COVID-19?
No. An mRNA vaccine is not a virus and can’t cause disease.
 Because it activates the immune system, it can cause mild symptoms in some people (e.g., fatigue, achiness, fever). Based on data from the clinical trials, the most common reactions to the vaccine are pain at the injection site, fatigue, headache, and muscle aches. These symptoms are very common with other vaccines, including the flu shot, and are a sign that the body is responding to the vaccine.

The other authorized vaccine uses a modified virus that can’t replicate and does not cause any disease, including COVID-19.

Additionally, the mRNA vaccines have been authorized for an additional dose following the primary series. Booster doses have been authorized for all three vaccines. Adults over age 18 can get any vaccine as a booster; adolescents 16-17 years old can only get Pfizer-BioNTech as a booster dose. Timing on when to get a booster depends on which vaccine you received as your primary series.

Moderna and Pfizer-BioNTech Boosters (mRNA 2-dose vaccines)

  • People who have completed a primary series (2 shots) of the mRNA vaccines at least 5 months ago.
  • 18 years of age can receive any vaccine as a booster dose, though mRNA is preferred in most cases.
  • 12-17 years of age can only receive Pfizer-BioNTech vaccine as a booster dose.

Janssen/Johnson & Johnson Booster (Single dose vaccine)  

  • A single Janssen COVID-19 vaccine booster dose may be administered at least 2 months after completion of the single-dose primary regimen to individuals 18 years of age and older.

Can I get a different vaccine booster dose than the vaccine I originally got?
Yes. You can “mix and match” booster dose in eligible individuals following completion of primary vaccination with a different COVID-19 vaccine. The FDA has indicated the potential benefits of a mixed vaccine dose outweigh the risks.

Some individuals may choose to get a different vaccine for their booster based on local availability, or individual risks and benefits. AAFP has compiled additional FAQs for booster doses. Vaccines are available in doctor offices, pharmacies, and community locations. Find out where to get the vaccine at

What is the difference between the emergency use authorization and licensure (approval) by the FDA?
Emergency use authorization (EUA) is a process by which the FDA can authorize use of a medication or vaccine with less data if the benefit of the vaccine has been shown to outweigh the risk. EUAs can be issued only during a declared emergency, such as the COVID-19 pandemic. Vaccines issued an EUA will continue to be studied to ensure they meet the high standards for safety, effectiveness, and manufacturing quality the FDA requires of an approved product.

The FDA approved the first COVID-19 vaccine, Pfizer/BioNTech, for individuals 16 years of age and older.

What are the differences between the COVID-19 vaccines?
Two of the vaccines are mRNA vaccines that have a piece of mRNA specific for the SARS-CoV-2 spike protein. They have similar efficacy and safety profiles. The third vaccine uses a non-replicating adenovirus to deliver the spike protein into nearby cells. None of the vaccines use live viruses so there is no risk of infection. The main differences between the vaccines include the ages of individuals eligible to get the vaccines, the length of time between doses, the number of doses, the cold chain requirements for storage, and the preparation of the vaccine.


Authorized Vaccine Overview

Pfizer-BioNTech Moderna Janssen (J&J)     
  • Approved for 12 and older
  • EUA for ages 5-15
  • additional (3rd) dose for immunocompromised individuals ages 5 and older
  • booster dose for individuals 12 and older 5 months after initial series
  • EUA for ages 18 and older
  • additional (3rd) dose for
    immunocompromised individuals 18 and older
  • booster dose for individuals aged 18 and older 5 months after initial series
  • EUA for ages 18 and older
  • EUA for a booster dose given at least 2 months later for all adults 18 years and older who received first dose


Ages eligible for vaccine 5 and older 18 and older 18 and older
Length of time between doses 21 days 28 days n/a single dose
  • COVID-19 vaccines currently approved or authorized by FDA are effective in preventing serious outcomes of COVID-19, including severe disease, hospitalization, and death.
  • COVID-19 vaccines are recommended for everyone aged 5 years and older in the United States.
  • In most situations, mRNA COVID-19 vaccines are preferred over the Janssen COVID-19 vaccine for primary and booster vaccination.


Why should I get a vaccine?
All COVID-19 vaccines are
 effective at preventing COVID-19, hospitalizations and deathBy getting vaccinated, you are reducing your risk of disease, hospitalization, severe complications, and even death. Reducing the risk of disease also prevents the health care system from being overwhelmed.

While it is still possible to pass the virus to others if you are vaccinated, the risk is significantly lower. However, to prevent the spread of the highly contagious Delta and Omicron variants, vaccinated individuals should wear masks in indoor public areas (schools, businesses, etc).

What does it cost to get the vaccine?
COVID-19 vaccines available at no cost
 to individuals, and clinicians administering the vaccine will be reimbursed for vaccine administration. See guidance on coding and payment.

Should I take any pain medications before getting the vaccine?
No. It is not recommended for people to take pain relievers before getting the vaccine as it is not known how these medications may affect how well the vaccine works. Get tips on relieving pain and discomfort after the vaccine.

How many doses are needed?
The mRNA vaccines require two doses
; the Pfizer-BioNTech vaccine should be given 21 days apart and the Moderna vaccine doses should be spaced 28 days apart for an effective immune response. Recipients should get the second dose from the same manufacturer as their first dose. However, if they get a dose of a different vaccine, no additional doses are needed, and the series is considered complete. The Johnson & Johnson vaccine is a single dose to complete the series.

Individuals with moderate to severe immunosuppression may receive an additional dose 28 days after to conclusion of their the primary series of either of the mRNA COVID-19 vaccines.

COVID-19 vaccines remain effective at preventing severe illness, hospitalization, and death, even against the widely circulating variants. However, public health experts have seen reduced protection, especially among certain populations, against mild and moderate disease. The FDA authorized, and the CDC recommended, a booster dose for adults aged 16 and older.

See AAFP’s booster dose FAQs for more information or the CDC’s updated clinical information.

What are the side effects of the vaccine?
Data from the clinical trials of all three candidates indicate that the most common reactions were pain at the injection site, fatigue, headache, and muscle aches. These symptoms are commonly seen with other vaccines. A few people also reported fever and nausea. These symptoms were observed in adolescents as well as adults. No serious side effects were seen in the data reported in the trials. The CDC and the FDA will continue monitoring adverse events or side effects.

There have been reports of a few cases of severe allergic reaction to one of the components of mRNA vaccines and one component in the Johnson & Johnson vaccine. Individuals receiving any of the vaccines should be monitored for 15-30 minutes after injection. If a person has an allergic reaction following the first dose of an mRNA vaccine, they can get the Johnson & Johnson vaccine for their second dose.  Additionally, there have been a few reports of blood clots along with low platelet counts after receiving the Johnson & Johnson vaccine. These reports were studied thoroughly. It was determined that these events were rare, and millions of people have been vaccinated without any major issues. Read more on the CDC website.

Should I be worried about experiencing Myocarditis?
Myocarditis, inflammation of the heart, has also been observed in some individuals following receipt of one of the mRNA COVID-19 vaccines. However, these have been rare events—only around 700 out of almost 200 million people who have received at least one dose of the vaccine. Most cases were mild and resolved within a few days after vaccination. Myocarditis is fairly common after viral infections, and the risk of myocarditis is much higher with viral infection from COVID-19.

What is TTS?
TTS stands for thrombosis plus thrombocytopenia, or blood clot with low platelet counts. TTS is a rare event that could occur after vaccination (typically within 2 weeks after receipt of the J&J vaccine. People should seek medical attention immediately if they develop any of the following symptoms:

  • shortness of breath
  • chest pain
  • leg swelling
  • persistent abdominal pain
  • severe or chronic headaches or blurred vision
  • easy bruising or tiny blood spots under the skin (not including the injection site)

People who have a history of TTS should not get a booster of the J&J vaccine and are recommended to get an mRNA COVID-19 vaccine as a booster at least 2 months (8 weeks) following their dose of the Janssen COVID-19 Vaccine and after their clinical condition has stabilized. Prior to booster vaccination, a conversation between the patient and their clinical team, including a hematologist or other specialists, may assist with decisions about using an mRNA COVID-19 vaccine as a booster and the timing of the booster vaccination.

See the CDC clinical considerations for more information.

How long does immunity last?
It is not known how long immunity will last from the COVID-19 vaccine. 
It’s also not known how long immunity from natural infection lasts; there are reports of waning antibody levels around three months after infection, and a few cases of reinfection have been reported. We do know that seasonal coronaviruses (a source for the common cold) do not induce a robust immune response, which leads to limited immunity to these viruses. Research is ongoing, but so far studies have demonstrated real world effectiveness against hospitalization and severe disease with all three authorized vaccines including against the variants.

Do I still need to wear a mask and physically distance if I have the vaccine?
While the vaccines provide protection against COVID-19 disease, it is still possible to become infected and transmit the virus. This means that fully vaccinated individuals may still get COVID-19, although they will be protected against severe disease, hospitalization, and death. In areas with substantial and high transmission, fully vaccinated individuals should wear a mask in public indoor settings to help prevent spread of variants and protect others. Universal indoor masking for all teachers, staff, students, and visitors to K-12 schools, regardless of vaccination status, is also recommended to allow for full-time in-person learning with proper prevention strategies in place.

If I am vaccinated against COVID-19, can I still spread the virus to others?
The vaccines are very effective at preventing illness, hospitalizations, and death in those receiving the vaccine. However, data are showing that with the increase in the circulation of variants, it is possible for people who have been fully vaccinated to get and spread COVID-19. It is extremely important for all individuals to wear masks and distance when in public spaces (schools, work, businesses) in order to prevent spread of the virus to people who have not been vaccinated, most importantly children under age 12.

Can I get the vaccine if I’ve already had COVID-19?
Data from the clinical trials showed that the vaccines are safe for individuals who were previously infected with SARS-CoV-2. The length of protection against reinfection is not known so it is recommended for all eligible individuals to get vaccinated.

Who can’t get the vaccine?
Children under age 5 are not eligible to receive a COVID-19 vaccine at this time. 
Those under age 18 are not eligible to receive the Moderna or Johnson & Johnson. Clinical trials have begun to determine safety and efficacy in younger individuals. patients who are pregnant, lactating, or immunocompromised are encouraged to receive the vaccine. No safety issues have been observed in individuals who were pregnant or lactating who also received the vaccine. These patients are recommended to receive the vaccine. There is no evidence that the COVID-19 vaccines interfere with fertility, so individuals interested in having children should get vaccinated.

As with other vaccines, anyone who has a fever or other symptoms may not be able to get the vaccine until their symptoms resolve. This includes those who have symptoms or have tested positive for COVID-19. There is also caution for people with documented anaphylactic reactions to vaccines or who had an allergic reaction to the first dose of COVID-19 vaccine.

Can I get other vaccines, like the flu shot, at the same time as the COVID-19 vaccine?
Yes! Substantial data have shown that the COVID-19 vaccines are safe.
 The CDC updated its clinical considerations so that COVID-19 vaccines can be given at the same time as other recommended vaccines, like the flu shot. This covers both adolescent and adult immunizations. Individuals who are behind in receiving recommended immunizations are encouraged to get caught up on their vaccines to protect themselves from preventable diseases.

Individuals who have a known allergy to any of the components of the vaccines should receive a different type of vaccine. Based on recent safety data, individuals with a history of thrombosis plus thrombocytopenia (TTS), or blood clot with low platelet count, have a contraindication to receiving the Janssen (J&J) vaccine. These individuals are recommended to get one of the mRNA vaccines.

If I have allergies, can I get the COVID-19 vaccine?
 Seasonal allergies and even food allergies, including allergies to shellfish and peanuts, do not exclude you from getting the COVID-19 vaccine. Some individuals who have known allergies to any components of the vaccine or a reaction to a first dose of the vaccine should talk to their family doctor as to which vaccine is right for them.

Can I get the COVID-19 vaccine if I am pregnant or breastfeeding?
Individuals who are pregnant or lactating are strongly recommended to get vaccinated against COVID-19. Data have demonstrated that the vaccines are effective and safe for people who are considering pregnancy, are pregnant, or recently pregnant. Vaccination provides protection for both the parent and baby against major issues seen after infection with COVID-19.

How do I report symptoms after the vaccine?
Vaccine recipients should report side effects (called adverse events) to the Vaccine Adverse Event Reporting System (VAERS). This nationwide program collects data to use as signals of unexpected events from a vaccine. If you have a question on what might be considered a side effect related to the vaccine, talk with your family physician.

In addition to VAERS, the CDC implemented a new, smartphone-based tool called v-safe that sends text messages to encourage adverse event reports or impact to quality of life. This system requires a smartphone, and recipients must opt into the system. Information on v-safe is provided to anyone who gets the vaccine, along with a card indicating which vaccine and dose was given, and the EUA fact sheet. Parents will be able to sign up for v-safe on behalf of their child who receives the vaccine.

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