• COVID-19 Vaccine FAQs

    Updated May 14, 2021

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

    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.

    Can the vaccines cause COVID-19?
    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.

    What vaccines are available?
    The FDA authorized three vaccines for COVID-19.
    The Pfizer-BioNTech mRNA vaccine was authorized for individuals 12 years and older. The Moderna mRNA vaccine and Janssen (Johnson & Johnson) adenovirus vaccine were authorized for individuals 18 years and older. All three vaccines were recommended by the CDC’s Advisory Committee on Immunization Practices (ACIP). They will review additional vaccines when authorized by the FDA. All states and jurisdictions have opened vaccine  appointments to eligible individuals.  Vaccines are available in doctor offices, pharmacies, and community locations. Find out where to get the vaccine at vaccines.gov.

    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, and have additional safety monitoring and informed consent and education associated with them.

    What are the differences between the vaccines authorized by the FDA?
    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. A side by side comparison is below.




    Janssen (J&J)

    Ages eligible for

    12 and older

    18 and older

    18 and older

    Length of time
    between doses

    21 days

    28 days

    n/a single dose

    Storage requirements

    -80 C; stable at
    4 C for 5 days

    -20 C; stable at
    4 C for 30 days

    -20 C; stable at
    4 C for 90 days

    Preparation of

    Reconstitution of lyophilized powder:
    5 doses per vial

    No dilution needed:  10 doses per vial

    No dilution needed:
    5 doses per vial

    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.

    What does it cost to get the vaccine?
    COVID-19 vaccines will be 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.

    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. However, the CDC and the FDA are monitoring the adverse events or side effects as the vaccines are distributed to the public.

    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

    How long does immunity last?
    It is not known how long immunity will last from the COVID-19 vaccine. In the clinical trials that have been conducted to date, the median length of follow-up was two months for vaccine recipients.

    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. It is likely that a vaccine will have a stronger and more lasting immune response, but data are limited and the research is ongoing.

    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 may still be possible to transmit the virus. In patients who received the Pfizer-BioNTech or Moderna vaccine, the high rate of efficacy in preventing disease was not observed until several weeks after the second dose of the vaccines. People who are vaccinated do not need to wear masks in public unless required by the business or organization or local public health measures. Masks will still be required in healthcare facilities. More data are needed to see how long immunity lasts. Additional rounds of immunizations may be needed.

    If I am vaccinated against COVID-19, can I still spread the virus to others?
    The vaccine trials conducted didn’t look at the vaccine’s ability to prevent virus spread. We know the vaccine is very effective at preventing illness in those receiving the vaccine. Because there aren’t data demonstrating the ability of the vaccines to prevent viral transmission, it is important to continue to wear a mask and socially distance even after getting vaccinated.

    Can I get the vaccine if I’ve already had COVID-19?
    Yes, although there aren’t enough data currently to determine how prior infection with COVID-19 affects the efficacy of the vaccine. It’s known that natural immunity to the virus decreases over time, so currently, under the EUA, individuals who were previously infected are eligible for the vaccine.

    Who can’t get the vaccine?
    Children under age 12 are not eligible to receive the Pfizer-BioNTech vaccine. 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. . Ppatients who are pregnant, lactating, or immunocompromised are able to determine if they wish 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 encouraged to have a discussion on the potential benefits and risks with their family physician. There is no evidence that the COVID-19 vaccines interfere with fertility, so individuals interested in having children should consider getting 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?
    Substantial data have shown that the COVID-19 vaccines are safe.
    Therefore, CDC has updated its clinical considerations so that COVID-19 vaccines can be given with other recommended vaccines. This covers both adolescent and adult immunizations. Individuals who are behind in receiving recommended immunizations are encouraged to get caught up on their vaccines. Additional information can be found here:  https://www.cdc.gov/vaccines/covid-19/info-by-product/clinical-considerations.html

    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. See more information at https://www.cdc.gov/vaccines/covid-19/info-by-product/clinical-considerations.html.

    Can I get the COVID-19 vaccine if I am pregnant or breastfeeding?
    There is currently limited data on the use of COVID-19 vaccines in pregnant or breastfeeding women. There have been no safety issues observed so AAFP and other organizations encourage individuals who are pregnant or lactating to consider getting vaccinated to prevent complications observed from COVID-19 infeciton. Individuals can talk with their family physician about the risks and benefits of being vaccinated.

    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.

    Because any COVID-19 vaccine will be provided under EUA, clinicians will have additional reporting requirements outlined in the EUA fact sheet from the FDA. Each state and jurisdiction has plans in place for reporting. 

    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. 


    1. Centers for Disease Control and Prevention. Understanding mRNA vaccines. https://www.cdc.gov/coronavirus/2019-ncov/vaccines/different-vaccines/mrna.html. Accessed Dec. 12, 2020.
    2. Centers for Disease Control and Prevention. Frequently asked questions about COVID-19 vaccine. https://www.cdc.gov/coronavirus/2019-ncov/vaccines/faq.html. Accessed Dec. 12, 2020.
    3. COVID-19 Vaccines for Children and Teens: https://www.cdc.gov/coronavirus/2019-ncov/vaccines/recommendations/adolescents.html        
    4. U.S. Food and Drug Administration. Vaccines and Related Biological Products Advisory Committee Meeting, Dec. 10, 2020 FDA Briefing Document, Pfizer-BioNTech COVID-19 vaccine. https://www.fda.gov/media/144245/download. Accessed Dec. 10, 2020.
    5. U.S. Food and Drug Administration. Vaccines and Related Biological Products Advisory Committee Meeting, Dec. 17, 2020 FDA Briefing Document, ModernaTX COVID-19 vaccine. https://www.fda.gov/media/144434/download. Accessed Dec. 17, 2020.
    6. U.S. Food and Drug Administration. Vaccines and Related Biological Products Advisory Committee Meeting. February 28, 2021. FDA Briefing Document. Janssen COVID-19 vaccine. https://www.fda.gov/media/146217/download. Accessed February 26, 2021.