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COVID-19 Vaccine FAQs

This information has been reviewed by Carrie Horn, MD (September 2022).

About the Vaccine

Q: Can the COVID-19 vaccine give you the disease? 

A: No! The vaccine cannot give you COVID-19. The mRNA vaccines from Pfizer and Moderna, the adenovirus vaccine from Johnson & Johnson and the protein-based vaccine from Novavax contain only a tiny part of the virus's genetic material. The mRNA vaccines contain instructions to make just one protein called "s" or "spike." None of the vaccines contain the pieces needed to make more virus. You can have side effects from any of the vaccines, including low-grade fevers, chills and body aches. However, these are the effects of your immune system responding to the vaccine while it is developing immunity.  

Q: How do the COVID-19 vaccines work?

A: Vaccines expose us to pieces of bacteria or a virus, which cause our bodies to recognize a potential threat and mount an immune response. The goal with a vaccine is to help the body prepare for an infection. It will be then be ready to effectively eliminate the SARS-CoV-2 virus.
There are different types of vaccines.

  • The Pfizer, Moderna, Novavax and Johnson & Johnson vaccines all carry genetic material. This is what the cells use to make copies of the viral spike protein.
  • The Pfizer and Moderna messenger RNA (mRNA) vaccines deliver genetic material inside minuscule oily bubbles.
  • The Novavax vaccine uses a traditional delivery, with an ingredient that stimulates the immune system, along with a version of the viral spike protein.
  • The Johnson & Johnson vaccine carries the coronavirus genetic material within a harmless cold virus, which has been altered so that it cannot copy itself inside a cell and therefore cannot make you sick.

Once your cells use that genetic material to make the coronavirus spike proteins, they will be displayed on the surface of those cells. Immune cells will encounter these proteins, recognize them as foreign invaders and generate an immune response specific to the spike protein. Then, if you are exposed to the real virus, your body activates that specific immune response so that antibodies can disable the virus and protect you from getting sick.

Q: How soon will someone be protected after taking the vaccine?

A: With the two mRNA vaccines, it is estimated that immunity develops around two to three weeks after completing the second shot in the series. The exact timing of immunity can vary because each vaccine works a little differently and every individual responds a little differently. Both the Pfizer and Moderna vaccines are two-shot vaccines. It is important that an individual have both shots in order to have the immunity demonstrated by the trials. 

The Johnson & Johnson (J&J) adenovirus vaccine requires only one shot. Most people develop peak immunity after two weeks of getting that shot. However, a second dose of an mRNA vaccine (Pfizer or Moderna) is recommended for people who have had the J&J vaccine. The J&J vaccine was shown to have a short period of effectiveness and severe side effects in some cases.

The Novavax vaccine provides protection within a similar timeframe (two to three weeks). Like the mRNA vaccines, the Novavax vaccine requires a primary series of two shots, administered three to eight weeks apart. According to the World Health Organization, the vaccine is 90% effective against mild, moderate and severe disease.

The booster doses of Moderna and Pfizer likely provide protection around one week after the booster has been given.

Q: Can I get the vaccine if I am not feeling well?

A: If you are not feeling well, it is recommended that you wait until you are feeling better to get the vaccine. If you have symptoms and/or signs of respiratory infection, evaluation and testing from health care resources in your community is recommended.

Q: If you have severe allergies or food allergies, including anaphylaxis, should you get the COVID-19 vaccine?

A: You should talk to your doctor, but for most people the answer is YES, you should get the vaccine. Your doctor may tell you to take medicine before and after the shot to decrease the risk of an allergic reaction. You should also tell people giving you the vaccine that you have allergies and remain at the vaccine site for at least 15-30 minutes after receiving the shot. Most people with allergies have not had a problem with the vaccine. The Center for Disease Control and Prevention (CDC) considers it a “precaution” to vaccination if you have had an allergic reaction to any injectable medicine or vaccine but not to other medications or environmental allergens. The only true warning signs are having had an allergic reaction to one of the vaccine’s components, or to a first dose of the vaccine itself.

Q: If you have cancer, should you get the COVID-19 vaccine?

A: You should talk to your oncologist. For most people, the answer is YES, you should get the vaccine. Cancer is a risk factor for having more severe COVID-19, and the vaccine can help prevent severe disease. 

Q: If you are pregnant or nursing, should you get this vaccine?

A: You should talk to your doctor and decide together. There is growing evidence that the COVID vaccine is safe and effective for pregnant women. The benefits of receiving the vaccine far outweighing potential risks.

Q: I recently received passive antibody therapy for COVID-19. Can I still get vaccinated with a COVID-19 vaccine? 

A: Yes, BUT vaccination should be postponed for at least 90 days for individuals who have received monoclonal antibodies or convalescent plasma as part of COVID-19 treatment to prevent those therapies from potentially interfering with the development of immunity from the vaccine.

Q: What are the side effects of the vaccine?

A: Side effects of the vaccines include flu-like symptoms such as aches, headache and fever. A small number of people with a history of allergies have developed severe allergic reactions after receiving the vaccine. As a result, people receiving vaccines are asked about their history of allergic reactions. They are also advised to remain at the vaccine site 15-30 minutes after receiving the shot to make sure they do not develop allergic reactions. National Jewish Health is equipped with medications to treat allergic reactions. A CDC analysis of 14 million Pfizer and Moderna vaccinations from January to February 2021 found a rate of 4.5 cases of anaphylaxis for every 1 million doses. This rate is similar to other vaccines that have been given for influenza or pneumonia.

Booster FAQs

Q: Who needs COVID-19 booster shots?

A: As of September 2022, the CDC recommends the Omicron booster shot for everyone over age 12 who has received all doses in the primary series of  the Pfizer, Moderna or Johnson & Johnson vaccines.

The latest vaccines protect against the original COVID strain and the Omicron subvariants BA.4/BA.5. This version of the vaccine is only approved as a booster dose. You should not get the Omicron booster if you have not received a primary dose of one of the original vaccines.

Currently, there is no recommendation for a Novavax booster.

Stay up to date with CDC guidelines on COVID vaccines and boosters, as recommendations may change. 

Talk to your health care provider about your medical condition and whether getting an additional dose is appropriate for you.

Q: Can I get the Omicron booster if I have not received any other booster shots?

Yes, as long as you have received two primary doses of the original Pfizer or Moderna vaccines or the single-dose Johnson & Johnson vaccine.

Q: When should I get the Omicron booster?

Both the Pfizer and Moderna boosters can be taken two months after primary vaccination or a previous booster. However, experts believe that waiting up to four months after your last dose may result in a slightly better immune response.

Q: Can I mix and match vaccine types/doses between the primary doses and the Omicron booster?

A: Yes. There is evidence that mixing vaccines may enhance the immune response.  The Johnson & Johnson (J&J) vaccine is NOT recommended as a booster dose. As of September 2022, the CDC recommends that everyone age 12 or older receive the Omicron booster. The Pfizer booster is approved for everyone age 12 and older. The Moderna booster is approved for adults 18 years and older.

Mixing Novavax doses with any of the vaccines listed above is not recommended at this time.

Make sure to check updates with the CDC.

Q: What do I do if I am immunocompromised and received Johnson & Johnson as my initial vaccine?

A: It is recommended that people initially vaccinated with J&J should receive an additional dose. Because of the J&J vaccine’s waning effectiveness and higher risk of side effects, the Pfizer and Moderna vaccines are the preferred options for the additional dose. However, in certain circumstances, an additional J&J shot can be considered.

Q: Why do I need a booster? What are the benefits?

A: The research trials around the vaccine suggest there may be decreased effectiveness over time. The booster is designed to re-prime your immune system to provide better immunity. Boosters may still not provide full immunity to COVID-19 in people who are immunocompromised. You should continue to maintain physical precautions to help prevent COVID-19. Your close contacts should also be vaccinated as appropriate.

Q: Is the vaccine dosage in the Omicron booster the same as the primary series?

A: The Pfizer booster dose is the same as the primary series. Booster doses for Moderna were authorized at a lower dose.

Q: Where can I get additional information?

A: Additional information can be found on the CDC website.

COVID, Flu & Pneumonia

Q: Does the COVID-19 vaccine take the place of the pneumococcal vaccine or the flu vaccine?

A: No, the COVID-19 vaccine specifically protects against infection by SARS-CoV-2. It does not take the place of the pneumonia vaccine or the flu vaccine, which are caused by other organisms. Please consult with your health care provider regarding which vaccines are recommended for you.

Q: Is COVID-19 a seasonal illness like the flu?

A: Scientists don’t yet know if COVID ultimately will be seasonal. Unlike influenza which drops to very low prevalence in the summertime, cases of COVID-19 have remained at a significant level even in warmer months. This ongoing “reservoir” of virus in the population allows the pandemic to persist. A larger amount of circulating virus in the population also allows for more opportunities for mutation over time and the creation of variants of SARS-CoV-2 (the virus that causes COVID-19). These variants can be more transmissible or elicit a less effective immune response in people who have already had COVID-19 or been vaccinated. As the virus continues to mutate, you may continue to see spikes in different places. The premature relaxing of mitigation measures (social distancing, masking) could also lead to similar spikes. The immunity you get from either “natural infection” (getting sick with COVID-19) or vaccination may fade over time. Repeated vaccination may be necessary to address regular spikes aggravated by the seasons.


Q: Are the vaccines effective against COVID-19 virus variants?

A: Yes. The vaccines are quite effective in preventing serious illness and death.

Q: Will the COVID-19 vaccine still work if the virus mutates?

A: Yes, they will work. Their effectiveness depends on the mutation. The current vaccines produce antibodies that bind the spike protein, which the virus uses to enter cells. Mutations that change the spike protein could affect vaccine effectiveness. Recent mutations, including Delta and Omicron, do affect the spike protein. Preliminary studies have shown that the vaccines still work against viruses with those mutations in terms of preventing serious illness and death. However, they are somewhat less effective at preventing mild-moderate disease for the certain variants. Pharmaceutical companies and health officials around the world are monitoring these mutations closely. They are developing plans to adjust the vaccines as necessary. 

Lessons from COVID

Q: What are some lessons infectious disease specialists have learned from COVID?

A: First, we need to be much better prepared for the threats posed by emerging pathogens, and particularly highly transmissible viruses. Despite the trial run of the H1N1 Swine Flu pandemic in 2009, our focus lapsed, and we were woefully underprepared for COVID-19. Secondly, we need to continue to question preliminary assumptions about new diseases. We also learned not to ignore the scientific method in the rush to find new therapies. For example, preliminary studies grounded on weak data were later disproven by more rigorous randomized controlled trials. Finally, we learned that the control of highly infectious diseases requires broad societal support for public health measures. 

Vaccine After Exposure & Recovery from COVID

Q: I had a known exposure in the last 14 days with a COVID-19 case. Can I get the vaccine to stop me from developing the disease? 

A: No. Current evidence suggests that the vaccine cannot be used for post-exposure prevention. Protection from the vaccine is not immediate. For the mRNA vaccines and Novavax, it requires two doses and may take one to two weeks from second dose to build immunity. 

If you have had a known exposure to COVID-19, you should not seek vaccination until the quarantine period has ended to avoid potentially exposing healthcare personnel and others during the vaccine visit. Make sure you are not sick with COVID-19 at the time of vaccination.

Q: If I had COVID-19 and recovered, do I need to get the vaccine?

A: Yes, the CDC recommends people who have had COVID-19 and recovered should get a vaccine. Experts do not yet know how long immunity lasts – or how strong the immunity is – after a case of COVID-19. Experts do know, however, that reinfection is possible.

Vaccine for Children

Q: Can children get the COVID-19 vaccine?

A: Yes. Children aged six months and older are eligible for the vaccine. The groups recommended to receive the vaccines could change in the future, as ongoing clinical trials in children are completed.

You can learn more about vaccination for children and teens on the CDC’s website here.  




The information on our website is medically reviewed and accurate at the time of publication. Due to the changing nature of the COVID-19 pandemic, information may have since changed. and your state’s health department may offer additional guidance.

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