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Advances in Care

Infectious Disease Specialist Answers Your mRNA COVID-19 Vaccine Questions

Plus, Why He Got Vaccinated

First Published December 2020 / Updated February 2021

The Food and Drug Administration (FDA) issued Emergency Use Authorization (EUA) for the Pfizer-BioNTech COVID-19 vaccine and Moderna COVID-19 vaccine in December 2020. What does this mean for you and your loved ones?

"As a medical professional who has spent a majority of my career focused on infectious disease and respiratory viruses, I received an mRNA COVID-19 vaccine as soon as it was offered to me at Northwestern Medicine," shares Michael G. Ison, MD, infectious diseases and organ transplantation specialist at Northwestern Medicine. "I chose to get a vaccine because the research suggests it will protect me."

Here, Dr. Ison answers the most commonly asked questions about the mRNA COVID-19 vaccines.

NOTE: As clinical trials expand upon data around vaccine safety and efficacy, the Centers for Disease Control and Prevention (CDC) provides the most up-to-date evidence-based recommendations.

How do the mRNA COVID-19 vaccines work?
Many other vaccines send a weakened or inactivated version of a virus into your body. However, the Pfizer-BioNTech and Moderna COVID-19 vaccines work differently. They each deliver a version of the COVID-19 genetic code, called messenger RNA (or mRNA), into your cells.

  • mRNA tells your cells to make a harmless protein called a "spike protein." It is similar to what is found on SARS-CoV-2, the virus that causes COVID-19.
  • Next, your body destroys the vaccine material.
  • Your immune system will know that the cells with the spike protein don't belong in your body. It will see them as intruders, or "antigens."
  • Then your body will develop "antibodies," which act as soldiers to destroy antigens. 

Thanks to the vaccine, your immune system's "memory cells" now know how to make these antibodies. So, if you are exposed to the SARS-CoV-2 virus in the future, your body will be able to help keep you from getting sick with COVID-19.

Are the two mRNA vaccines from Pfizer-BioNTech and Moderna interchangeable?
No. Both vaccines require two doses, but you should receive the same vaccine for each dose. For example, you cannot get one dose of the Pfizer-BioNTech COVID-19 vaccine followed by one dose of the Moderna COVID-19 vaccine. The safety of mixing the two vaccines has not been evaluated.

What are the benefits of getting vaccinated with the mRNA COVID-19 vaccines?
Current data shows that 94% to 95% of people who receive two doses of the Pfizer-BioNTech or Moderna COVID-19 vaccines will not get COVID-19. For people who do get infected despite vaccination, the disease is generally mild.

Who should and should not get vaccinated?
Vaccination is key to ending the COVID-19 pandemic. The goal is for every adult patient to have the option to be vaccinated as soon as the supply is available. For the Pfizer-BioNTech COVID-19 vaccine, EUA from the FDA was granted for those age 16 and older. For the Moderna COVID-19 vaccine, EUA from the FDA was granted for those age 18 and older.

Clinical trials leading to the EUA for these mRNA vaccines did not include pregnant and lactating women, children, nor people with severe allergies. Talk to your primary care physician to weigh the risks and benefits as outlined by the FDA if any of the following apply to you:

  • You have HIV infection or another immunocompromising condition
  • You are pregnant or plan to be pregnant
  • You are breastfeeding
  • You have any allergies
  • You have a fever
  • You have a blood disorder or are on a blood thinner
  • You have received a different COVID-19 vaccine

You should not receive the mRNA COVID-19 vaccine within two weeks of receiving any other vaccine, including the flu vaccine.

What if I have allergies?
According to the CDC, those who have had severe reactions to other vaccines or injectable drugs are still approved to receive the Pfizer-BioNTech or Moderna COVID-19 vaccine. Patients with allergies who get vaccinated will need to be monitored for 30 minutes following vaccination, compared to patients without allergies who will be monitored for 15 minutes.

How many doses do I need to get?
You should receive two doses of the Pfizer-BioNTech or Moderna COVID-19 vaccine.

  • For the Pfizer-BioNTech vaccine, the second dose should be given 21 days after the first.
  • For the Moderna vaccine, the second dose should be given 28 days after the first.

Can the mRNA COVID-19 vaccines give me COVID-19?
You cannot get COVID-19 from the Pfizer-BioNTech nor Moderna COVID-19 vaccine. The mRNA vaccines do not contain the virus that causes COVID-19, so there is no way for you to get infected through vaccination.

Are there side effects of the mRNA COVID-19 vaccines?
Side effects are similar to what you might experience with other vaccines. The most common side effects are mild, and include fatigue, headache, fever and chills. These side effects may occur as a result of your body's work to create immunity to the virus and are a sign that the vaccine is working. Such side effects should go away within a few days.

Please note that some people will not have any side effects, and some people may experience significant side effects. It is advised to consult your provider if:

  • You are experiencing any side effects for more than a few days.
  • You notice that the area where you received the vaccine increases in redness or tenderness over 24 hours.
  • You become worried by your side effects.

The CDC recommends that patients report any side effects via V-safe, a smartphone-based tool.

Can I get the COVID-19 mRNA vaccines if I have had the flu vaccine this year?
Yes, but the CDC recommends that the Pfizer-BioNTech and Moderna COVID-19 vaccine be given at least two weeks before or after any other vaccine.

If I have had COVID-19 and/or tested positive for antibodies to the virus that causes COVID-19, do I need the vaccine?
According to the CDC, more research is needed to determine how long patients who have recovered from COVID-19 will be protected by natural immunity. Therefore, even if you have had COVID-19 or positive detection of antibodies, data shows the mRNA COVID-19 vaccine will likely benefit you: It could boost your natural immunity or build immunity if your natural immunity has decreased over time. In most cases, a 90-day period after your COVID-19 diagnosis or positive antibody test is recommended before getting a COVID-19 vaccine.

How long should I wait to get vaccinated if I believe I am exposed to COVID-19? Should I get tested for COVID-19 first?
It is advised that you wait at least until you are no longer required to isolate or quarantine before getting the mRNA COVID-19 vaccine. At this time, it is not recommended to get tested for COVID-19 before getting vaccinated.

Can I spread COVID-19 once I receive an mRNA COVID-19 vaccine? Do I still need to wear a mask?
Once you have been vaccinated, you should continue behaving as though you have not been vaccinated: Wear a mask, maintain physical distance and wash your hands.

  • These vaccines are not 100% effective, so there is a chance you could still get and spread the disease after vaccination.
  • These vaccines may take a week or two to help your body build immunity, so you could still become infected before the vaccine is able to protect you.
  • Clinical trials tested the ability of the Pfizer-BioNTech and Moderna vaccines to prevent and decrease symptoms of COVID-19, but they did not measure whether those vaccinated were carriers of SARS-CoV-2, the virus that causes COVID-19. That means you may still be able to infect others.

Use all tools available to you to protect both yourself and others from COVID-19.

Is vaccination covered by my insurance?
The federal government has indicated that as soon as the supply is available, every person in the U.S. should be able to be vaccinated with no out-of-pocket expense, regardless of insurance coverage. Your insurance provider may be billed to cover the cost to administer the vaccine.

Vaccine supply remains limited across the U.S. Northwestern Medicine is closely monitoring the roll-out of vaccines as well as advice from public health officials. Patients at Northwestern Medicine can learn about our vaccination plan on our COVID-19 Vaccine page. Prepare yourself for vaccination by reading more about the benefits of getting vaccinated and the science behind vaccines.

Michael G. Ison, MD
Michael G. Ison, MD
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