Physician giving older woman a vaccine

COVID-19 Vaccine FAQs

Private: Paul Edward Sax, MD
Contributor Paul Edward Sax, MD
Private: Daniel R. Kuritzkes, MD
Contributor Daniel R. Kuritzkes, MD

Widespread vaccination is our best way to end the COVID-19 pandemic. Mass General Brigham experts answer your top questions about the COVID-19 vaccines. Daniel R. Kuritzkes, MD, and Paul Sax, MD, share the latest vaccine guidance on booster shots, vaccination during pregnancy and more. Dr. Kuritzkes is the chief of the Division of Infectious Diseases at Brigham and Women’s Hospital and Dr. Sax is the clinical director of the Infectious Disease Clinic.

The Federal Drug Administration (FDA) has approved the Pfizer-BioNTech COVID-19 vaccine (also known as Comirnaty) for use in people age 5 and older. Children ages 5 through 11 years old will receive a separate vaccine that has one-third of the dose given to adolescents and adults. The Moderna and Johnson & Johnson vaccines are approved for use in those 18 and older.

Dr. Kuritzkes answers the most commonly searched COVID-19 vaccine questions in the video below.

How can I schedule a vaccine appointment?

As COVID-19 cases decline, Mass General Brigham will be closing most of our central vaccine clinics. There are many places to get vaccinated in your community. Many sites offer booster shots. To find a location near you, visit or You can text your zip code to 438829 or call 1-800-232-0233.  

Mass General Brigham offers vaccines at many primary care and some specialty offices alongside visits. You can find out if a COVID-19 vaccine is available at an upcoming visit by contacting your doctor’s office.   

Mass General Brigham will also continue to distribute vaccines from our mobile community vans. Check the schedule to see if the vans will be near you. Check the schedule to see when the vans will be near you.

Why is it important to get a COVID-19 vaccine?

“There are two compelling reasons to recommend vaccination over herd immunity,” said Dr. Sax. “The first is that immunization protects people from COVID-19, which is a potentially life-threatening disease. COVID-19 has a much higher case fatality rate than flu, and even young healthy people can become severely ill.“

Some people with COVID-19 have symptoms that may persist for weeks or months. Common complaints include fatigue, shortness of breath, feeling a racing or pounding heart (also called palpitations), chest pain, trouble concentrating and “brain fog.”

“The second reason to advocate for getting the vaccine is that it will protect others. People with COVID-19 — the actual disease — are highly infectious during the early phases of their illness. This means they can readily transmit (spread) the infection to others,” explained Dr. Sax.

Can I get sick from the vaccine?

No. The vaccine does not contain the whole or live virus. This means it can’t cause COVID-19.

Why is it important that people get their second or third dose of the Pfizer or Moderna vaccines?

“The first dose of the vaccine “primes” the immune system to respond to the vaccine. The second dose or third dose “boosts” it, greatly increasing antibody responses,” explained Dr. Sax.

“Such a strong antibody response is thought to correlate with how well and how long we’ll be protected from getting COVID-19 in the future,” he added. “It’s likely especially important in those who might have a less robust immune response to vaccinations, such as older people and those who have weakened immune systems.”

People who are moderately to severely immunocompromised are especially vulnerable to COVID-19. They are more likely than others to have a serious, prolonged illness. CDC recommends people who are immunocompromised receive a third dose of the Pfizer or Moderna vaccines. Immunocompromised people include cancer patients who are actively receiving treatment, organ transplant recipients who are taking medications to suppress their immune system, people with HIV and people who take high-dose corticosteroids or other medications to suppress their immune response.

Am I eligible for a booster shot?

You’re eligible for a first booster shot if:

  • You received the J&J vaccine at least 2 months ago and you’re 12 or older.
  • You received a Pfizer series at least 5 months ago and you’re 12 or older.
  • You received a Moderna series at least 5 months ago and you’re 18 or older.

Can I get a second booster shot?

On March 29, 2022, the CDC and FDA approved a second booster for certain patients:

What are mRNA vaccines?

Both the Pfizer and Moderna vaccines are mRNA vaccines. While the mRNA vaccine is a new kind of vaccine, researchers have been studying and working on them for many years.

When you get an mRNA vaccine, it gives your cells directions on how to make the COVID-19 proteins found on the outside layer of the coronavirus (also called spike proteins). Your immune system can then make antibodies to these proteins, which protects you from getting infected with COVID-19.

The mRNA from the vaccine never enters the nucleus of your cells and doesn’t get into your DNA. Your cells break down and get rid of the mRNA soon after they’re finished using the instructions. The Centers for Disease Control and Prevention (CDC) has information about mRNA vaccines.

Did the Brigham lead any of the COVID-19 vaccine trials?

Yes. Leadership at the COVID-19 Prevention Network (CoVPN) chose Lindsey Baden, MD, director of the Brigham’s Center for Clinical Investigation (CCI), as the co-principal investigator of an mRNA Phase 3 vaccine trial. The trial evaluates the effectiveness and safety of the Moderna vaccine and its ability to prevent COVID-19 illness. Watch our video to learn more.

What are COVID variants?

Viruses constantly change through mutation. Sometimes new variants emerge and disappear. Other times, new variants emerge and persist. Multiple variants of the virus that causes COVID-19 have been documented in the U.S. and around the world during this pandemic.

The CDC reports that multiple variants are spreading around the world. Some of these variants seem to spread more easily and quickly than other variants, which may lead to more cases of COVID-19.

How can widespread vaccination help prevent the spread of variants?

Dr. Sax expects widespread immunization will lead to decreased numbers of people with COVID-19 and less circulating virus. With less virus out in the community reproducing, the virus has less of an opportunity to evolve into variants.

I already had COVID-19. Should I get vaccinated?

Yes. You can still get the vaccine if you’ve had COVID-19 and have recovered. If you’re actively sick with COVID-19 or have symptoms that could be from COVID-19, you shouldn’t get the vaccine at this time.

What are the side effects of the COVID-19 vaccine?

Some people have side effects after receiving the vaccine. Most mild side effects resolve within a day or so.

The most commonly reported symptoms are:

  • Pain at the site of vaccination
  • Swollen lymph nodes in the same arm as the injection
  • Chills
  • Fatigue (feeling very tired)
  • Fever
  • Headache
  • Muscle pain or joint pain
  • Nausea or vomiting

If you get the vaccine but don’t have side effects, there’s no reason to worry.

“As far as we know, this protection applied across all the study participants, even those who had few or no side effects from the vaccine,” said Dr. Sax.  “As a result, at present there is no recommendation to repeat the vaccine or do additional testing afterwards if a person has no side effects.”

Last reviewed: April 5, 2022

Private: Paul Edward Sax, MD
Paul Edward Sax, MD

Paul Edward Sax, MD, is the clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital.

Private: Daniel R. Kuritzkes, MD
Daniel R. Kuritzkes, MD

Dr. Kuritzkes is chief of the Division of Infectious Diseases at Brigham and Women's Hospital

Before you go,

Get additional tips on keeping your family healthy and safe during the COVID-19 pandemic. Read more COVID-19 articles.