Health
gloved hands with a vile and needle
gloved hands with a vile and needle
gloved hands with a vile and needle

COVID-19 Vaccine: What You Need to Know

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The U.S. Food and Drug Administration (FDA) has authorized use of vaccines for the coronavirus that causes COVID-19. Lisa Maragakis, M.D., M.P.H., senior director of infection prevention, and Gabor Kelen, M.D., director of Johns Hopkins Office of Critical Event Preparedness and Response, address common questions and explain how a vaccine could affect the current pandemic.

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COVID-19 Vaccines and How They Work

How do vaccines work?

Vaccines help people develop immunity to a virus or other germ. A vaccine introduces a less harmful part of that germ — or something created to look or behave like it — into a person’s body. The body’s immune system develops antibodies that fight that particular germ and keep the person from getting sick from it. Later, if the person encounters that germ again, their immune system can “recognize” it and “remember” how to fight it off.

Is there a vaccine for the coronavirus disease?

Several COVID-19 vaccines have been authorized for emergency use among specific age groups by the U.S. Food and Drug Administration (FDA). Johns Hopkins Medicine views all authorized COVID-19 vaccines as highly effective at preventing serious disease, hospitalization, and death from COVID-19.

Learn more about coronavirus vaccine safety.

How will a vaccine prevent COVID-19?

The coronavirus that causes COVID-19 has spikes of protein on each viral particle. These spikes help the viruses attach to cells and cause disease. Some of the coronavirus vaccines are designed to help the body “recognize” these spike proteins and fight the coronavirus that has them.

An effective vaccine will protect someone who receives it by lowering the chance of getting COVID-19 if the person encounters the coronavirus. More important is whether the vaccine prevents serious illness, hospitalization and death.  At this time, all three vaccines are highly efficacious at preventing serious illness, hospitalization and death from COVID-19.  Widespread vaccination means the coronavirus will not infect as many people. This will limit spread through communities and will restrict the virus’s opportunity to continue to mutate into new variants.

When can I get the coronavirus vaccine?

Now that the Food and Drug Administration has issued emergency use authorizations for COVID-19 vaccines, vaccines are being distributed across the United States.

If you are a Johns Hopkins Medicine patient, visit our COVID-19 Vaccine Information and Updates page for current information on getting vaccinated. Your state’s health department website can also provide updates on vaccine distribution in your area.

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Getting the Vaccine

How many shots are in the COVID-19 vaccine?

Pfizer’s and Moderna’s vaccines require two doses given several weeks apart (Pfizer’s second shot is given three weeks after the first, and Moderna’s is four weeks after the first).The COVID-19 vaccine from Johnson & Johnson requires only one dose.

What does “fully vaccinated” mean?

According to CDC guidelines, you are fully vaccinated when it has been:

  • Two weeks after your second dose in a two-dose series, such as the Pfizer or Moderna vaccines.
  • Two weeks after a single dose of the Johnson & Johnson vaccine. However, data from clinical trials are clear that there is further improvement four weeks after the single-shot vaccine, especially for preventing severe COVID-19 or having asymptomatic infection. For this reason, Johns Hopkins Medicine recommends four weeks after the single-dose vaccine to be considered fully vaccinated.

If you don’t meet these requirements, you are not fully vaccinated.

For the two-dose COVID-19 vaccines, what happens if my second dose is delayed?

If you have received the first dose of the Pfizer/BioNTech COVID-19 vaccination, the Centers for Disease Control and Prevention (CDC) states that the second shot should take place three weeks after the first one.

If your first coronavirus vaccine was from Moderna, the CDC says your second shot should be given to you four weeks after the first one.

If something happens that prevents you from getting the second dose of either COVID-19 vaccine on time, you can still receive it up to six weeks (42 days) after the first dose. We do not recommend delaying your second dose, but the data from clinical trials support this range. There are currently limited data on the efficacy of mRNA COVID-19 vaccines administered beyond this window. According to the CDC, however, if the second dose is administered beyond these intervals, there is no need to restart the series.

What if I received my second vaccination earlier than recommended?

You should not get the second vaccine dose earlier than the recommended times. But, if you’ve already received your second shot, and it was early by 4 days, or less than the recommended time window, your vaccinations are OK, and you do not need to repeat the vaccination series.

Should I get a COVID-19 vaccine if I already had COVID-19?

Yes, evidence continues to indicate that getting a COVID-19 vaccine is the best protection against getting COVID-19, whether you have already had COVID-19 or not.

  • A study published in August 2021 indicates that if you had COVID-19 before and are not vaccinated, your risk of getting reinfected is more than two times higher than for those who were infected and got vaccinated.
  • While evidence suggests there is some level of immunity for those who previously had COVID, it is not known how long you are protected from getting COVID-19 again. Plus, the level of immunity provided by the vaccines after having COVID-19 is higher than the level of immunity for those who had COVID but were not subsequently vaccinated.
  • Getting vaccinated provides greater protection to others since the vaccine helps reduce the spread of COVID-19.

At the time of vaccination, be sure to tell your care provider about your history of COVID-19 illness, including the kind of treatment, if any, you received and when you recovered. Wait until your isolation period ends before making an appointment to get the vaccination.

Protection and Immunity

If I get a coronavirus vaccination, do I still have to wear a mask? Physical distance?

The CDC continues to monitor the spread of COVID-19 and makes recommendations for wearing face masks, both for those who are fully vaccinated as well as those who are not fully vaccinated.

The CDC also recommends that masks and physical distancing are required when going to the doctor’s office, hospitals or long-term care facilities, including all Johns Hopkins hospitals, care centers and offices.

Johns Hopkins Medicine’s current mask safety guidelines have not changed, and we still require all individuals to wear masks inside all of our facilities.

Does taking over-the-counter medications before receiving the vaccine lessen its effectiveness?

Some studies have suggested that taking medications such as Tylenol or Advil before getting a vaccination might reduce your body’s ability to mount an immune response to the vaccine. It’s unclear if these findings have any clinical significance, though, and other studies did not find any effect of anti-inflammatory medications on the immune reaction to vaccines.

If you regularly take aspirin or other over-the-counter medications such as acetaminophen (e.g., Tylenol), ibuprofen (e.g., Motrin, Advil) or naproxen (e.g., Aleve) for other medical conditions, please continue to do so as directed by your physician or as needed. Otherwise, it’s probably best to not take over-the-counter medications such as acetaminophen or ibuprofen that reduce fever or inflammation before receiving a vaccine. If you are uncomfortable or have symptoms after vaccination, that is the time to take an over-the-counter medication to help you feel better. 

Vaccine Safety and Side Effects

How will we know if a COVID-19 vaccine is safe and effective?

In order to be declared safe and effective, a COVID-19 vaccine must pass certain tests and standards. Organizations such as the National Academy of Sciences, the National Institutes for Health, and the Food and Drug Administration (FDA) use scientific data from research to help decide if and when new drugs and vaccines can become available to the public. It is important to note that you cannot get COVID-19 from a vaccine. The vaccines contain proteins or other biological substances to stimulate the immune response, but not the coronavirus itself.

Learn more about the safety of the COVID-19 vaccines.

Demographics of the COVID-19 Vaccine Trials

What are the coronavirus vaccine side effects?

You may have pain in the arm where you got the shot, and you might run a fever and experience body aches, headaches and tiredness for a day or two. Chills, swollen lymph nodes can also occur.

Read more about possible COVID-19 vaccine side effects

Is it OK to get the COVID-19 vaccine if I have allergies?

While there have been reports of severe allergic-type reactions in a very small number of patients, the CDC says that people with allergies to certain foods, drugs, insects, latex and other common allergens can still get the COVID-19 vaccine.

If you have had a severe allergic reaction (anaphylaxis) to injectables or other vaccines, be sure to discuss the COVID-19 vaccination with your doctor, who can evaluate you and assess your risk. The vaccine provider should observe you for 30 minutes rather than the routine 15 minutes after vaccination, and if you have an allergic reaction to the first shot, you may not receive the second.

The CDC says that at this time, anyone who has a severe allergy (such as anaphylaxis) to any of the vaccine ingredients should not get that vaccine.

How Do We Know a COVID-19 Vaccine Will Be Safe and Effective?

Women and Children

Can children receive a COVID-19 vaccine?

The Centers for Disease Control and Prevention (CDC) now recommends a COVID-19 vaccine for children ages 5 and older. Johns Hopkins Medicine encourages all families to have eligible children vaccinated with the COVID-19 vaccine. Currently, Pfizer’s vaccine is the only approved COVID-19 vaccine for children.

Read more about what parents need to know about the COVID-19 vaccine.

Does the COVID-19 vaccine interfere with getting a mammogram?

Getting a mammogram too soon after your second dose of the coronavirus vaccine could result in a false positive and a callback due to temporarily swollen lymph nodes.

The Johns Hopkins Division of Breast Imaging supports the recommendation from the Society of Breast Imaging: When possible, and if it does not delay care your doctor recommends, you should schedule screening mammograms before your first dose of a COVID-19 vaccine or four to six weeks after the second dose.

Read more about the COVID-19 vaccine and mammograms.

Why is it Important that I Consider Getting the COVID-19 Vaccine?

Updated November 23, 2021.