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

Featured Experts:

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?

Yes, the FDA has granted emergency use authorization (EUA) for COVID-19 vaccines. Pfizer’s vaccine was authorized Dec. 12, 2020; Moderna’s version received authorization Dec. 18, 2020; and Johnson & Johnson’s was authorized Feb. 27, 2021 but its use was paused on April 13. (Read more about the J& J pause.)

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 in development 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.

Is it OK if my two COVID-19 vaccinations are from different manufacturers?

Once you’re given one type of vaccine, the second vaccine dose should be the same type, from the same manufacturer as the first vaccine dose. The Pfizer/BioNTech and Moderna mRNA COVID-19 vaccines are not interchangeable with each other or with other COVID-19 vaccine products. Each vaccine is manufactured differently — even those that use similar underlying technology, such as Pfizer and Moderna.

Shortly after your first vaccination, your record will show which type you received, so you can schedule your second shot with the same brand and at the proper time interval between doses.

These recommendations may change as further information becomes available or as other vaccine types are authorized for use. In rare cases, such as in the event of an allergic reaction, an exception may be made to allow for the second dose from a different manufacturer.

Protection and Immunity

How long will it protect me? Will I have to get a COVID-19 shot every year?

This is a question researchers are eager to answer. People who are infected with the coronavirus show a decline in protection within a few months, but their immunity may last significantly longer than that. (A few people appear to have caught COVID-19 twice, but this is unusual). Data from the vaccine trials indicate strong immunity at least months after vaccination, indicating possible long-term immunity.

As the SARS-CoV-2 coronavirus has begun to change (mutate), studies are exploring how these changes affect the virus’s characteristics, including its ability to spread between people. A substantial mutation might have an impact on the effectiveness of the vaccines, and vaccine manufacturers are preparing to make adjustments where necessary.

Will the vaccine work if I’ve already had COVID-19 or tested positive for the coronavirus?

The U.S. Centers for Disease Control and Prevention (CDC) notes that people who have already had COVID-19 or tested positive may still benefit from getting the COVID-19 vaccination. There is not enough information currently available to say if or for how long people are protected from getting COVID-19 after they have had it (natural immunity). Early evidence suggests natural immunity from COVID-19 may not last very long, but more studies are needed to better understand this.

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

The CDC has updated its recommendations on resuming some activities once you have completed your COVID-19 vaccines and allowed at least two weeks for your immune system to respond to vaccination to the last shot. Here are some highlights of what fully vaccinated people may do now:

  • If you are fully vaccinated, you can resume activities that you did prior to the pandemic.
  • Fully vaccinated people can resume activities without wearing a mask or physically distancing, except where required by federal, state, local, tribal, or territorial laws, rules, and regulations, including local business and workplace guidance.

There is a rumor that taking over-the-counter medications for pain relief before receiving the vaccine lessens its effectiveness. Is this true?

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.

For the vaccines that use two doses, if you have not had COVID-19, the chance of having noticeable side effects is higher after the second shot. Those who have had COVID-19 may experience stronger side effects after the first dose.

Experiencing side effects does not mean that you have COVID-19, but signals that your immune system is responding to the vaccine. These side effects are considerably less risky to your health than having COVID-19, but if they persist, call your doctor and ask about taking over-the-counter pain and fever reducers to help you feel better.

Johnson & Johnson Vaccine

Although rare, a serious, but treatable blood clot condition has been observed in a few cases after taking the J&J vaccine. Please read our full article on this to learn more.

For three weeks after receiving the J&J vaccine, you should watch for possible symptoms of a blood clot with low platelets.

If you experience any of the following symptoms, call your primary care provider immediately or visit your closest urgent care center. If your symptoms are severe, call 911.

  • severe or persistent headaches or blurred vision
  • shortness of breath
  • chest pain
  • leg swelling
  • persistent abdominal pain
  • easy bruising or tiny blood spots under the skin near the injection site

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

Will children receive a COVID-19 vaccine?

The Food and Drug Administration has authorized the Pfizer-BioNTech vaccine for distribution to individuals ages 16 years and older. 

No COVID-19 vaccine has been authorized for children under 16. At least one trial including participants between age 12 and 18 with their parents’ permission is in progress. But it may be late 2021 or even 2022 before there is enough evidence on the timing, safety, effectiveness and practical aspects of vaccinating children for the coronavirus, especially children under age 12.

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 May 14, 2021.