COVID-19 Vaccine Frequently Asked Questions

How do vaccines work?

Vaccines trigger the body’s natural response to fight illnesses.  Vaccines work by training the immune system to be ready to recognize viruses or bacteria when they enter the body. Then, if a person is exposed to that virus or bacteria, their immune system knows how to respond. Most people vaccinated for COVID-19 will become immune, meaning they will not  get sick if exposed to the virus in the future. A small number of people might still get infected, but they usually have mild symptoms and are protected against serious illness.

Is the COVID-19 vaccine safe?

Yes. The Federal Drug and Administration (FDA) works to make sure that all vaccines are as safe as possible through careful testing. So far, the FDA has approved the Moderna, Pfizer-BioNTech, and Johnson & Johnson COVID-19 vaccines. Although the vaccines are new, the process for ensuring vaccine safety is not new. All vaccines must meet strict safety standards before being released to the public. Many thousands of adults of different ages and races volunteered for clinical trials to test the COVID-19 vaccines, and more than 100 million Americans are fully vaccinated. As more people get vaccinated, the FDA and others continue to track side effects and watch for any safety concerns.

Is the COVID-19 vaccine effective?

Yes. All three vaccines have been shown to be very effective at protecting people from getting extremely sick or requiring hospitalization. The Moderna and Pfizer-BioNtech vaccines require two doses, spaced several weeks apart, to be fully effective. The Johnson & Johnson vaccine works with just one dose instead of two.  Public health and medical providers are strongly urging people to take whatever vaccine they are able to get. The most important thing is to get immunized as soon as possible.

What is herd immunity?

Herd immunity is when most people are protected from a disease because they were vaccinated or had the disease. With herd immunity, the disease won’t spread as easily from person to person. Wide-scale vaccination (over 70%) leads to herd immunity, which could end the pandemic and protect those who cannot be immunized. Measles, mumps, polio, and chickenpox are examples of childhood diseases that were once very common. These diseases are now rare in the U.S. because of successful vaccination programs leading to herd immunity.

What’s in the COVID-19 vaccine?

There is no coronavirus in the vaccines, which means the vaccines cannot cause COVID-19. The active ingredient in the Moderna and Pfizer-BioNTech vaccines is called messenger RNA (mRNA). These COVID-19 vaccines do not have preservatives, which is why they must stay frozen until they ready to use. The Johnson & Johnson vaccine does not use mRNA. Instead, it has a modified adenovirus that delivers instructions for the body to attack the coronavirus' spike protein. The vaccines also include lipids, salts, sugars, acids, and acid stabilizers to help keep them stable and deliver the active ingredient to your cells. The vaccines do not contain ingredients that are poisonous or harmful.

Will mRNA change my DNA?

No. When the mRNA enters a cell it provides instructions to make a spike protein similar to that of the SARS-CoV-2 virus (the virus that causes COVID-19). Your body responds by making antibodies to recognize the spike protein. After you have the spike protein in your body, the mRNA is destroyed. It will not mix into your DNA.

Are there side effects?

Yes. The most common side effect is pain in the arm where you get the shot. Some people also have one or more of these symptoms: tiredness, headache, muscle pain, chills, joint pain, and fever. Another side effect is swollen lymph nodes in the neck or under the arm where the shot was given. These are all signs of the body’s immune system working! More people had these side effects after the second dose than after the first dose.

A very small number of the many people vaccinated have had severe allergic reactions. After you get your vaccine, you’ll be asked to stay where you got your shot for about 15 minutes to monitor for an allergic reaction. People who have had a severe allergic reaction after a previous dose of the vaccine, or who have had a severe allergic reaction to any ingredient of the vaccine they are offered, should not get the vaccine. Talk to your health care provider if you have had a severe allergic reaction to other vaccines. Your health care provider can help you decide if it is safe for you to receive the COVID-19 vaccine.

Should people who have already had COVID-19 get the vaccine?

Yes. People who have had COVID-19 may have some immunity against the coronavirus, but this immunity can be weak and may not last long. Most people develop more antibodies to the coronavirus after they are fully vaccinated than after getting COVID-19. The vaccine gives your body stronger protection against getting COVID-19 again.  People with COVID-19 should wait until they are no longer infected and have fully recovered before getting the vaccine.

Am I protected as soon as I receive the vaccine?

No.  It takes time to build immunity in response to the vaccines. The CDC considers people “fully vaccinated” against COVID-19 when at least 2 weeks have passed since their final dose. For the Pfizer and Moderna vaccines this is two weeks after the second dose, and for the Johnson & Johnson vaccine this is two weeks following the single dose.

Do I still need to wear a mask after I’ve had the vaccine?

Yes. The vaccine is an important way to stay safe, but for now, continue to follow all public health orders including wearing masks, maintaining physical distance, avoiding crowds, and washing your hands. The vaccine is not 100 percent effective, and not everyone will be vaccinated for a while. We are still learning about COVID-19 and don’t know when community spread of the virus will end. Getting vaccinated is a very important step to help that time come sooner, but in the meantime, we must continue other precautions.

Who can get the vaccine?

Anyone 12 years and older can get the vaccine. Californians can go to to sign up for an appointment and receive updates about new appointments and locations.

Where will I get the vaccine?

This will vary by county. It's best to check with your local public health department or primary care provider about where the vaccine will be available in your location.

What does the vaccine cost?

Most health care plans and county health departments will provide the vaccine free of charge.

Is the vaccine required for licensed child care providers?

No. Getting the COVID-19 vaccine is voluntary. But employers may encourage child care staff to be vaccinated. Some people are not eligible to get the vaccine because of their health status.

Are children eligible to get the COVID-19 vaccine?

No. At this point, the COVID-19 vaccine has not been approved for children, although testing is underway. Current vaccine laws for child care enrollment do not include the COVID-19 vaccine.

How long will immunity last after the vaccine?

It is unknown exactly how long immunity will last after getting fully vaccinated. Scientists are actively studying if and when a booster vaccine may be needed. This would not be unusual, many vaccine-preventable diseases require boosters. For example, an influenza vaccine is recommended every year.

Is the vaccine safe for pregnant women?

If you are pregnant, you can receive a COVID-19 vaccine. Pregnant women are more likely to get severely ill if they get COVID-19 compared with non-pregnant people. Getting a COVID-19 vaccine during pregnancy can protect you from getting severely ill if you get COVID-19.   Pregnant women should talk to their health care providers about their individual risks and decisions about getting the vaccine..

Does the vaccine work against the new variants of the coronavirus?

Probably yes.  All viruses change over time, and scientists are studying the new COVID-19 variants. Most scientists believe the currently approved vaccines should protect against new variants. At this time, there is no evidence that the new variants cause more severe illness or increased risk of death. However, the new variants seem to spread more easily. Quickly vaccinating people is one of the best strategies to decrease the development of new variants.

Once I’m vaccinated, do I need to quarantine if I’m exposed to someone with suspected or confirmed COVID-19?

No. You won’t need to quarantine if you meet these three criteria: 1. You are fully vaccinated (two weeks after you have received the last dose of the Moderna or Pfizer vaccines and two weeks after one Johnson & Johnson shot); 2. You are within three months after getting the last dose; 3. You have haven’t had symptoms since the COVID-19 exposure.

Summary Note: The COVID-19 vaccine can help end the pandemic and reduce the number of lives lost to COVID-19.  This could especially help people in the highest risk groups, including people with chronic health problems, those who work closely with others, and people living in large households. Discuss your personal risks of COVID-19 with your health care provider. If you are able to get the vaccine, consider the benefits of being vaccinated in reducing the risk for yourself, your family, your community, and the children in your care.


This collection of questions and answers was made possible with funding from the Heising-Simons Foundation.

updated 5/12/2021