COVID-19 Vaccine

Medically Reviewed by Dan Brennan, MD on June 20, 2022

As the coronavirus continues to cause illness and death around the world, vaccines are seen as one of the best ways to stop it.

Coronavirus Vaccine Progress


Vaccines are available for children as young as 6 months old.  The Pfizer-BioNTech COVID-19 vaccines aren’t as strong as those that have been authorized for older people. As in the adult version, the children’s version requires two doses taken 3 weeks apart. In addition, children 6 months to 4 years old will also get a third dose after at least 8 weeks from their second dose.

A two-dose Moderna vaccine is also approved. Children 6 months and older can now get the Moderna vaccine too.

Top health experts have a preference for the type of vaccine that you choose. They recommend that you choose a vaccine made with mRNA (like the ones from Pfizer and Moderna) rather than the J&J vaccine, which is made differently. The recommendation is endorsed by the CDC and comes from the Advisory Committee on Immunization Practices, which reviewed the latest evidence on the effectiveness, safety, and rare side effects of the available vaccines.

That said, if you can’t get an mRNA vaccine or you don’t want to, you should definitely get the J&J vaccine. Receiving any COVID-19 vaccine is better than being unvaccinated, experts say.

Booster shots of the Pfizer and Moderna vaccines are recommended for everyone 18 years or older at least 5 months after getting the first two doses. But people 12-17 years old who got their first two doses from Pfizer or Moderna may only get the Pfizer booster.

If you got the J&J vaccine as your original dose, the CDC recommends a booster shot, preferably an mRNA vaccine, at least 2 months after your last jab.

If you’re 18 or older, you can get a booster dose of any of the COVID vaccines authorized in the U.S. That means you don’t have to stick with same the vaccine you got at first. For example, if your initial doses came from Moderna, you can get a booster dose from Pfizer.

Also, the CDC recommends a second booster shot made from mRNA for certain people with weakened immune systems or if they’re 50 or older. You may get this at least 4 months after your first booster shot.

Based on new research, if you got the J&J vaccine as your original vaccine and booster, the CDC recommends a second booster using an mRNA vaccine 4 months after your last jab.

Health care workers and the elderly were the first to receive the vaccine. But after the success of mass production and distribution, the vaccines were made available to a broader population.

The CDC also says the vaccines are safe for pregnant women and there is no indication they pose any danger to the fetus. There have been reports of adverse allergic reactions to some of the vaccines, so at present time, people who have a history of severe allergies are advised not to get vaccinated

Both the Pfizer and Moderna vaccines have been shown to be more than  90% effective for adults. Johnson & Johnson is more than 66% effective.

Scientists are continuing to look at how effective the vaccines are against COVID variants, like the highly infectious Omicron. But the bottom line is this: Anyone who’s eligible to get vaccinated and boosted should do so because it lowers your chances of getting severely sick -- or dying -- from COVID-19. 

The Pfizer vaccine uses messenger RNA (mRNA). This is what carries the instructions for making the “spike” protein that lets the virus enter human cells. The mRNA vaccine tells your immune cells to make just the protein and act as if they’ve already been infected with the coronavirus, giving you some immunity against it.

The J&J vaccine uses DNA that’s designed to trigger an immune response to the virus.

Several COVID vaccines, like Johnson & Johnson’s, have weakened versions of the adenovirus, one of the viruses that causes the common cold. It’s been combined with genes from the new coronavirus’ spike protein to trigger your immune system to fight it. 

Yet other vaccines teach your immune system to target the coronavirus by using versions of the spike protein or the virus itself.

This version of the coronavirus, which causes the disease COVID-19, only surfaced in late 2019. Normally, developing a new vaccine for a new virus takes years, but scientists were able to get a boost from research on similar coronaviruses that cause severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS).

Experts say this coronavirus could eventually turn out to be seasonal, like colds and the flu. A vaccine would be vital to helping control it.

What Does a COVID-19 Vaccine Do?

When you come into contact with viruses or bacteria, your body’s immune system makes antibodies to fight them off.

A vaccine forces your immune system to make antibodies against a specific disease, usually with a dead or weakened form of the germs. Then, if you come into contact with them again, your immune system knows what to do. The vaccine gives you immunity, so you don’t get sick or so your illness is much milder than it otherwise would have been.

The vaccine should slow the spread of COVID-19 around the world. Fewer people should get sick, and more lives can be saved.

Can You Get the Flu Vaccine and the COVID-19 Vaccine at the Same Time?

Yes. The CDC says you no longer have to wait 14 days between vaccinations. Experts say that after getting the COVID-19 vaccine, your immune response -- the process in which your body builds antibodies to protect you against the virus -- is basically the same whether you take it alone or with the flu vaccine. Flu season in the U.S. typically lasts from October to May.

Possible common side effects like pain, redness, and swelling may last a day or so at the injection site. These won’t change much if you get the flu vaccine, too. Call 911 or head to the nearest hospital if you have a severe allergic reaction.

How Are Vaccines Developed?

The development of a vaccine against COVID-19 has taken place in an unparalleled pace. Usually such a process takes years, but the scope of the pandemic triggered round-the-clock work by thousands of researchers working on over 100 different forms of the vaccine.

Effectiveness and safety were key concerns and the Pfizer vaccine approved in the U.S. for emergency use has been found to have 95% efficacy after its second dose.

Before any vaccine can be used widely, it must go through development and testing to make sure that it’s effective against the virus or bacteria and that it doesn’t cause other problems. The stages of development generally follow this timeline:

  • Exploratory stage. This is the start of lab research to find something that can treat or prevent a disease. It often lasts 2 to 4 years.
  • Pre-clinical stage. Scientists use lab tests and testing in animals, such as mice or monkeys, to learn whether a vaccine might work. This stage usually lasts 1 to 2 years. Many potential vaccines don’t make it past this point. But if the tests are successful and the FDA signs off, it’s on to clinical testing.
  • Clinical development. This is a three-phase process of testing in humans. Phase I usually lasts 1 to 2 years and involves fewer than 100 people. Phase II takes at least 2 years and includes several hundred people. Phase III lasts 3 or 4 years and involves thousands of people. Overall, the clinical trial process may stretch to 15 years or more. About a third of vaccines make it from phase I to final approval.
  • Regulatory review and approval. Scientists with the FDA and CDC go over the data from the clinical trials and sign off.
  • Manufacturing. The vaccine goes into production. The FDA inspects the factory and approves drug labels.
  • Quality control. Scientists and government agencies keep tabs on the drug-making process and on people who get the vaccine. They want to make sure it keeps working safely.


How to Volunteer

If you're interested in volunteering for a COVID-19 vaccine trial, here are some sources for more information: 

Government-sponsored sites:

  • COVID-19 Prevention Network (CoVPN). This is funded by the National Institute of Allergy and Infectious Diseases and coordinated by the Fred Hutchinson Cancer Research Center in Seattle. Its goal is to enroll thousands of volunteers into COVID vaccine trials nationwide. Many research centers are using this site to find volunteers.
  • This is a government database of public and private clinical studies done worldwide. The site also offers considerations for joining a clinical trial.

Sites that link volunteers with trials nationwide include:

Individual hospitals, universities, research centers, and others may also provide opportunities to enroll in a COVID-19 vaccine clinical trial. Some include:

You can also call or visit the website of your local hospital or research institution to find out if it is taking part in any trials.

Show Sources


FDA: “Coronavirus (COVID-19) Update: FDA Authorizes Pfizer-BioNTech COVID-19 Vaccine for Emergency Use in Adolescents in Another Important Action in Fight Against Pandemic,” “Coronavirus (COVID-19) Update: FDA Authorizes Moderna and Pfizer-BioNTech COVID-19 Vaccines for Children Down to 6 Months of Age.”

Reuters: “Canada allows Pfizer COVID-19 vaccine for children aged 12-15.”

Pfizer: “Pfizer-Biontech Announce Positive Topline Results of Pivotal Covid-19 Vaccine Study in Adolescents.”

CDC: “Getting Your COVID-19 Vaccine,” “Possible Side Effects After Getting a COVID-19 Vaccine,” “The Flu Season,” “COVID-19 Vaccine Booster Shots,” “CDC Endorses ACIP’s Updated COVID-19 Vaccine Recommendations,” “What You Need to Know About Variants,” “Johnson & Johnson’s Janssen COVID-19 Vaccine Overview and Safety,” “Moderna COVID-19 Vaccine Overview and Safety,” “Pfizer-BioNTech COVID-19 Vaccine (also known as COMIRNATY) Overview and Safety,” “CDC Recommends Additional Boosters for Certain Individuals.”

UpToDate: “COVID-19 adenovirus vector vaccines (United States and Canada: Authorized for use): Drug information.”

Yale Medicine: “COVID-19 Vaccines for Kids Under 5: What Parents Need To Know.”

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