COVID-19 Vaccines

Medically Reviewed by Melinda Ratini, DO, MS on July 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.

Types of COVID-19 Vaccines

The virus that causes COVID-19 only surfaced in late 2019. Normally, developing a 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 is vital to helping control it.

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 recommends everyone ages 6 months and older get vaccinated against COVID-19. And everyone 5 years and older should also get a COVID-19 booster, if eligible.

The CDC says the vaccines are safe for pregnant people, and there’s no indication they pose any danger to a fetus. There have been reports of adverse allergic reactions to some of the vaccines, but these are extremely rare.

The CDC advises that anyone who had a severe allergic reaction after getting Pfizer-BioNTech or Moderna vaccines should not get another dose of either. Anyone who had a severe allergic reaction after receiving the Johnson & Johnson vaccine should not get another dose of it.

There are four types of vaccines approved in the United States:

Pfizer-BioNTech. This vaccine uses messenger RNA (mRNA). This is what carries 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.

This vaccine is approved for children and adults. The children’s dose is not as strong as the adult version. Children from 6 months to 4 years old get a three-dose primary series, and everyone age 5 and older gets a two-dose primary series.

The CDC also recommends a first booster of Pfizer-BioNTech vaccine for most children, teens, and adults at least 5 months after their final dose in the primary series.

Children and teens who are moderately or severely immunocompromised should get their first booster at least 3 months after their final dose in the primary series.

A second booster of Pfizer-BioNTech COVID-19 vaccine is also recommended at least 4 months after the first booster for children and teens who are moderately or severely immunocompromised and for all adults ages 50 and older. It’s also recommended for other groups of adults considered to be at high risk.

Moderna. This is an mRNA vaccine given in two doses, 4 to 8 weeks apart. Those ages 6 months to 17 years can get the Moderna vaccine, but a booster of Moderna is not recommended for them at this time. People 18 years or older can get the Moderna booster at least 5 months after getting the first two doses.

If you’re 18 or older, you can get a booster dose of any of the mRNA COVID vaccines authorized in the U.S. That means you don’t have to stick with same vaccine you got first.

For example, if your first doses came from Moderna, you can get a booster dose from Pfizer. In some cases, it may be OK to get the Jonson & Johnson vaccine as a booster.

A second booster dose of Moderna may be given at least 4 months after the first booster dose of any approved COVID-19 vaccine. It may be given to those 18 and older who are eligible because of their age or risk for severe illness from COVID-19.

Johnson & Johnson vaccine. 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’s spike protein to trigger your immune system to fight it.

Top health experts recommend you choose a COVID vaccine made with mRNA (like the ones from Pfizer and Moderna) rather than the J&J vaccine, which is made differently. Their 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 available vaccines.

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

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.

Also, the CDC recommends a second booster shot made from mRNA for certain people with weakened immune systems or those 50 and 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.

Novavax. The FDA has approved the emergency use of the Novavax COVID-19 vaccine for people 18 years and older. Experts continue to study the vaccine for children as young as 12.

This vaccine offers another option for those who haven’t gotten the Pfizer, Moderna, or J&J vaccine. The Novavax shot is like other vaccines that have been used in the U.S. for more than 30 years. It uses protein subunits, just like the hepatitis B, influenza, and whooping cough vaccines.

The CDC suggests Novavax if you prefer to get a vaccine that’s built on different technology than previous COVID-19 vaccines.

It includes a two-dose series. You’ll get your second shot 3 weeks after your first. The Novavax vaccine is different from the other three available COVID-19 vaccines. It contains:

  • The SARS-CoV-2 spike protein. Scientists make this in lab-grown insect cells. It helps your body create an immune response. Unlike other COVID-19 shots, the Novavax vaccine puts a version of this spike protein directly into your body to create antibodies and T cells.
  • Matrix-M adjuvant. This is a compound that boosts effectiveness of vaccines and other drugs.

Clinical trials show that the Novavax vaccine helped prevent some COVID-19 infections and lowered the likelihood of severe illness from the virus in 90% of cases.

At this time, there is no Novavax booster available in the U.S. Experts continue to look at how well the vaccine may work as a booster, and whether you can use Novavax with other types of COVID-19 vaccines.

What Does a COVID-19 Vaccine Do?

When you come in 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 in 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.

The Pfizer, Moderna, and Novavax vaccines have been shown to be at least 90% effective for adults. Johnson & Johnson’s COVID vaccine is more than 66% effective. But these numbers were from before new COVID-19 variants became common.

While these vaccines may not completely prevent you from getting infected with a COVID variant, they’ll likely protect you from getting severely ill.

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 where you got the shot may last a day or so. 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 forms of the vaccine.

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 of 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 are:

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,” “Coronavirus (COVID-19) Update: FDA Authorizes Emergency Use of Novavax COVID-19 Vaccine, Adjuvanted.”

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,” “mRNA Vaccines,” “CDC Recommends Novavax’s COVID-19 Vaccine for Adults.”

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,” “Novavax's COVID-19 Vaccine: Your Questions Answered.”

University of Maryland Medical System: “COVID Variants and the Vaccine.”

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