Vaccines have been a critical tool in managing the COVID-19 pandemic. Researchers have been working on vaccines against the novel coronavirus, known as SARS-CoV-2, since it was first identified and characterized.

In fact, the World Health Organization (WHO) estimated that more than 200 COVID-19 vaccine candidates were in development in December of 2020. Since then, several vaccines have been authorized or approved for use.

Generally speaking, there are four different types of COVID-19 vaccines that are being used throughout the world. Keep reading to learn what these are, how they work, and more.

The four types of COVID-19 vaccines being used around the world are:

  • messenger RNA (mRNA) vaccines
  • viral vector vaccines
  • protein subunit vaccines
  • whole virus vaccines

The table below provides a brief summary of the different types of COVID-19 vaccines and the branded names that they’re associated with.

Vaccine typeBranded names
mRNAPfizer, Moderna
Viral vectorJohnson and Johnson (J&J), AstraZeneca, Sputnik V
Protein subunitNovavax
Whole virusSinopharm, Sinovac

The mRNA vaccines work by teaching your body to make a protein from the novel coronavirus. This protein is called the spike protein. Normally, the virus uses it to attach to and enter cells.

These vaccines contain a molecule called mRNA that’s surrounded by a protective lipid (fat) layer. The function of mRNA is to tell cells how to make proteins. Your cells use mRNA every day to make proteins that are vital for life.

Here’s how the mRNA vaccines work:

  1. After being injected into the upper arm muscle, the vaccine mRNA is able to enter nearby cells, shedding its protective fat layer.
  2. Once inside the cell, the vaccine mRNA teaches the cell how to make the spike protein. After doing so, the vaccine mRNA is destroyed.
  3. When the cell has made the spike protein, it displays the protein on its surface.
  4. Cells of the immune system can see this spike protein and recognize it as foreign.
  5. The immune system becomes active and creates an immune response, which can include both antibodies (which target foreign objects) and T cells (which protect against infection) that specifically recognize the spike protein.

There are currently two mRNA vaccines in use. These are the Pfizer-BioNTech and Moderna vaccines. Both of these vaccines are given as two doses. The Pfizer-BioNTech doses are spaced out over 21 days (3 weeks). The Moderna doses are spaced out over 28 days (4 weeks).

Effectiveness of mRNA vaccines

Large-scale clinical trials found that both mRNA vaccines were very effective. Vaccine effectiveness was found to be 95 percent and 94.1 percent for the Pfizer and Moderna vaccines, in that order.

However, much has changed since these trials. Variants of the novel coronavirus have emerged, such as the highly infectious Omicron variant. The mRNA vaccines are less effective against these variants.

Because of the rise of variants as well as naturally decreasing immunity, public health organizations around the world have recommended booster doses.

As a result, research has focused on vaccine effectiveness in terms of variants and booster doses. Let’s see what some of this research says.

Research studies

A 2022 study examined the effectiveness of the Pfizer vaccine between November 2021 and January 2022. This study’s findings about the effectiveness against the Omicron variant were as follows:

  • Two doses of Pfizer. Vaccine effectiveness was 65.5 percent in the 2 to 4 weeks after the second dose but dropped to 8.8 percent after 25 or more weeks.
  • Two doses plus Pfizer booster. Vaccine effectiveness rose to 67.2 percent in the 2 to 4 weeks after the booster dose but dropped to 45.7 percent after 10 or more weeks.

Another 2022 study looked at the effectiveness of the Moderna vaccine against the Omicron variant. This study reported the following:

  • Two doses of Moderna. Vaccine effectiveness was 44 percent in the 14 to 90 days after the second dose but dropped quickly after 90 days.
  • Two doses plus Moderna booster. Vaccine effectiveness was 71.6 percent in the 14 to 60 days after a booster dose but dropped to 47.4 percent after 60 days.

Viral vector vaccines for COVID-19 use a modified virus to deliver instructions to your cells on how to make the spike protein. The modified virus is harmless and can’t make copies of itself or cause disease.

The viral vector vaccines for COVID-19 all use an adenovirus vector. In nature, adenoviruses can cause cold- or flu-like symptoms.

Viral vector vaccines work in the following way:

  1. After being injected into the upper arm muscle, the vector virus enters into nearby cells.
  2. Once inside the cell, the vector virus releases its genetic material, which contains instructions on how to make the spike protein. After doing this, the remaining pieces of the vector virus are destroyed.
  3. When the cell has made the spike protein, it displays the protein on its surface. The genetic material released by the vector virus is also quickly destroyed.
  4. Cells of the immune system can see the spike protein on the cell surface and recognize it as foreign.
  5. The immune system becomes active and creates an immune response, which can include both antibodies and T cells that specifically recognize the spike protein.

There are a few examples of viral vector vaccines in use throughout the world. These include:

Viral vector vaccine effectiveness

The large-scale clinical trials of the J&J vaccine found that a single vaccine dose was 66.9 percent effective for preventing moderate to severe or critical COVID-19.

Clinical trials of the AstraZeneca vaccine found that overall vaccine effectiveness after two doses was 70.4 percent.

The arrival of the Omicron variant has hit viral vector vaccines pretty hard. However, receiving a booster with an mRNA vaccine can help.

One of the 2022 studies discussed earlier also looked at the effectiveness of the AstraZeneca vaccine against the Omicron variant. This study’s findings were as follows:

  • Two doses. No effect against the Omicron variant was seen from 20 weeks after the second dose.
  • Two doses plus Pfizer booster. Vaccine effectiveness rose to 62.4 percent 2 to 4 weeks after a Pfizer booster dose but dropped to 39.6 percent after 10 or more weeks.
  • Two doses plus Moderna booster. Vaccine effectiveness increased to 70.1 percent 2 to 4 weeks after a Moderna booster dose but dropped to 60.9 percent in weeks 5 to 9.

Protein subunit vaccines are pretty straightforward. They contain a purified protein from a virus that the immune system can see and respond to. In the case of the novel coronavirus, this protein is the spike protein.

Protein subunit vaccines work in the following way:

  1. The purified spike protein enters the body after being injected into the upper arm muscle.
  2. Cells of the immune system encounter the spike protein and recognize it as foreign.
  3. The immune system becomes active and creates an immune response, which can include both antibodies and T cells that specifically recognize the spike protein.

There are various protein subunit vaccines in development. One that you may have heard of is the Novavax vaccine, which is given in two doses spaced 21 days (3 weeks) apart.

The spike protein in the Novavax vaccine is made in cells in a laboratory and is purified before being stuck onto a tiny, round particle called a nanoparticle. This design imitates the shape of the novel coronavirus and also helps to group many spike proteins together so that the immune system can see them.

Protein subunit vaccine effectiveness

A large-scale clinical trial of the Novavax vaccine found that its effectiveness was 90.4 percent.

However, this trial was performed in early 2021, before the arrival of the Delta and Omicron variants. Detailed data on the Novavax vaccine’s effectiveness against these variants haven’t been published yet.

So far, Novavax has released a statement based off of early data that antibodies from the first two-dose vaccine series have some effectiveness against the Omicron variant. Protection also increased after a booster dose.

The last type of COVID-19 vaccines are whole virus vaccines. These vaccines contain whole virus particles, known as virions, of SARS-CoV-2, the virus that causes COVID-19.

The only whole virus vaccines that are in use are inactivated. In an inactivated vaccine, the virus has been treated so that it remains whole but can’t cause disease. This is typically accomplished by using chemicals or heat.

An inactivated whole virus vaccine works in the following way:

  1. The inactivated virus enters the body after being injected into the upper arm muscle.
  2. Cells of the immune system encounter the inactivated virus and recognize it as foreign.
  3. The immune system becomes active and creates an immune response, which can include both antibodies and T cells that specifically recognize the spike protein.
  4. Because the inactivated virus in the vaccine can’t make copies of itself, it’s destroyed by the immune system.

Two examples of inactivated whole virus vaccines are the Sinovac and Sinopharm vaccines.

Whole virus vaccine effectiveness

A 2021 study of the Sinovac vaccine, called CoronaVac, found that vaccine was only 46.8 percent effective against symptomatic SARS-CoV-2 infection after the second vaccine dose.

The Omicron variant has greatly impacted the effectiveness of the available inactivated vaccines.

Overall, researchers are finding that these vaccines provide little to no protection against this variant. However, boosters with another type of vaccine may help to restore this protection.

Before being used on a wide scale, all vaccines need to be shown to be both safe and effective in large-scale clinical trials.

In the United States, the Food and Drug Administration (FDA) reviews the data from these trials before approving the vaccine or issuing an emergency use authorization.

Generally speaking, some of the most common side effects of COVID-19 vaccines are:

These side effects typically come on within a day of receiving a vaccine dose. They only last a few days before going away on their own.

If you have side effects such as fatigue, fever, and muscle pain, you may feel as if the vaccine is making you sick. However, these symptoms are completely normal and are actually a sign that your body is creating an immune response to the vaccine.

Who shouldn’t receive a vaccine?

There are some people who shouldn’t receive a COVID-19 vaccine. This is called a contraindication to vaccination. For the vaccines that are currently in use in the United States, the only contraindications to COVID-19 vaccines are:

  • a known allergy to an ingredient in the vaccine
  • a history of a serious allergic reaction, called anaphylaxis, after a previous dose of the vaccine
  • a history of thrombosis with thrombocytopenia syndrome (TTS), which involves blood clots combined with a low number of cells called platelets, after a previous dose of the J&J vaccine (J&J vaccine only)

mRNA vaccine notable side effects

In rare situations, mRNA vaccines can lead to myocarditis, or inflammation of the heart muscle. The Centers for Disease Control and Prevention (CDC) notes that this is more common:

  • in teens and young adults assigned male at birth
  • after the second vaccine dose
  • within a week of being vaccinated

According to a 2021 study of 139 teens and young adults with suspected myocarditis after vaccination, the condition was typically mild and resolved quickly when treatment was given.

Additionally, a 2022 study found that a person was more likely to develop myocarditis after contracting SARS-CoV-2 than after receiving a COVID-19 vaccine.

Viral vector vaccine notable side effects

Although very rare, serious side effects such as TTS and Guillain-Barré syndrome (GBS) have been reported after vaccination with viral vector vaccines such as the J&J and AstraZeneca vaccines.

Based off of an updated risk-benefit analysis, the CDC is now recommending that people receive an mRNA vaccine over the J&J vaccine. This recommendation was made based on the fact that the J&J vaccine:

  • carries a risk of TTS and GBS, which isn’t associated with the mRNA vaccines
  • has a lower effectiveness than the mRNA vaccines

Similarly, the United Kingdom offers alternatives to the AstraZeneca vaccine in individuals under the age of 40. These alternatives were offered because people in this age group, particularly people assigned female at birth, are at a higher risk of TTS.

There are several different types of COVID-19 vaccines. These vaccines all work in different ways to prepare your immune system to respond to the novel coronavirus, should you be exposed to it.

Before being widely used, vaccines must go through a rigorous clinical trial process to assess their safety and effectiveness. As such, vaccines that have been authorized or approved have been shown to be safe and effective.

To increase your protection against COVID-19, it’s important to stay up to date on your COVID-19 vaccinations. Never hesitate to talk with a doctor or other healthcare professional if you have any concerns or questions about vaccination.