Overview of COVID-19 Vaccines

Published: June 11, 2021
Revised: February 16, 2023
Samantha Lewiston
By Samantha Lewiston, PharmD

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The Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices currently recommends COVID-19 vaccination for all eligible people.

What is an Emergency Use Authorization (EUA)?
Emergency Use Authorizations allow the Food and Drug Administration (FDA) to authorize the use of unapproved medical products in the case of a public health emergency. These products must meet several criteria to be eligible for an EUA. The criteria include a lack of already approved, adequate and available alternatives and the product’s ability to diagnose, treat or prevent serious illness from a public health threat, including an infectious disease such as COVID-19. Experts at the FDA review all available evidence for a product before issuing an EUA. Several COVID-19 vaccines have received an Emergency Use Authorization by the FDA.

How do the COVID-19 vaccines work?
There are three types of COVID-19 vaccines authorized for use in the United States: mRNA vaccines (Pfizer-BioNTech, Moderna), protein subunit vaccine (Novavax), and a viral vector vaccine (Johnson & Johnson’s Janssen).

The mRNA vaccines contain instructions for human cells to produce a small fragment of the spike protein. This spike protein is usually found on the outside of the COVID-19 virus. Once the mRNA vaccine is in the body, the immune system uses the instructions to produce this spike protein and breaks down the mRNA. The immune system then recognizes the spike protein and develops a response to get rid of the protein. This response guides the body to fight off future infections from the COVID-19 virus.

The viral vector vaccine contains the instructions to produce a small fragment of the spike protein as well, but the instructions are contained in a different, harmless virus to get them into the body. Like the mRNA vaccines, a part of the spike protein is produced once the vaccine is in the body. The immune system develops a response to get rid of the protein, which prepares the body to fight off future COVID-19 infections.

The protein subunit vaccine contains a version of the spike protein and another inactive ingredient that stimulates the immune system. The immune system recognizes the spike protein and develops a response to get rid of the protein. The body will then remember this response to fight future COVID-19 infections.

Are the COVID-19 vaccines safe?
COVID-19 vaccines are evaluated for safety during clinical trials that studied the vaccines in tens of thousands of people. Since the EUAs have been granted, millions of people have safely received the COVID-19 vaccines. The most common side effects are swelling and pain at the injection site, fever, headache, tiredness, muscle pain, chills, and nausea. A severe allergic reaction to a COVID-19 vaccine is very rare.

What is herd immunity?
Herd immunity occurs once a vast majority of a population is protected from a disease through previous infection or vaccination. Once most people are protected, and the disease is not as easily spread, those who are not already protected (often those who cannot receive the vaccine due to a severe allergic reaction or medical condition) receive indirect protection from the disease due to reduced infection in the community. Each disease requires a different percentage of people to be immune to achieve herd immunity. The percentage required for COVID-19 is still unclear and continues to be studied by experts.

What are variants?
Viruses, including COVID-19, may change slightly over time as they reproduce inside the human body and throughout the population. These changes lead to variants of the virus, which may have slightly different characteristics from the original virus. Several variants of the COVID-19 virus have been found in the United States and in other countries.

As new variants are identified, experts study them to understand any new characteristics they may have, such as how it spreads and what happens to people who are infected with the variant virus. 

Early studies suggest the protection offered through vaccination protects against some variants, especially in preventing severe disease, hospitalization, and death. Recent studies show the variants are reducing the ability of COVID-19 vaccines in preventing infection and therefore boosters will be needed for those previously fully vaccinated. A bivalent mRNA booster shot contains updated protection against type kinds of the Omicron variant. It is important to get a bivalent booster after the initial vaccine series and previously available boosters to maintain optimal protection.

COVID-19 vaccines are safe and effective, as shown by clinical trials and millions of doses given throughout the United States. Herd immunity is the indirect protection offered once many people are vaccinated, but a specific goal percentage is still unknown. Variants are new versions of a virus with small changes, and preliminary studies show COVID-19 vaccines and updated bivalent boosters do offer a range of protection from them.

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