Woman working with vaccine in lab

Many people were astonished at how quickly the COVID-19 vaccines were developed.

Although the first vaccines were created, evaluated and authorized for emergency use in under a year, rest assured that no steps were skipped in ensuring their safety and effectiveness. They went through the same layers of review and testing as other vaccines.

Scientists have been working for many years, long before the start of the COVID-19 pandemic, to develop vaccines against coronaviruses. Knowledge gained through past research helped speed up development of the COVID-19 vaccines.

After their initial development in laboratories, all vaccines go through three phases of clinical trials to make sure they are safe and effective. (The trials compare outcomes, including how many people get sick, between those who are vaccinated and those who are not.) Then, the U.S. Food and Drug Administration (FDA) reviews the findings from the clinical trials before deciding whether to grant its authorization or approval.

While the three phases of vaccine clinical trials are normally performed one at a time, they overlapped during development of the COVID-19 vaccines to speed up the process so the vaccines could be used as quickly as possible to help fight the pandemic. The trials showed no serious safety concerns within eight weeks following vaccination, which is significant as it is unusual for adverse reactions to vaccines to occur after that period of time. Clinical trials for COVID-19 vaccines have involved tens of thousands of volunteers of different ages, races and ethnicities.

The FDA and Centers for Disease Control and Prevention (CDC) continue to monitor the safety of the vaccines and alert the public about health problems that are reported after vaccination.

Still deciding about the vaccine or know someone who is? See these reasons to get the vaccine.

The Traditional Vaccine Testing Process

The testing of vaccines is a complex process. There are many layers of study and review that need to take place before a vaccine can be made available to the general public.

Discovery Phase

Before any vaccine testing or development begins, scientists study the structure of a virus and how it causes disease in the body. This allows them to identify potential ways of creating an effective vaccine.

This process usually takes several years. In contrast, new technology allowed the genetic makeup of COVID-19 to be shared with researchers worldwide just a few weeks after the first case of the virus.

Once scientists select the type of vaccine that looks the most promising, the vaccine is then created in the lab.

Pre-Clinical Testing

In a laboratory setting, scientists study the vaccine, looking at the safety and efficacy of the vaccine in animals before moving to testing in humans.

Clinical Trials

If enough research supports a vaccine candidate, it can then be tested using clinical trials involving people. These clinical trials, done in three phases, are tightly regulated by the FDA.

In the case of the COVID-19 vaccine, all three phases were planned simultaneously to prevent delays if a vaccine candidate was proven effective. But usually, each phase is planned after the previous one is completed since companies don't want to put time and money behind a vaccine if it won't be successful. This may result in one- to two-year gaps between clinical trial phases. 

Phase 1 Clinical Trial

A study of usually less than 100 people is designed to determine if the vaccine is safe, the best dosage and if there are any serious side effects.

Phase 2 Clinical Trial

A larger study with a few hundred volunteers focuses on how well the vaccine works. These studies also continue to investigate its safety and side effects.

Phase 3 Clinical Trial

An even larger study, often including thousands of volunteers, allows scientists to compare people who receive the vaccine to those who did not. They can better determine if the vaccine is safe and effective in a larger population of individuals.

Volunteers are randomly selected to receive either the vaccine or an alternative "control" medication. The control medication is usually a placebo or another vaccine that is already FDA-approved.

FDA Review of Clinical Trials

The FDA looks for evidence that the vaccine is unsafe, ineffective or has side effects that outweigh the benefits of receiving the vaccine.

If there is substantial evidence that the vaccine is effective and does no harm, then the FDA approves the vaccine for general public use. If a vaccine is not proven safe, has significant side effects or does not show efficacy, the vaccine is not distributed to the public.


Traditionally, the manufacturing of a vaccine begins when Phase 3 of clinical trials are being planned. In the case of the COVID-19 vaccine, manufacturing began in parallel with the clinical trials to ensure that FDA-approved vaccine candidates could be distributed quickly.

Throughout the manufacturing process, batches of vaccine are tested to ensure that they're packaged and distributed in a way that maintains their effectiveness. The FDA routinely reviews the results of these tests.

Additionally, all manufacturing facilities are routinely inspected and expected to meet strict quality and safety standards.

Continued Safety Monitoring

After a vaccine is approved, both the CDC and the FDA continue monitoring the safety of the vaccine. Experts can quickly determine if vaccine recommendations need to be reevaluated.

How COVID Vaccine Testing Moved So Fast

The speed of COVID-19 vaccine development has given some people pause, but this vaccine was created under unique circumstances:

Vaccine Research Funding

Vaccines can be expensive to create. Each step of the testing process requires significant funding from companies.

To reduce the chance of losing money, companies usually want to see overwhelming evidence that the vaccine will ultimately be effective and worthy of FDA approval before moving on to the next step of the testing process. This can lead to gaps in funding as well as a hesitancy to fund vaccine development efforts.

In the case of the COVID-19 vaccine, public and private entities united to fund development efforts. Billions of dollars from the government and nonprofits allowed development to take place unimpeded by cost constraints.

Pre-Approval Vaccine Manufacturing

To cut costs, companies usually wait until a vaccine is FDA-approved before starting the manufacturing and distribution process.

In the case of the COVID-19 vaccine, companies received funding from the government and federal agencies to cover manufacturing costs upfront. Millions of potential vaccine doses were created before they were actually approved, ensuring that FDA-approved vaccines would be distributed significantly faster.

Vaccines that are not proven safe and effective during clinical trials and aren't approved by the FDA will not be distributed.

Administrative Changes

Developing a vaccine is a top priority for the government, FDA and other regulatory entities. To speed up the review of vaccine efficacy data, a number of administrative changes were made to prioritize COVID-19 vaccine work. For example, all COVID-19 vaccine clinical trial phases were planned at once to prevent delays that usually occur.

But none of those changes have affected the way scientists or the FDA evaluate vaccine safety.

When a safety concern arises at any stage of the process, scientists halt the study to investigate it further before continuing the study. A few times during the development process, COVID-19 vaccine clinical trials were paused so concerns could be safely addressed.

A Familiar Virus

While SARS-CoV-2, the virus that causes COVID-19, was new to the world, the family of viruses it came from was familiar to infectious disease scientists.

Scientists were already researching potential vaccines for other coronaviruses, such as SARS and MERS, so they applied much of what they already knew to developing a COVID vaccine.

Additionally, SARS-CoV-2 causes an acute illness, which makes it inherently easier to vaccinate against. Vaccines against viruses that cause chronic illnesses, such as HIV, are usually more difficult to develop.

Global Communication and Collaboration

More advanced ways of communicating and sharing information globally have also significantly sped up the vaccine creation process. Nations across the globe have united to develop an effective COVID-19 vaccine as quickly and safely as possible.

For example, the discovery phase of vaccine development is usually a very long process. However, the genetic makeup of COVID-19 was shared with researchers worldwide just a few weeks after the first case of the virus. This allowed researchers and health organizations around the world to begin researching the best type of vaccine quickly.

New Vaccine Development Technologies

There are many different ways to develop a vaccine, but in the last decade, scientists have made significant advancements in this area.

For example, older vaccines were typically created by using weakened or inactivated viruses. Today, companies can simply read a virus's genetic code and synthesize it in a lab, significantly shortening development times.

Other recent advances in immunology, genetics and other fields have enhanced scientists' understanding of how to create an effective vaccine quickly.

Updated 9/29/2022

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