When you hear about a new vaccine being approved, it might seem like it happened very fast. But behind every shot is years of research, testing, and safety checks. Vaccines go through a careful process to make sure they are safe, effective, and well-studied before anyone gets them.

Let’s take a look at that process step by step.

In the beginning, a virus or bacteria is discovered that infects humans and causes harm. Because of the harm scientists set out to learn as much as they can about the disease, how humans body’s react to it, and ways to prevent it from infecting us.

Testing vaccines is called Clinical Trials and they have 4 phases.

Phase 0/Pre-Clinical: In the beginning vaccine are tested using animal models such as mice, rats, and the non-human primates. This step can last anywhere from a year to 10 years. This is because it needs to be proven to work in animals without causing serious effects before it can begin the clinical trial process. When an animal shows signs of protection from the vaccine then it can begin phase 1.

Phase 1: Researchers will spend months looking at the effects of the vaccines in 20-80 participants who have no health conditions. The purpose of this phase is to determine the highest dose that can be given without serious side effects. For example, the participants will be given 2mL to see if that small dose is effective. If the dose has no effect then the amount will be increased. Eventually the researchers will determine how much they can give to protect from disease without serious side effects. Investigators can also look at how the vaccine is delivered either orally, intravenous, or intranasally and observe what version works best.

Phase 2: This phase involves hundreds of participants. Participants are recruited through multiple routes such as online through ads, community outreach and engagement, or through their healthcare providers. This phase can take months to years depending on results or type of medication. The FDA has reported that only about 33% of medications ever pass this phase.

Phase 3: The third phase usually involves up to 3,000 participants who have the condition the new medication is made to treat. This trial phase commonly can last years. This phase treats older versions of the vaccine, if they exist, against the new one to show that it works better and is just as safe. The investigators will use randomization to select which participants get which treatment. These trials are also double-blinded meaning no one knows which treatment they are getting. This helps eliminate potential bias in the results. This phase will produce more evidence for side effects due to the larger number of participants. The FDA will review the data and will only approve drugs if they are shown to be as safe and effective as others on the market. About 20-30% of medications ever pass to the next phase.

Phase 4: Now that the vaccine is available to the public and approved, phase 4 can begin. Investigators now are collecting more data on a larger scale. They also are able to identify long term effects of the vaccines. Side effects are recorded in the Vaccine Adverse Event Reporting System (VAERS) either by individuals or by a physician. Anyone can input and record their side effects within this system. The CDC and FDA also work to monitor large sets of the population.

During this phase if any serious side effects are observed the company will be forced to recall the vaccine. Historically, most vaccines are successful and never get pulled from market. However, sometimes due to manufacturing errors multiple vaccines throughout history have been pulled. (Look out for a future post about the history of vaccines.)

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The clinical trial process is long and strenuous. Often times most medications never make it to market. Each phase is an important step to making sure it is both safe and able to treat the condition it is meant for. If you are interested you can even join clinical trials yourself!

Next time you’re considering vaccinating you or your child think how much research has gone into making a safe and effective way to protect us.