What You Should Know About Vaccines

Skip to Content

What You Should Know About Vaccines

What you should know about vaccines

Immunization is our best defense against COVID-19. With a COVID-19 vaccine now available to anyone ages 12 and older in the U.S., it’s natural to have questions about how vaccines work, the approval process, vaccine safety and more.

How do vaccines work?

Vaccines stimulate the body’s immune system to create immunity through antibodies and some kinds of white blood cells. Vaccine development is a complex process based on more than 80 years of experience. Safe and effective vaccines have been developed for serious illnesses such as smallpox, diphtheria, pertussis, tetanus, polio, measles, mumps, rubella, hepatitis B, varicella, hepatitis A, influenza, rotavirus and pneumococcal pneumonia. Before a vaccine is approved, it must undergo a series of studies and trials and be thoroughly tested for safety and efficacy.

What are the phases for approving a vaccine?

Vaccines are subject to a rigorous review, which usually starts in the laboratory and moves to testing through clinical trials. The FDA requires success through three trial phases before approval for widespread use.

  • Phase I tests safety, dosage and immunogenicity in a small number of healthy volunteers.
  • Phase II is randomized, placebo controlled and involves several hundred volunteers including people at risk of acquiring the disease. The goals are to better understand the vaccine’s safety, immunogenicity, proposed doses, schedule of immunizations and method of delivery.
  • Phase III is a much larger trial to assess safety and efficacy in tens of thousands of people from groups that are at risk of the infection. Phase III is expected to show if a vaccine can protect at least 50 percent of people who receive it and to identify any rare side effects that may have not been identified earlier.

How do we know if a new vaccine is safe?

Throughout the development process, there are several safeguards to promote safety:

  • Adverse events are actively monitored and, if severe, lead to pausing new enrollment so an extensive review can be done to assess for any link with the vaccine.
  • Vaccine side effects are sought regularly and measured in pre-determined categories and degrees of severity using routine visits, calls and review of self-report logs weekly. After each vaccine dose, volunteers are followed daily, usually for 14 days.
  • If serious adverse events occur, clinicians will evaluate the volunteer and assure timely medical care. Serious adverse events are reported to the independent Data and Safety Monitoring Board, which has access to all study results, including who received the vaccine and who did not. The board monitors for adverse events and takes action to alert the vaccine sponsor and the FDA.

It's common to experience benign side effects from a vaccine, such as soreness or fatigue, which indicate the body is responding the way it’s supposed to. Because of the rigorous vaccine safety measures in place, serious adverse reactions to vaccines are rare.

What are the types of vaccine approval?

  • Full approval: This is traditional way we think of new therapies getting approved by the FDA. Vaccines and biologics are approved through a biological license application.
  • Expanded investigational access: Sometimes called “compassionate use,” this process is used in life-threatening conditions for a patient to get access to a therapy outside of a clinical trial. An example would be convalescent plasma.
  • Emergency use authorization: EUA is the fastest way to get a treatment or vaccine in the hands of people who need it most. This approval path requires a lower standard for use because it’s needed quickly in an emergency. EUAs only require FDA to determine a product “may” be effective and that benefits are likely to outweigh risks.

What officials oversee COVID-19 vaccine distribution?

The federal government is centralizing the process for ordering, distributing and tracking COVID-19 vaccines. The CDC is providing guidance to state and local jurisdictions, including how the vaccine should be distributed. State health departments are essential parts of the vaccine distribution system, as is the private sector.

How do we know if a COVID-19 vaccine is effective?

There are several ways to measure a vaccine’s effectiveness. The main way effectiveness is measured is comparing volunteers who received the vaccine with those who received placebo for development of COVID-19 infection. Other measures may include the vaccine’s ability to prevent severe COVID-19, symptomatic COVID-19 infection or asymptomatic COVID-19 infection.

The COVID-19 vaccine has been shown to be both very effective and safe.

What is herd immunity?

Herd immunity is a form of indirect protection from infectious disease. It occurs when a sufficient percentage of a population becomes immune to an infection, whether through vaccination or previous infections. When this happens, it greatly reduces the probability of encountering the virus because chains of transmission get broken by immune people. As a result, there is a decreased risk of infection for individuals who lack immunity and a new outbreak or pandemic cannot occur.

Easier Than Ever to get the vaccine

Is vaccine safe for pregnancy

The Delta Variant Solo Panel

COVID 19 Vaccine Resource Center Panel Button

What you should know about vaccines Panel Callout

What you should know about vaccines Panel Callout


Coronavirus Info Center Panel Button

Safe in Our Care Panel Callout

In the News

Covid 19 Hits Home

Dont post your vaccination card

COVID-19 Breakthrough Cases