COVID-19 Vaccine Effectiveness
What We Know about How Well the Vaccines Are Working
COVID-19 vaccination reduces the risk of getting COVID-19 and helps protect you from severe illness. CDC continuously monitors how well the vaccines are working. So far, studies that have looked at how COVID-19 vaccines work in real-world conditions (vaccine effectiveness studies) have shown that these vaccines are working well, although more data are needed.
In the United States, most available vaccine effectiveness data are related to mRNA vaccines (Pfizer-BioNTech and Moderna) because these vaccines have been available longer and are used more widely. CDC and other experts continue to study the effectiveness of both mRNA vaccines and Johnson & Johnson’s Janssen (J&J/Janssen) COVID-19 vaccine in real-world conditions.
COVID-19 vaccines are protecting people in the real world
Vaccine effectiveness studies provide growing evidence that mRNA COVID-19 vaccines protect as well in real-world conditions as they have in clinical trial settings. These studies show that the vaccines reduce the risk of COVID-19, especially severe illness, among people who are fully vaccinated.
The COVID-19 vaccines offer protection against symptoms, but also help avoid people get infected with the virus that causes COVID-19 at all. Vaccination can reduce the spread of disease, which helps protect you and the people around you.
Two doses are better than one for mRNA vaccines: If you are getting the Pfizer-BioNTech or Moderna vaccine, be sure to get both doses.
Real-world data from vaccine effectiveness studies have shown that receiving only one dose of these mRNA COVID-19 vaccines gives some protection against COVID-19, at least in the short term. However, these studies have also shown that for mRNA vaccines, two doses provide better protection than one dose. To get the most benefit from vaccination, people should get both doses of the mRNA vaccines.
It usually takes about 2 weeks for the body to build full protection after vaccination. You are considered fully vaccinated 2 weeks after your second dose of the Pfizer-BioNTech or Moderna vaccine or 2 weeks after your single dose of the J&J/Janssen vaccine.
COVID-19 vaccines help protect against severe illness, even among people infected after being vaccinated (vaccine breakthrough cases)
Right now, most cases of COVID-19 are in people who are not fully vaccinated. Although COVID-19 vaccines currently appear very effective against severe disease and death, no vaccine is perfect. Some people who are fully vaccinated will still get COVID-19; these are called vaccine breakthrough cases. Some evidence shows that vaccination may make illness less severe for those who still get sick. This includes people aged 65 years and older, who are at higher risk of severe COVID-19 illness.
- Get a COVID-19 vaccine as soon as you can.
- Get the most protection by receiving all recommended doses of a COVID-19 vaccine. Keep taking all precautions until you are fully vaccinated.
- Keep taking some precautions even after you are fully vaccinated, if you live in an area with substantial or high community transmission
COVID-19 vaccines and new variants of the virus
Viruses are constantly changing, and new types of the virus, called variants, occur. New variants of the virus that causes COVID-19 are spreading in the United States and in other parts of the world. Research shows that the COVID-19 vaccines used in the United States protect against severe disease, hospitalization, and death from known variants of concern; they may not be as effective in preventing asymptomatic infection. CDC will continue to monitor how vaccines are working to see if variants have any impact on how well COVID-19 vaccines work in real-world conditions.
CDC has several ways to monitor COVID-19 vaccine effectiveness
Along with studies performed by vaccine manufacturers, by other government agencies, and by academic investigators, CDC also works on many types of studies to determine COVID-19 vaccine effectiveness in real-world conditions. Some examples of the systems CDC uses are listed in the table below; you can learn more about each system through the links provided:
Examples of CDC’s systems for monitoring vaccine effectiveness include:
|Outcome monitored||Population monitored||Monitoring system|
|Infection||Long-term care facility residents||NHSN|
|Symptomatic illness||Healthcare providers and frontline workers||HEROES/RECOVER|
|Hospitalization and deaths||Hospitalized adults||IVY|
|Hospitalization and deaths||Hospitalized people (all ages)||COVID-NET and VISIONpdf icon|