Learn to spot the difference
When it comes to health and medicine, it’s sometimes hard to understand the difference between correlation and causation. But correlation does not equal causation.
Causation
Causation is when one thing leads to another thing happening. For example, a measles infection is known to cause a blistery rash in many patients.source: 1 Or we know that cigarette smoking is known to cause lung cancer, especially in many longtime smokers.source: 2
Correlation
Correlation is when one thing happens around the same time as or following another, but one did not necessarily cause the other to happen. For example, a toddler may fall down after being vaccinated and the parent worries the vaccine affected the child’s balance. But the more likely reason for the tumble is that at the same time toddlers are being vaccinated they are also learning to walk, and therefore fall down frequently.
Examples of correlation vs. causation
Check out the graphic to the right, which explains correlation and causation using the sun as an example.
Here’s another example: A dog barks before a bolt of lightning in the sky, then a clap of thunder can be heard. The dog did not cause the thunder, it just happened to bark around the same time. However, the bolt of lightning did cause the noise of thunder.
Correlation ≠ causality
Just because one thing happens around the same time as something else doesn’t mean they are related.
Correlation
The sun did not cause the skinned knee. They just fell while playing outside.
Causality
The sun caused the ice cream to melt.
The same with vaccines and autism. Autism can show up around the same time a child gets their vaccines (correlation). But vaccines don’t cause autism.
Correlation, causation, and vaccine safety
People may link negative health issues to vaccinations because they happen around the time vaccines were given.source: 3 For instance, some people have concerns that vaccines can cause autism. Rigorous studies have proven that vaccines have nothing to do with autism.source: 4 Autism is often diagnosed in early childhood when children get their first routine vaccines.source: 5 But if vaccines truly caused autism, there would be many more people with autism. More than 9 out of every 10 children in the United States get their vaccines on time for kindergarten.source: 6 Not even 1 out of every 10 children have an autism diagnosis.source: 7
Scientists continually monitor safety data to determine whether a health issue correlated with a vaccine is actually caused by the vaccine. One of the systems they use is the Vaccine Adverse Event Reporting System (VAERS). Anyone can submit a report to VAERS. For example, let’s say a bee stings you on the arm after you got your vaccine and later in the day your arm becomes sore and swollen. Since those are side effects of both a bee sting and a vaccine, you can’t be sure what is causing the soreness and swelling. You should report it to VAERS anyway.
VAERS reports correlations—things that happen around the same time as a vaccine being given. But VAERS does not determine causation.source: 5 Once a report is submitted, researchers then evaluate it to determine if the vaccine caused something.source: 9
Share the facts
Spreading the truth saves lives, so use our resources to help parents understand how vaccines work, why they’re safe, and how they help protect kids.
Understanding risk
The benefits of getting vaccinated outweigh the risks of getting a vaccine-preventable disease.
Five reasons to vaccinate your child
Keeping your child up to date on their vaccines is the most important thing you can do to protect their health.
Doing your own research
Independent learning helps us learn more about topics we don’t always understand.
Sources
- CDC: Measles Symptoms and Complications
- CDC: Lung Cancer Risk Factors
- CDC: How Vaccine Safety Monitoring Works
- CDC: Autism and Vaccines
- CDC: Screening for Autism Spectrum Disorder
- CDC: Vaccination Coverage by Age 24 Months Among Children Born During 2018–2019 — National Immunization Survey–Child, United States, 2019–2021
- CDC: Autism Prevalence Higher, According to Data from 11 ADDM Communities
- HHS: VAERS FAQ
- FDA: Vaccines Protect Children From Harmful Infectious Diseases
Our Affiliated Sites
Disclaimer Policy: Links with this icon () mean that you are leaving the HHS website.
Disclaimer Policy: Links with this icon () mean that you are leaving the HHS website.
- The Department of Health and Human Services (HHS) cannot guarantee the accuracy of a non-federal website.
- Linking to a non-federal website does not mean that HHS or its employees endorse the sponsors, information, or products presented on the website. HHS links outside of itself to provide you with further information.
- You will be bound by the destination website's privacy policy and/or terms of service when you follow the link.
- HHS is not responsible for Section 508 compliance (accessibility) on private websites.
- For more information on HHS's web notification policies, see Website Disclaimers.