Did a South Korean study that COVID-19 vaccines cause cancer?

News about health and medicine travels fast. Sometimes, this speed can lead to mistakes or fear. A major talking point has been a claim that a South Korean study proved that COVID-19 vaccines cause cancer. This claim is very serious and has spread widely.

But what did the study really say? Did the researchers actually find that the COVID-19 vaccine directly causes cancer? It is important to look closely at the science to understand the truth. The short answer is: No, the study did not claim that the vaccines cause cancer. People sharing misinformation often twist the true findings.

What the South Korean Study Actually Found

The journal Biomarker Research published the large-scale population-based cohort study. The scientists looked at health data from millions of people in South Korea. They aimed to see if getting the COVID-19 vaccine associated statistically with a cancer diagnosis within one year.

The study did find that vaccinated people were more likely to be diagnosed with six specific types of cancer within a year compared to unvaccinated people. These types included thyroid, gastric, colorectal, lung, breast, and prostate cancers.

The Big Difference: Association vs. Causation

This is the most important part to understand. Finding an association is not the same as proving causation.

Association means two things happen together. For example, people who wear rain boots might be more likely to carry umbrellas. The boots do not cause the umbrella, but the two things are connected by a third factor: it is raining.
Causation means one thing directly makes the other happen. For example, smoking causes lung cancer.

The South Korean researchers were clear about this. They stated in their own report that their “findings do not establish causal relationships.” They only found a link, not a cause.

When news outlets and social media users claimed the study was “proof” that the vaccines cause cancer, they were wrong. They ignored what the scientists themselves said.

Why the Link is Likely Misleading

Scientists and fact-checkers quickly pointed out that the link found in the study is most likely due to two things that have nothing to do with the vaccine itself causing cancer:

1. Surveillance Bias

This is the most likely reason for the link. People who get vaccinated are often more likely to be health-conscious. They are more likely to visit doctors, get routine check-ups, and have cancer screenings. When a person sees a doctor more often, they are more likely to have a cancer diagnosed early. This is called surveillance bias.

Increased screening likely led to doctors diagnosing the vaccinated group with cancer earlier, not the vaccine causing the cancer to grow.

2. Reverse Causation

Another factor is reverse causation. The people who were prioritized for vaccines early on were often older or had other health problems. These same groups are also at a naturally higher risk of getting cancer.

It is possible that people who already had a higher cancer risk were more likely to get vaccinated. In this case, the underlying health risk (not the vaccine) is what led to the cancer diagnosis.

3. The Time Problem

Solid cancers (like those mentioned in the study) take a very long time to develop—often many years. The study only looked at cancer diagnoses within one year after vaccination. Medical experts explain that a vaccine cannot medically cause a solid tumor to develop, grow, and receive a diagnosis in such a short time.

The researchers themselves admitted that their one-year follow-up period was “relatively short for evaluating cancer incidence.”

The Bottom Line on Vaccine Safety

The overall scientific consensus and large-scale, real-world data are clear. There is no good evidence that COVID-19 vaccines cause cancer.

Major health organizations, including Cancer Research UK and global health agencies, have stated that the vaccines are safe and effective. They do not increase the risk of cancer. In fact, for people undergoing cancer treatment, the vaccines are an important tool to protect them from severe COVID-19 illness.

The claim that the South Korean study proved a link between the vaccine and cancer is a case of misrepresenting scientific findings.

The headlines suggesting a South Korean study proved that COVID-19 vaccines cause cancer are false and misleading. The study only showed a statistical link, or association, between vaccination and diagnosis of certain cancers within one year. The researchers did not claim to find a causal link. This association is most likely due to vaccinated people getting more health screening and being more likely to find a pre-existing cancer (surveillance bias).

The medical evidence from around the world consistently shows that COVID-19 vaccines are safe and do not cause cancer.

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