Covid-19 vaccination did not affect cancer rates in Korea, study finds

Mass Covid-19 vaccination did not lead to more cancer cases in Korea, according to a study of more than 36 million people.

The result runs counter to an earlier Korean study that linked the shots to higher risks of six cancers.

A team led by Jung Jae-hun, a professor of preventive medicine at Korea University College of Medicine, published the findings Tuesday in the journal Biomarker Research. The same journal carried the earlier study in September last year.

The team linked National Health Insurance Service data with Covid-19 vaccination records to track cancer incidence among 36,024,394 people born between 1955 and 2004. The observation period ran from February 2011 to September 2025, and 89.3 percent of the people studied received at least one dose.

The researchers set August 2021, when Korea’s first-dose vaccination rate passed 50 percent, as the turning point. They used cancer trends from roughly 10 years before that time to project incidence for the following period, then compared the projections with actual rates. The aim was to see whether cancer cases jumped or began rising faster after mass vaccination.

Over the 50 months from August 2021 to September 2025, none of the 13 cancer categories showed a statistically significant change in either level or trend. Separate analyses of stomach, colorectal, lung and prostate cancer produced the same result. Cancer rates simply kept to the trends seen before vaccination.

The new study responds to research published in September last year by a team including Chun Eun-mi, a professor of pulmonology at Ewha Womans University Mokdong Hospital. Chun’s team analyzed health insurance data on about 8.4 million people in Seoul and reported that vaccinated people were more likely than unvaccinated people to be diagnosed with stomach, colorectal, lung, breast, thyroid or prostate cancer within a year of vaccination.

Researchers disputed whether a gap in cancer rates between the two groups could show that the vaccine caused cancer. Jung pointed out that the earlier study applied different selection criteria to the vaccinated and unvaccinated groups. Its supplementary material states that 499,572 people with cancer history in the year before observation began were excluded from the vaccinated group, but it lists no such exclusion for the unvaccinated group. Jung also questioned whether the study adequately accounted for differences between the two groups.

“In Korea, only 10 percent were unvaccinated, and vaccinated people reached 90 percent,” Jung said. “The unvaccinated should be seen as having characteristics different from the general population, but [the study] did not adjust for that.”

Vaccinated and unvaccinated people can differ not only in health but also in how often they get checkups or visit hospitals. Those habits affect how likely a person is to be diagnosed with cancer. Without accounting for them, differences that have nothing to do with the vaccine can be read as its effects, Jung argued.

These problems combined, Jung said, explain why the earlier study’s own graphs show the two groups’ cancer rates diverging right after vaccination and then converging over time.

Rather than comparing vaccinated and unvaccinated people directly, the new study examined whether cancer trends across the entire study population shifted around the point when vaccination rates climbed quickly.

“To verify the earlier study, we looked at whether cancer incidence in Korea changed after mass vaccination, and the actual analysis found no clear change,” Jung said.

BY RHEE ESTHER [[email protected]]