Since the rollout of COVID-19 vaccines, billions of people worldwide have been protected against severe illness and death. But as vaccination campaigns matured, researchers also began to ask: what are the long-term health outcomes of vaccination?

A new population-based study from Seoul, South Korea, published in Biomarker Research (2025), examined whether COVID-19 vaccination was linked to cancer diagnoses within one year after vaccination. Using data from over 8.4 million people, this is one of the largest studies of its kind.

Study Highlights

• Data Source: Korean National Health Insurance database (2021–2023)

• Sample Size: 8.4 million individuals, with matched vaccinated and unvaccinated groups

• Follow-up: 1 year after vaccination

Researchers looked at overall cancer risk, specific cancer types, and how risks varied by vaccine type, sex, and age.

Key Findings

The study found increased risks for six cancer types within a year of vaccination (HR – Hazard Ratios):

• Thyroid cancer (HR 1.35)

• Gastric cancer (HR 1.34)

• Colorectal cancer (HR 1.28)

• Lung cancer (HR 1.53)

• Breast cancer (HR 1.20)

• Prostate cancer (HR 1.69)

By Vaccine Type

• mRNA vaccines (Pfizer, Moderna): Higher risk of thyroid, colorectal, lung, and breast cancers

• “cDNA” vaccines (DNA-based platforms): Higher risk of thyroid, gastric, colorectal, lung, and prostate cancers

• Heterologous vaccination (mix-and-match): Higher risk of thyroid and breast cancers

By Sex and Age

• Men: more vulnerable to gastric and lung cancers

• Women: more vulnerable to thyroid and colorectal cancers

• Under 65: higher risk of thyroid and breast cancers

• Over 75: higher risk of prostate cancer

Booster Doses

Booster shots were especially linked to increased risks of gastric and pancreatic cancers.

What This Means (and Doesn’t Mean)

• These are associations, not proof of causation. The vaccines are not shown to directly cause cancer.

• Possible explanations include vaccine-induced inflammation, immune responses, or detection bias such as more health checkups after vaccination.

• The authors strongly caution that further research is needed to confirm these findings and explore mechanisms.

COVID-19 vaccines have saved millions of lives. But as the virus becomes less severe, the balance of benefits versus risks may shift for certain populations. This study suggests doctors should be alert, especially regarding gastric cancer risks after booster doses, and that vaccination strategies might need to be tailored by age and health status.

Reference

Kim HJ, Kim MH, Choi MG, Chun EM. 1-Year Risks of Cancers Associated With COVID-19 Vaccination: A Large Population-Based Cohort Study in South Korea. Biomark Res. 2025;13:114.