More or Less – “No, a study has not shown that the covid jab causes cancer”

BBC Radio 4 | Host: Tim Harford | Date: January 17, 2026

Episode Overview

This episode of More or Less tackles a widely circulated claim that a recent study has shown a dramatic increase in cancer risk after COVID-19 vaccination. Listener Lyn asks host Tim Harford to scrutinize a viral statement made by content creator Kayla Betts, who referenced a peer-reviewed study from South Korea. With the help of Professor Justin Fendos, an expert in cancer biophysics and health informatics, Tim dissects the statistical methods and interpretations behind the alarming claim and explains why the conclusion is unfounded.

Key Discussion Points & Insights

1. The Origin of the Claim (01:18–02:20)

• Kayla Betts’ Statement:
  Betts claimed a peer-reviewed study of 8.4 million people (published September 2025 in Biomarker Research) found “a 27% overall increase in cancer risk in the vaccinated group in just the first year.”
• Tim Harford’s Response:
  The referenced publication was a “correspondence,” not a full research report. Biomarker Research does use peer review for regular papers, but “correspondence” may not undergo the same scrutiny.

2. Scrutinizing the Study’s Methods (03:14–05:29)

• Guest Expert:
  Professor Justin Fendos (Xian Jiaotong-Liverpool University, China) specializes in cancer, biophysics, and health informatics.
• Core Variables Considered:
  • Demographics: Age, gender, income.
  • Other Variables: Prior disease history (Charlson Comorbidity Index), COVID infection status, vaccination status, and cancer incidence (the outcome).
  • Key Omissions: Genetics, smoking status, previous cancer diagnoses, and health-seeking behaviors were not included.
• Health-Seeking Behavior Bias:
  Fendos points out that people proactive about their health tend to both get vaccinated and participate in cancer screenings, increasing diagnosis rates incidentally.

“People who really care about their health and engage in one or more health activities are also more likely to engage in other types of health activities.”
— Prof. Justin Fendos (04:53)

3. The Flawed Study Time Frame (05:29–06:08)

• Short Analysis Window:
  • Vaccinated: Cancer diagnoses checked within a year of vaccination.
  • Unvaccinated: Diagnoses checked within a year from Jan 1, 2022.
• Problem:
  Cancers typically take years, even decades, to develop.

“Any new diagnosis of cancer within a year of receiving the jab is very unlikely to have been caused by the vaccine, at least so far as we know.”
— Tim Harford (06:08)

4. Population Statistics & National Trends (06:34–06:44)

• Extremely High Vaccination Rates:
  South Korea’s vaccination uptake exceeded 96%.

• Real-World Data Mismatch:
  If the claim were valid, South Korea should be seeing a nationwide cancer spike, which isn’t happening.

5. Statistical Pitfalls: Hazard Ratios & Multiple Outcomes (07:11–08:34)

• Hazard Ratio Explained:
  Used to compare event rates (e.g., cancer) between groups.
• Multiple Outcomes:
  Study examined 29 cancer types. Checking many outcomes increases chances of finding significance just by chance (multiple comparisons problem).
• Critical Missing Correction:
  The study didn’t adjust (“statistical correction”) for checking so many outcomes. Such a correction substantially lowers the number of statistically significant findings.

“The more times you go fishing, the more likely you are just by random chance to catch something. And so you have to make an adjustment, a correction for this possibility. It looks like they did not do this kind of a correction.”
— Prof. Justin Fendos (08:38)

6. Findings Disappear With Proper Analysis (09:10–09:35)

• Correcting the Stats:
  Once the appropriate corrections are applied, the purported link between vaccination and increased cancer risk vanishes.

“If you do correct for the fact that so many outcomes are being studied, the relationship between the vaccines and cancer disappears.”
— Tim Harford (09:10)

• Conclusion:
  The analysis does not show the COVID vaccine increases cancer risk. The data was misinterpreted and possibly miscommunicated by those lacking statistical expertise.

Notable Quotes & Memorable Moments

• [02:32] “I do not believe that this paper decisively, definitively or even very convincingly shows that COVID vaccine uptake causes or is strongly related to cancer incidence.” — Prof. Justin Fendos
• [06:08] “Any new diagnosis of cancer within a year of receiving the jab is very unlikely to have been caused by the vaccine, at least so far as we know.” — Tim Harford
• [08:38] “The more times you go fishing, the more likely you are just by random chance to catch something. And so you have to make an adjustment, a correction for this possibility.” — Prof. Justin Fendos
• [09:35] “So, no, we do not have evidence that the vaccine causes cancer risks to rise.” — Tim Harford

Timestamps for Important Segments

• 01:18 — Host introduces the viral vaccine-cancer claim
• 01:51 — Kayla Betts presents the original claim
• 02:20–03:53 — Tim clarifies the study type, introduces guest expert
• 03:53–05:29 — Justin Fendos explains missing variables & health-seeking bias
• 05:37–06:08 — Prof. Fendos details flaws in the study’s time frame
• 06:34–06:44 — Vaccination rates in South Korea
• 07:11–08:34 — Explanation of hazard ratios and statistical significance
• 08:38–09:19 — Importance of statistical correction (multiple comparisons)
• 09:35 — Summary and conclusive dismissal of the claim

Closing

Tim Harford and guest Justin Fendos expertly debunk the claim that COVID-19 vaccines cause increased cancer risk, showing how important statistical rigor and domain knowledge are when interpreting medical studies. Careful statistical correction and context make clear there is no evidence supporting this alarming claim.

Listeners with further statistical questions are encouraged to write in to the show.