- Health
South Korea study does not show COVID-19 vaccines weaken immune system
Key takeaway
Associations can be the product of multiple factors and don’t always represent cause-and-effect. Though imperfect, COVID-19 vaccines are effective at reducing the risk of severe disease and death from COVID-19. Claims that the vaccines weaken the immune system aren’t substantiated by credible evidence.
Reviewed content
Verdict:
Claim:
South Korea study shows COVID-19 vaccines “eroded immune function across an entire country—and likely the entire globe”
Verdict detail
Inadequate support:
The study found COVID-19 vaccination was associated with a higher risk of influenza-like illness and pertussis. But it didn’t look at immunologic or clinical biomarkers related to immunity, therefore claims it shows change in immune function are unfounded.
Misrepresents a complex reality:
The study didn’t rule out certain confounders such as healthcare-seeking behavior, which tend to differ between vaccinated and unvaccinated people, and could affect how often cases of respiratory illnesses are recorded in both groups.
Full Claim
“BREAKING: 51 Million-Person Study Finds COVID-19 ‘Vaccines’ Increase Risk of Respiratory Infections by up to 559%”; A South Korea study “has delivered a striking population-level signal suggestive of vaccine-acquired immunodeficiency syndrome (VAIDS)”; “These findings point to immune exhaustion, IgG4 dominance, and secondary immunodeficiency following repeated mRNA exposure — consistent with vaccine-acquired immune dysregulation (VAIDS)”
Review
In November 2025, social media posts claimed that a study from South Korea showed COVID-19 vaccination weakened the immune system and increased the risk of respiratory infections[1].
One such post on X by Nicolas Hulscher received more than 430,000 views. The post claimed “51 Million-Person Study Finds COVID-19 ‘Vaccines’ Increase Risk of Respiratory Infections by up to 559%”, a claim also propagated through his articles posted on the websites Vigilant Fox and The Focal Points.
Hulscher is the administrator of the McCullough Foundation, a non-profit founded by cardiologist Peter McCullough. McCullough has repeatedly claimed that COVID-19 vaccines are harmful. Incidentally, he is also chief scientific officer for The Wellness Company, which sells “spike detox” supplements.
Hulscher himself has spread misinformation about COVID-19 vaccines on several occasions, including co-authoring an analysis claiming that COVID-19 vaccines led to many deaths. The analysis turned out to contain numerous methodological flaws.
The claim builds on the existing narrative that COVID-19 vaccines cause immunodeficiency, which proponents call VAIDS. Science Feedback covered iterations of this claim in previous reviews and found them to be unsubstantiated. Scientific evidence shows that COVID-19 vaccination actually strengthens the immune system and reduces the risk of death from COVID-19. The South Korea study that forms the basis of Hulscher’s claims doesn’t show otherwise. We explain below.
How was the study conducted and what did it find?
For context, most of South Korea’s population received at least one dose of COVID-19 vaccine. According to data from the World Health Organization, roughly 87% of the population completed the primary vaccination series as of 31 December 2023 (see Figure 1 below). This means that only 13% of people remained unvaccinated or didn’t complete the primary series. On top of that, 66% received a booster dose.
This data suggests that people who didn’t get vaccinated or complete the primary vaccination series make up a small minority. More importantly, this group is likely to behave quite differently from the rest of the population[2].
The authors of the study wanted to assess trends in respiratory illnesses during and after the COVID-19 pandemic, as well as determine whether COVID-19 vaccination status was associated with the risk of various respiratory illnesses.
To do this, the researchers drew on a national database that integrated both health insurance claims as well as COVID-19 vaccination records for the entire Korean population. The health insurance claims enabled researchers to identify medical diagnoses related to seven non-COVID-19 respiratory infections that were established in a hospital setting, namely upper respiratory tract infection, pneumonia, influenza-like illness, common cold, scarlet fever, pertussis, and tuberculosis.
In the study, the authors stated that “Nationwide polymerase chain reaction testing for SARS-CoV-2 was conducted across hospitals and medical centers […] between January 2020 and May 2023, establishing comprehensive epidemiological data, including daily SARS-CoV-2-infected cases”. What we can take away from this is that in general, COVID-19 cases could be reliably distinguished from non-COVID respiratory infections for most, though not all, of the study period.
The authors compared the incidence of these conditions during and after the pandemic with the predicted incidence based on pre-pandemic data from 2017 to 2019. They also stratified results based on age, number of vaccine doses received, and history of SARS-CoV-2 infection. Note that the authors grouped unvaccinated people with those who received only one vaccine dose.
Several trends emerged from this analysis. Firstly, influenza-like illnesses decreased during the pandemic (between 2020 and 2022), likely due to pandemic measures such as physical distancing, lockdowns, and widespread use of face masks. But in 2023, the incidence of upper respiratory tract infections and the common cold resurged. COVID-19 vaccination status was associated with a lower risk of influenza-like illness and pertussis, but a higher risk of upper respiratory tract infections and the common cold.
Results don’t support claim of weakened immunity in vaccinated people
To better understand the implications of the study, Science Feedback reached out to Helen Petousis-Harris, an associate professor at the University of Auckland and co-director of the Global Vaccine Data Network, a research network looking at vaccine safety and effectiveness.
In an email, she explained that the study only identified “associations, not causes” and that the study didn’t examine immunity. These caveats were also mentioned by the authors of the study themselves (“as this was an observational study, causal inferences between COVID-19 vaccination and the risk of subsequent respiratory infections cannot be definitively established”; “our findings should be interpreted as associative in the absence of immunologic and clinical biomarkers”).
The study design also comes with several important limitations, one of which is residual confounding. Confounding factors are variables that influence the results in a study but aren’t the variables being studied. Failing to account for confounding factors can lead to incorrect conclusions.
For example, the group that received four or more doses was considerably older compared to groups that received fewer or no doses, Petousis-Harris noted. Indeed, the study’s supplementary data showed that the average age of the group which received four or more doses was 67 years old. Contrast this with 38 and 47 years old for the groups that received two and three doses, respectively.
While the study authors made adjustments for some variables including age, Petousis-Harris said that “remaining confounding is likely—especially healthcare-seeking intensity, testing, and access,” which would influence the number of insurance claims related to upper respiratory tract infection. Other factors that could influence claims are coding practices and post-pandemic care patterns.
Another limitation of the study comes from the potential for exposure misclassification and change in risk over time, she said. Specifically, the study considered only the total number of doses received until 1 June 2023, which would miss out on any doses given after that date, as well as waning of COVID-19 immunity.
We reached out to the authors of the study for comment and will update this article if new information becomes available.
Claims of “immune exhaustion” and VAIDS aren’t substantiated by the study
Hulscher claimed that the findings show “immune exhaustion, IgG4 dominance, and secondary immunodeficiency following repeated mRNA exposure — consistent with vaccine-acquired immune dysregulation (VAIDS)”.
However, the study contains no data to back up these claims. This is also clarified by the study’s authors who stated they did not test for immunologic or clinical biomarkers, therefore further extrapolation about the potential mechanisms underpinning the associations seen—based solely on these results—wouldn’t be possible.
Furthermore, the study’s findings are inconsistent with regards to the risk of respiratory illness. As described earlier, COVID-19 vaccination was associated with a higher risk of upper respiratory tract infections and the common cold, but a lower risk of influenza-like illness and pertussis. If COVID-19 vaccination resulted in immunodeficiency as Hulscher claimed, we should expect to see the risk for all respiratory illnesses increase, not just one or two.
Hulscher argued that the lower risks for influenza-like illness and pertussis were simply “statistical artifacts” due to “many mild respiratory illnesses” being misclassified as COVID-19 in the post-vaccine era, “artificially deflating their apparent incidence in vaccinated groups”. He didn’t provide evidence to support the assertion about misclassification. Moreover, if this was true, the misclassification should also have affected claims for upper respiratory tract infections and the common cold, since both are also typically mild respiratory infections that can be confused with COVID-19.
The terms “immune exhaustion” and “IgG4” have been at the center of other unsubstantiated claims about the dangers of COVID-19 vaccines.
However, there’s currently no evidence indicating that COVID-19 vaccination induces “immune exhaustion”, a phenomenon mainly seen in chronic viral infections like HIV and cancer, but not vaccination. And while some published studies reported a rise in a type of antibody called IgG4 after COVID-19 booster doses[3,4], IgG4 isn’t intrinsically harmful; in fact at least one study found that this increase in IgG4 could potentially be beneficial in antiviral responses against SARS-CoV-2[5]. At the moment, the clinical implication of the IgG4 increase remains to be established[6].
We reached out to Hulscher asking for information to support his claims. He did not respond to the request for comment.
Conclusion
A South Korea study found that COVID-19 vaccination was associated with a lower risk of influenza-like illness and pertussis but a higher risk of upper respiratory tract infections and common cold. However, the study didn’t examine the potential mechanisms behind the associations. For instance, it didn’t examine biomarkers for immunity. Therefore, it provides no data to support claims of immunodeficiency in vaccinated people. Claims that COVID-19 vaccination weakens the immune system are unfounded.
Reviewers’ feedback
Helen Petousis-Harris
Associate Professor, University of Auckland
What the study shows:
- Design: national, retrospective observational cohort linking Korea’s insurance claims and COVID-19 vaccination records; exposure fixed as “number of COVID-19 doses received by 1 June 2023”; follow-up to 30 September 2024. Outcomes were claims-based ICD-10 groupings (e.g., upper respiratory tract infection (URTI) J00–J06; “common cold” J00).
- Main findings: Compared with 2017–2019 expectations, post-pandemic (2023–2024) URTI/common cold diagnoses were higher; influenza-like illness (ILI) and pneumonia plunged during 2020–2021 then rebounded. In adjusted models, ≥4-dose recipients had lower risk of ILI and pertussis, but higher hazard ratios for URTI and common cold vs the least-vaccinated group. There is no analysis of immunologic endpoints.
- The study’s analysis identifies associations, not causes. It’s far more plausible that the apparent rise in common colds among heavily vaccinated groups reflects differences in age, healthcare use, and post-pandemic viral rebound than any biological effect of the vaccine. Observing both higher and lower risks across infections is typical of residual confounding, not immune dysfunction.
Key limitations that severely limit causal interpretation:
- Residual and structural confounding: Dose groups differ significantly in age/comorbidity/healthcare behaviour (≥4-dose group is significantly older). Although models adjust for covariates, remaining confounding is likely—especially healthcare-seeking intensity, testing, and access, which drive claims for mild URTIs.
- Potential collider/over-adjustment bias: Adjusting for COVID-19 severity, prior infection phase, and time since last dose—variables on the causal pathway and influenced by vaccination—can bias coefficients upward for URTI/colds. Note that crude hazard ratios for URTI/common cold are ≤1 but become >1 after adjustment—an instability that flags model concerns rather than biology. This all-over-the-map flip-flop suggests a problem.
- Exposure misclassification & time-varying risk: “Total doses by 1 June 2023” then fixed through follow-up ignores later boosters and waning; no vaccine type/interval analysis.
- Outcome measurement bias: URTI/common cold claims are highly sensitive to age, care-seeking, coding practices, and post-pandemic care patterns. The paper provided no virologic confirmation or severity stratification.
- Multiple testing & ecological backdrop: Post-pandemic rebound (“immunity debt”), pathogen interference, and policy changes complicate attribution to vaccination status. The paper itself cites these as alternative explanations.
This is descriptive epidemiology for trends, but the dose-response associations with URTI and common colds are hypothesis-generating only. The design does not establish that vaccination increases susceptibility to infections. The authors report both increased and decreased risks for different respiratory outcomes (e.g., lower ILI and pertussis, higher URTI/common cold) across vaccine dose strata. In an observational dataset of 39 million people, this kind of bidirectional pattern is a red flag that non-causal factors are driving the signals—not the vaccine itself.
Is there credible evidence that COVID-19 vaccination weakens the immune system?
No credible clinical evidence. Large bodies of data show that COVID-19 vaccination reduces severe COVID-19 and doesn’t cause generalised immunodeficiency. Post-pandemic surges of non-SARS-CoV-2 respiratory infections are well-documented internationally and are primarily attributed to behavioural/policy shifts and population “immunity debt”, rather than vaccine-mediated immune damage. Independent literature on post-pandemic respiratory rebounds supports this[7,8,9].
References:
- Song et al. (2025) Incidence of Respiratory Infections after the COVID-19 Pandemic (2023–2024) and Its Association of Vaccination Among Entire Populations in Korea. International Journal of Infectious Diseases.
- Thomson et al. (2016) The 5As: A practical taxonomy for the determinants of vaccine uptake. Vaccine.
- Irrgang et al. (2022) Class switch toward noninflammatory, spike-specific IgG4 antibodies after repeated SARS-CoV-2 mRNA vaccination. Science Immunology.
- Buhre et al. (2023) mRNA vaccines against SARS-CoV-2 induce comparably low long-term IgG Fc galactosylation and sialylation levels but increasing long-term IgG4 responses compared to an adenovirus-based vaccine. Frontiers in Immunology.
- Aurelia et al. (2025) Increased SARS-CoV-2 IgG4 has variable consequences dependent upon Fc function, Fc receptor polymorphism, and viral variant. Science Advances.
- Kadkhoda K. (2023) Post-COVID mRNA-vaccine IgG4 shift: worrisome? mSphere.
- Lenglart et al. (2025) Surge of Pediatric Respiratory Tract Infections after the COVID-19 Pandemic and the Concept of “Immune Debt”. Journal of Pediatrics.
- Gosert et al. (2025) Rebound of Respiratory Virus Activity and Seasonality to Pre‐Pandemic Patterns. Journal of Medical Virology.
- Hong et al. (2025) Post-pandemic resurgence of respiratory virus infections: age and department-specific patterns in a Chinese tertiary hospital (2020–2024). BMC Infectious Diseases.
