No large-scale study has established that COVID-19 vaccines cause lymphoma. The concern circulates widely online, fueled by a real and visible side effect: swollen lymph nodes after vaccination that can look alarming on imaging scans and, in rare cases, persist for months. Dozens of case reports describe lymphoma diagnosed in the weeks or months following a COVID shot, but whether the vaccine played any role or these are coincidences in a population of billions of vaccinated people remains genuinely unresolved. The evidence leans heavily toward coincidence, though a few biological threads keep the question from being fully closed.
Why Lymph Nodes Swell After Vaccination
Swollen lymph nodes after a COVID vaccine are one of the most commonly reported side effects, and they are a sign the immune system is doing exactly what it should. The mRNA vaccines in particular drive a strong and sustained response in the lymph nodes closest to the injection site. A study examining draining axillary lymph nodes found that germinal center B cells targeting the spike protein were present in all participants sampled after their primary shots, and high levels of these cells persisted for at least 12 weeks after the booster dose.1Nature. SARS-CoV-2 mRNA vaccines induce persistent human germinal centre responses That prolonged germinal center activity is what makes the vaccines effective at generating durable immunity, but it also means lymph nodes can stay enlarged for a surprisingly long time.
In imaging studies, the swelling was not always brief. One radiology study documented cases of persistent lymph node enlargement up to 43 weeks after vaccination.2PubMed Central. Axillary Adenopathy after COVID-19 Vaccine: No Reason to Delay Screening Mammogram After booster doses, resolution came faster on average, around 102 days, compared to roughly 129 days after a first dose.3PubMed. Axillary Lymphadenopathy After a COVID-19 Vaccine Booster Dose: Time to Resolution on Ultrasound Follow-Up and Associated Factors Neither age, vaccine brand, nor a history of breast cancer significantly changed how quickly the nodes returned to normal. For most people, the swelling goes unnoticed. But for anyone getting a mammogram, a PET scan, or a physical exam during that window, the enlarged nodes can trigger a cascade of worry and follow-up testing.
How Vaccine-Related Swelling Mimics Cancer on Scans
This is where the lymphoma question gets its legs. PET/CT scans, which are standard for monitoring cancer patients, detect metabolic activity. Swollen, actively working lymph nodes light up on these scans, and a post-vaccine node can look virtually identical to a cancerous one. Among 104 vaccinated patients who underwent PET/CT, 85% showed tracer uptake in axillary lymph nodes or the deltoid region on the vaccination side, with scans performed a median of 11 days after injection.4PubMed Central. COVID-19 vaccine related hypermetabolic lymph nodes on PET/CT: Implications of inflammatory findings in cancer imaging These are false positives in the cancer context, but they are very convincing ones.
The mimicry is not limited to nodes near the injection site. One striking case involved a patient in remission from Hodgkin lymphoma whose PET/CT showed hypermetabolic lymph nodes in the abdomen and neck, in exactly the same locations as the original disease, raising fears of relapse. A follow-up scan two months later showed the nodes had shrunk and their metabolic activity had normalized, consistent with a vaccine reaction rather than cancer returning.5PubMed Central. Hypermetabolic abdominal and cervical lymph nodes mimicking Hodgkin lymphoma relapse on FDG PET/CT after adenovirus-vectored COVID-19 vaccine The fact that vaccine-associated lymph node activity can appear far from the arm has been flagged across multiple radiology publications as a source of diagnostic confusion.6PubMed Central. The systemic impact of different COVID-19 vaccines in 2-[18F] FDG-PET/CT
This false-positive problem is not unique to COVID vaccines, either. A systematic review of vaccine-associated lymph node pathology found that other vaccines, including H1N1 and tetanus shots, have historically produced florid immune reactions that raised concern for lymphoma under the microscope. In one case, an H1N1 vaccine produced lymph node features resembling Hodgkin lymphoma; in another, a tetanus shot triggered a proliferation of immune cells initially read as an atypical T-cell population suspicious for cancer. Both were ultimately diagnosed as benign vaccine reactions.7PubMed Central. Pathological Findings in COVID-19 and Non-COVID-19 Vaccine-Associated Lymphadenopathy: A Systematic Review The COVID vaccines produce stronger and longer-lasting germinal center responses than most other vaccines, which makes the mimicry more frequent and more persistent, but the basic phenomenon has been around for decades.
Case Reports of Lymphoma After Vaccination
Case reports are the main fuel for the claim that COVID vaccines might cause lymphoma. These are detailed accounts of individual patients who developed lymphoma in a timeframe suggestive of a connection to their vaccination. They are real medical events, documented by treating physicians, and some of them are genuinely difficult to dismiss as pure coincidence.
One involved a 60-year-old woman who developed axillary lymph node swelling right after her first Pfizer dose. After her second dose, nodes in her neck and groin enlarged. A biopsy eight months later showed angioimmunoblastic T-cell lymphoma (AITL), a rare cancer. Molecular testing revealed pre-existing mutations in genes associated with this lymphoma subtype, suggesting the patient had a genetic predisposition. The authors proposed that the vaccine’s stimulation of certain immune cells could have tipped an already mutation-carrying cell population toward malignancy.8PubMed Central. Diagnosis of Angioimmunoblastic T Cell Lymphoma After Receiving First Dose of Pfizer/BioNTech (BNT162b2) Vaccine: A Case Report
Another case described an 80-year-old woman who noticed a mass near her temple the morning after her first mRNA vaccine. The mass shrank but persisted, and imaging revealed multiple enlarged lymph nodes. Initially assumed to be a vaccine side effect, the mass was eventually biopsied nine weeks later and diagnosed as marginal zone B-cell lymphoma. The authors emphasized that while observation of post-vaccine swelling is reasonable, persistent or growing nodes should prompt biopsy to rule out malignancy.9PubMed Central. Rapid progression of marginal zone B-cell lymphoma after COVID-19 vaccination (BNT162b2): A case report
A third report documented a 75-year-old woman who developed widespread skin plaques after her first mRNA dose, with rapid progression to tumors and lymph node involvement after the second dose. The diagnosis was mycosis fungoides, a type of cutaneous lymphoma. The authors suggested that shared signaling pathways enhanced by the mRNA vaccine could drive the development of this specific lymphoma subtype.10PubMed Central. Rapid Progression of Cutaneous Lymphoma Following mRNA COVID-19 Vaccination: A Case Report and Pathogenetic Insights
A systematic review compiled 143 newly diagnosed lymphoma cases following COVID vaccination, drawn from nine published articles. Roughly half involved Pfizer and about 43% Moderna. Non-Hodgkin lymphoma made up about 71% of cases, with Hodgkin lymphoma accounting for the remainder.11Authorea. A systematic review of lymphoma secondary to COVID-19 vaccination Those numbers sound alarming until you consider that billions of COVID vaccine doses have been administered worldwide, and lymphoma is diagnosed in hundreds of thousands of people every year regardless of vaccination. The 143 compiled cases represent a vanishingly small fraction of vaccinated individuals.
Why Case Reports Cannot Prove Causation
Case reports describe what happened to a specific patient. They cannot tell you why it happened. A person diagnosed with lymphoma two months after vaccination might have been diagnosed at the same time regardless of the vaccine. Lymphoma typically develops over months to years before symptoms appear, meaning many of these cancers were already growing when the patient received their shot. One case report review stated this directly: it is most likely that lymphoma in these cases is a chance phenomenon, and there is no evidence to support the idea that COVID vaccination increases the risk of lymphoma.12PubMed Central. Non-Hodgkin Lymphoma Developed Shortly after mRNA COVID-19 Vaccination: Report of a Case and Review of the Literature
There is also a detection bias at play. The vaccine causes lymph node swelling, which prompts patients and doctors to investigate with imaging and biopsies. That investigation can uncover a lymphoma that was already present but had not yet been noticed. In other words, the vaccine may reveal lymphoma rather than cause it. The 80-year-old patient with marginal zone lymphoma is a plausible example: her cancer was detected because the vaccine drew attention to nodes that turned out to be malignant.
The strongest theoretical mechanism for actual causation involves people who carry pre-existing genetic mutations that make certain immune cells prone to malignant transformation. The AITL case described above is the clearest illustration. The patient had mutations in TET-2, IDH-2, and RHOA genes before vaccination; the intense immune stimulation from the vaccine may have pushed those already-abnormal cells across the line into cancer. But even here, the case report authors acknowledged that more studies are needed and that the association remains speculative.
What Safety Databases Show
The U.S. Vaccine Adverse Event Reporting System (VAERS) is a passive surveillance tool where anyone, patients, doctors, or the general public, can report health events that happen after vaccination. A recent disproportionality analysis of VAERS found that lymphoma appeared in reports at a higher rate after mRNA COVID vaccines compared to other routine vaccines: roughly 35.5 reports per 100,000 mRNA vaccine reports versus 17.1 per 100,000 for comparator vaccines. The reporting odds ratio was about 2.08.13Clinical Lymphoma Myeloma and Leukemia. Aggressive B-Cell Lymphoma Reporting After COVID-19 mRNAVaccination: A Pharmacovigilance Signal in VAERS
That kind of signal is worth investigating, but it does not mean the vaccine doubles your lymphoma risk. VAERS data has well-known limitations. Reports are not verified. Anyone can submit one, and high-profile vaccines attract more reporting. During the COVID vaccine rollout, public awareness of VAERS was far higher than for routine immunizations, creating a reporting bias. Doctors monitoring post-vaccine lymphadenopathy were also more likely to submit reports when they found something unexpected. A disproportionality signal in VAERS is a flag to investigate further with controlled epidemiological studies; it is not a risk estimate.
On the safety side, a controlled study using the self-controlled case series method (which compares each person to themselves before and after vaccination, reducing confounding) found no significant difference in the risk of adverse events in the 28 days after any COVID vaccine dose between people with blood cancers and people without.14PubMed. Effectiveness and safety of COVID-19 vaccination in people with blood cancer This design is stronger than passive reporting, and it found no elevated risk.
How Past Vaccines Relate to Lymphoma Risk
The idea that vaccines could cause lymphoma is not new to COVID. Researchers have studied the relationship between vaccination history and lymphoma risk for other vaccines, and the results are reassuring. A large population-based study found that polio and smallpox vaccination were associated with a lower risk of non-Hodgkin lymphoma, though results varied between studies and between men and women.15PubMed Central. Vaccination history and risk of non-Hodgkin lymphoma: a population-based, case-control study
A separate study examined influenza, hepatitis A, hepatitis B, and yellow fever vaccinations. Influenza vaccination was associated with reduced lymphoma risk, with the association growing stronger with more doses and longer time since vaccination. Yellow fever vaccination was also inversely associated with risk. The study’s conclusion was direct: vaccinations against these pathogens are unlikely to increase lymphoma risk.16Cancer Epidemiology Biomarkers and Prevention. Vaccination History and Risk of Lymphoma and Its Major Subtypes None of this directly addresses mRNA COVID vaccines, which work differently from traditional vaccines, but it establishes that immune stimulation from vaccination has not historically been a lymphoma risk factor. If anything, the pattern runs the other direction.
COVID Infection Itself and Lymphoma
One dimension of this question that often gets overlooked is whether COVID-19 infection, the thing the vaccine prevents, carries its own lymphoma risk. Repeated COVID infections have been proposed as a trigger for lymphoma in patients with rheumatic diseases, who already carry elevated baseline risk for blood cancers. The mechanism suggested is chronic immune stimulation and dysregulation from recurrent viral infection pushing susceptible immune cells toward malignancy.17Annals of Case Reports. Lymphoma as a Complication of Recurrent COVID-19 Infection in Patients with Rheumatic Disease This is early-stage evidence, also based on case reports rather than large studies, but it reframes the risk question. If avoiding the vaccine means increased risk of severe or repeated COVID infection, and if repeated infection itself may promote lymphoma in vulnerable people, the risk calculus of vaccination looks different than the case reports alone suggest.
Practical Guidance for Cancer Screening and Imaging
For anyone undergoing cancer screening or monitoring, the practical concern is not whether the vaccine caused your cancer. It is whether the vaccine is making your scan unreadable. The answer is that timing matters. Radiology experts recommend scheduling PET/CT scans at least two weeks after vaccination, and ideally four to six weeks out, to minimize the chance that vaccine-related lymph node activity will be mistaken for cancer.18PubMed. COVID-19 Vaccination-Related Uptake on FDG PET/CT: An Emerging Dilemma and Suggestions for Management When imaging cannot be delayed, clinicians should note vaccination timing and injection side so that radiologists can factor this into their interpretation.19PubMed Central. Interference of COVID-19 Vaccination With PET/CT Leads to Unnecessary Additional Imaging in a Patient With Metastatic Cutaneous Melanoma-Case Report
If lymph nodes remain swollen well beyond the expected window, a biopsy is warranted. A review of vaccine-associated lymph node biopsies found that the vast majority turned out to be reactive (benign immune responses): 28 cases of reactive lymphadenopathy, 13 of follicular hyperplasia, and a handful of rarer benign conditions. Only two metastases were identified among all reviewed cases.20PubMed Central. COVID-19 vaccine-associated lymphadenopathy: a review The key takeaway for patients is that persistent swelling should not be ignored, but it also should not be assumed to be cancer. Biopsy settles the question definitively.
For People Already Living With Lymphoma
A different but related concern applies to people who already have lymphoma: should they get vaccinated, and will the vaccine work for them? The answer to the first question is yes. The safety data shows no elevated adverse event risk for people with blood cancers compared to the general population after COVID vaccination.14PubMed. Effectiveness and safety of COVID-19 vaccination in people with blood cancer
The answer to the second question is more complicated. Lymphoma patients, especially those receiving anti-CD20 therapies like rituximab, mount weaker antibody responses to the vaccine. One study found that seroconversion (developing detectable antibodies) occurred in only about 8% of lymphoma patients after a first dose and 21% after a second dose, compared to 93% and 100% of healthy controls.21PubMed Central. CORSA study finds spike-specific blunted immune responses in lymphoma patients after SARS-CoV-2 vaccine The antibody picture is discouraging, but the T-cell response tells a different story. About 69% of patients who failed to produce antibodies still showed measurable T-cell activity against the spike protein. Another study confirmed that the breadth of T-cell responses in anti-CD20-treated patients was comparable to healthy controls, spanning the full spike protein including the receptor binding domain.22PubMed Central. The immune response to Covid-19 mRNA vaccination among Lymphoma patients receiving anti-CD20 treatment Vaccination still provides partial protection for most lymphoma patients, even when standard antibody tests suggest otherwise.
When Cutaneous Lymphoma Follows Vaccination
One subset of the case reports deserves separate attention because it has a slightly different profile. Cutaneous lymphomas, cancers that originate in the skin rather than in lymph nodes, have been reported following COVID vaccination in small numbers. A systematic review of these cases found something the broader lymphoma case reports do not consistently show: the majority responded well to standard treatment, with lesions going into remission even in cases classified as aggressive subtypes.23PubMed Central. Rare COVID-19 vaccine side effects got lost in the shuffle. Primary cutaneous lymphomas following COVID-19 vaccination: a systematic review The proposed mechanism involves immunomodulatory effects of the mRNA vaccine on shared signaling pathways that can drive certain skin lymphomas, but the authors stopped well short of claiming the vaccine caused the cancers. What they did suggest is that if a connection exists, it does not appear to worsen outcomes. This is cold comfort if you are the patient, but it distinguishes the cutaneous lymphoma pattern from the more alarming framing that sometimes circulates online, where the implication is that the vaccine causes aggressive, untreatable cancer.
The broader question of whether mRNA vaccines have immunomodulatory effects beyond the intended immune response is an active area of research. The persistent germinal center activity documented in lymph node studies confirms that these vaccines interact with the immune system in sustained and sometimes unexpected ways. Whether that interaction can, in rare individuals with specific genetic vulnerabilities, tip the balance toward lymphoma development is a question that case reports alone cannot answer. It would require large, controlled cohort studies comparing lymphoma incidence in vaccinated and unvaccinated populations over several years, and those studies are only now becoming possible as enough time has passed since the mass vaccination campaigns.