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Immunotherapy Side Effects · Whole-Body Effects
Fever During Immunotherapy — Infection or an Immune Reaction?
A fever during immunotherapy can come from an ordinary infection or from the immune system itself — and for the first few hours, the two can look identical. NCCN and ASCO patient-safety guidance uses one clear temperature threshold to decide when a fever needs same-day medical review, regardless of which cause it turns out to be.
Medically reviewed by Dr. T. Raghavender Reddy, Medical Oncologist, MBBS · DM (Medical Oncology) · MD (Radiation Oncology) · Last reviewed August 2026
- Two causes, one instruction — infection and an immune-related reaction can look the same at first, and both need the same urgent review.
- Most fevers turn out ordinary — but immunotherapy can lower your defences, so even a 'simple' infection needs to be checked, not guessed.
- Timing is a clue, not proof — immune-related fever has a broad typical window across weeks to months, while infection can strike at any time.
- One number decides the next step — 100.4°F (38°C) or higher, especially with chills, means calling the same day.
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The core question
Is a Fever During Immunotherapy an Infection or an Immune Reaction?
It can be either, and for the first few hours the two often look identical. Immunotherapy can lower the body's usual defence against ordinary infections, and it can also trigger a systemic immune reaction that produces fever entirely on its own. Only same-day blood tests can safely tell the two apart.
This is why a fever during immunotherapy is never something to diagnose by watching and waiting at home — the treatment for an infection and the treatment for an immune reaction are different, and starting the wrong one late carries real risk.
Side by side
Infection or Immune Reaction — What Typically Differs?
No single row proves anything on its own. Look at the whole pattern — and if it doesn't clearly fit one column, treat the fever as urgent regardless.
| Feature | Infection-related fever | Immune-mediated fever |
|---|---|---|
| Typically starts | Any time, often with a clear trigger — a cold, a wound, a dental infection | Weeks to months into treatment, without an obvious source |
| Often comes with | A localised symptom — cough, burning on urination, wound redness, sore throat | Other new immune symptoms — rash, loose stools, joint pain, unusual fatigue |
| Typical pattern | Often rises quickly, sometimes with a clear peak | Can be low-grade and persistent, or spike suddenly |
| What confirms it | Blood cultures, a urine test, or an identified infection source | Ruling out infection first, then immune markers and specialist review |
| Safe next step | Same-day medical review, either way | Same-day medical review, either way |
This table is a reasoning aid, not a diagnosis. Only tests reviewed by your care team can confirm which one you actually have.
Whichever column it fits, call the same day if you notice:
- A reading of 100.4°F (38°C) or higher, with or without chills
- Shaking chills, confusion, a racing heartbeat, or breathlessness alongside the fever
- A fever that keeps climbing over a few hours rather than settling
- Any fever appearing alongside a new rash, severe diarrhoea, or yellowing of the skin or eyes
The threshold
What Temperature During Immunotherapy Means Call Now?
A single reading of 100.4°F (38°C) or higher is the threshold used across NCCN and ASCO patient-safety guidance for anyone on cancer treatment — call the CION helpline the same day, and be ready to go to the nearest emergency room, whatever the cause turns out to be.
| Reading | Chills? | What to do |
|---|---|---|
| Below 100.4°F (38°C) | No | Recheck within the hour; call the CION helpline if it keeps climbing or you're unsure. |
| 100.4°F (38°C) or higher | With or without chills | Call the CION helpline now and prepare to go to the nearest emergency room. |
| High fever | Shaking chills, confusion, or breathlessness | Same-day emergency — call an ambulance or go to the nearest ER immediately. |
This threshold applies the same way whether the fever started an hour ago or has been building for a day, and whether it "looks like" an ordinary infection to you or not.
The exception, not the rule
Is It Ever Safe to Simply Recheck a Fever at Home First?
A low reading — under 100.4°F (38°C), without chills, and with an obvious everyday cause such as a cold going around at home — can sometimes be rechecked within the hour rather than treated as an emergency. This is the only situation where a short wait is reasonable, and it stops applying the moment the reading rises or chills appear.
If you are managing a low fever this way, our guide on regular medicines during immunotherapy covers what is generally safe for everyday symptoms — but that guidance does not extend to a fever at or above the threshold above.
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Don't Wait Out a Fever on Immunotherapy
Whether it's infection or an immune reaction, the same team that manages your treatment can tell you which — reach them directly.
What to expect
What Happens When You Call About a Fever on Immunotherapy?
Calling early does not commit you to a hospital visit you don't need — it gets a specialist opinion on your specific reading and symptoms within minutes, and a clear next step either way.
- 1
Describe the reading
Share the exact temperature, when it started, and any chills, rash, breathlessness or diarrhoea alongside it.
- 2
Same-day triage
A member of the oncology team assesses whether this needs an immediate ER visit or a same-day clinic review.
- 3
Blood tests if needed
Blood counts and cultures check for infection; further tests follow based on your specific symptoms.
- 4
Treatment starts on findings
If infection looks likely, treatment usually starts before every result is back; if an immune reaction is suspected, the appropriate specialist is looped in.
- 5
A plan for immunotherapy
Your tumour board decides, once the cause is clearer, when treatment can safely continue.
Common causes
Can an Ordinary Illness Also Cause Fever During Immunotherapy?
Yes — most fevers during cancer treatment turn out to have an everyday cause, such as a viral cold, a urinary infection, or a minor wound infection, rather than a serious immune reaction. Immunotherapy does not remove the possibility of ordinary illness; it adds a second possible cause on top of it.
This is exactly why the same threshold applies to every fever, not just ones that "feel unusual". An ordinary infection in a person on immunotherapy can also progress faster than it otherwise would, since the body's usual defences may already be affected by the cancer and its treatment.
A common worry
Does a Fever Mean the Immunotherapy Is Working?
No — a fever is not a reliable sign that treatment is working, and it should never be treated as reassuring rather than assessed. Response to immunotherapy is judged through scans and your oncologist's review, not through side effects like fever.
See our full explainer on whether side effects mean the immunotherapy is working for a fuller answer — but treat any fever at or above the threshold as urgent regardless of what it might mean for your response.
Care during treatment
Fevers Are Manageable When Caught Early
Patients and families who act on the threshold promptly are supported by the same multidisciplinary team throughout treatment.
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Common questions
Fever During Immunotherapy: Your Questions Answered
Is a fever during immunotherapy an infection or an immune reaction?
It can be either, and in the first few hours they often look the same. Immunotherapy can lower the body's usual defence against ordinary infections, so a fever may simply be a chest, urinary or wound infection that needs prompt antibiotics. Immunotherapy can also switch on a broad immune response that produces fever on its own, with no infection anywhere in the body. Because the two causes need different treatment but look alike at first, a fever during immunotherapy is always checked with blood tests rather than guessed at home.
What temperature during immunotherapy needs same-day medical attention?
A reading of 100.4°F (38°C) or higher, particularly with chills, shivering, or feeling generally unwell, is the threshold used in NCCN and ASCO patient-safety guidance for anyone on cancer treatment. At or above this reading, call the CION helpline the same day and be prepared to go to the nearest emergency room — do not wait overnight to see if it settles. A lower reading without chills can usually be rechecked within the hour, but call sooner if it keeps climbing or you are unsure.
Can a mild fever during immunotherapy ever be watched at home first?
A low-grade reading — under 100.4°F (38°C), without chills, and with an obvious everyday cause such as a cold going around at home — can sometimes be rechecked within an hour rather than treated as an emergency. This is the exception, not the rule, and it stops applying the moment the reading crosses the threshold or chills appear. If you are ever unsure whether a fever counts as 'mild', call the helpline and describe it rather than deciding alone.
What tests confirm whether it is infection or an immune reaction?
Doctors typically start with blood tests — a complete blood count and blood cultures — to look for infection, alongside markers that can flag a broader immune reaction. Depending on other symptoms, additional tests such as a chest X-ray, urine test, or liver and thyroid panels may follow. Because both causes can coexist or evolve over the following day, a single normal test does not always end the assessment — your care team will tell you what to watch for next.
Can fever from immunotherapy start after treatment has already finished?
Yes. Both a weakened defence against infection and immune-related reactions can continue for weeks to months after the last dose, so the same threshold and the same same-day response apply even if immunotherapy has recently stopped. Always mention recent immunotherapy to any doctor assessing a fever, even months later — it changes how quickly and how carefully they investigate.
Does having a fever mean the immunotherapy is working?
No — fever is not a reliable sign that treatment is working, and it should never be treated as reassuring rather than assessed. A fever needs the same urgent evaluation whether it turns out to be an unrelated infection or an immune reaction; response to treatment is judged separately, through scans and your oncologist's review. See our page on whether side effects mean the immunotherapy is working for a fuller answer.