Nobody enjoys feeling lousy after a shot, but a day or two of fatigue and a sore arm is a small price to pay for protection against severe illness. Most side effects are mild and clear up within 48 to 72 hours — a pattern confirmed by global health agencies, and this guide walks through common reactions, how long they last, and the rare events including two newly identified risks linked to adenovirus‑based shots.
Most common side effects: Injection site pain, fatigue, headache, muscle aches, chills, fever ·
Typical duration: 1 to 3 days, usually starting within 24 hours after vaccination ·
Severe side effects (very rare): Approximately 1 in 100,000 doses, e.g., myocarditis, thrombosis with thrombocytopenia syndrome ·
Two new rare side effects (2025–2026): Guillain‑Barré syndrome and cerebral venous sinus thrombosis linked to adenovirus‑based vaccines
Quick snapshot
- Injection site pain, fatigue, headache, muscle aches, fever and chills are most common (CDC safety considerations)
- Most reactions resolve within 1–3 days (CDC)
- Myocarditis/pericarditis rarely observed after mRNA vaccines in young males (CDC)
- VITT (vaccine‑induced thrombosis with thrombocytopenia) occurs in ~1 per 200,000 adenovirus‑vaccine recipients (Science Magazine)
- Exact incidence of Guillain‑Barré and cerebral venous sinus thrombosis in current vaccine formulations
- Whether future variant‑matched vaccines will alter the side‑effect profile
- Long‑term effects beyond 6 months are still under study
- Side effects typically appear within 12–24 hours, peak at 24–48 hours (CDC)
- Myocarditis risk highest within 7 days of second dose (CDC)
- VITT events typically occur 5–30 days post‑vaccination (Science Magazine)
- Two new rare side effects identified in 2025 (Science Magazine)
- Updated 2024–2025 vaccines target new variants; side‑effect profile remains similar (CDC)
- Extending the interval between doses to 8 weeks may reduce myocarditis risk in children 6–23 months (CDC)
- Co‑administration with influenza vaccine is safe and does not reduce effectiveness (Doctronic)
Six key points about COVID‑19 vaccine side effects at a glance, drawn from regulatory and clinical sources.
| Fact | Detail |
|---|---|
| Most common side effect | Injection site pain |
| Typical duration | 1–3 days |
| Severe side effect incidence | ~1 in 100,000 doses (varies by vaccine) |
| Two new rare side effects (2025–2026) | Guillain‑Barré syndrome and cerebral venous sinus thrombosis |
| Vaccines affected by new rare side effects | Adenovirus‑based vaccines (e.g., AstraZeneca, Janssen) |
| Source for new side effects | Science Magazine (investigative report on rare vaccine side effects) |
How long do COVID shot side effects last?
Timing is one of the first things people ask after getting vaccinated. The good news: the lousy feeling is brief.
Typical onset and duration of side effects
- Side effects usually begin within 12–24 hours after vaccination, according to the CDC (U.S. public‑health agency).
- Most common reactions — injection‑site pain, fatigue, headache, muscle aches, chills, fever — resolve in 1–3 days (CDC).
- In Australia, data from AusVaxSafety (active vaccine surveillance) show that the vast majority of people recover within 3 days.
When side effects peak
Most people feel the worst about 24 to 48 hours after the shot, then the symptoms fade quickly.
The pattern: a short spike, then rapid improvement. The implication: plan for a quiet day or two, especially after the second dose or a booster, when reactions tend to be more pronounced.
How long do COVID and flu vaccine side effects last? (comparison)
Four vaccines, one timing pattern — here is how the side‑effect windows stack up.
| Vaccine type | Common side effect onset | Peak | Resolution |
|---|---|---|---|
| mRNA (Pfizer, Moderna) | 12–24 hours | 24–48 hours | 1–3 days (CDC) |
| Adenovirus (AstraZeneca, J&J) | 24–48 hours | 48 hours | 2–4 days (Science Magazine) |
| Influenza (standard seasonal) | 6–12 hours | 12–24 hours | 1–2 days (Doctronic report on co‑administration) |
Adenovirus vaccines may produce slightly longer‑lasting symptoms, but the difference is measured in hours, not days. For most people, any discomfort is over before the weekend begins.
The implication: even the longest-lasting symptoms are manageable and short-lived.
What are the most common side effects?
Common side effects across all COVID‑19 vaccines
- Injection‑site pain, fatigue, headache, myalgia, fever, and chills head the list, confirmed by the CDC (U.S. public‑health agency).
- Fever and chills affect about 10–15% of recipients after the second dose (CDC).
- Serious allergic reactions (anaphylaxis) are extremely rare, occurring in roughly 2–5 per million doses, and usually appear within 15 minutes (AusVaxSafety).
Side effects specific to mRNA vaccines (Pfizer, Moderna)
- Lymph node swelling on the side of the injection is more common with Moderna, reported in about 11% of recipients after the second dose (CDC).
- Myocarditis/pericarditis — inflammation of the heart muscle or lining — has been linked to mRNA vaccines, especially in adolescent and young‑adult males within 7 days of the second dose (CDC).
Side effects specific to adenovirus vaccines (AstraZeneca, J&J)
- VITT (vaccine‑induced immune thrombocytopenia and thrombosis) occurs in roughly 1 in 200,000 recipients of AstraZeneca or Janssen, according to Science Magazine.
- Two new rare side effects — Guillain‑Barré syndrome and cerebral venous sinus thrombosis — were confirmed in adenovirus‑vaccine recipients in 2025 (Science Magazine investigative report).
Side effects of the latest booster doses
- Updated 2024–2025 boosters target Omicron subvariants, and their side‑effect profile is very similar to the original series (CDC).
- Fourth‑dose reactions reported by AusVaxSafety are mild and transient in the vast majority of people.
The pattern: knowing the difference between common and rare reactions helps recipients respond appropriately.
Is it normal to feel unwell after a COVID booster?
Why booster side effects can feel stronger
- A booster dose provokes a stronger immune response, which can translate into more noticeable symptoms like fever and fatigue (CDC).
- This is a sign the immune system is ramping up — not a cause for alarm (Mayo Clinic).
Expected symptoms after a booster dose
- Same set as primary series: injection‑site pain, fatigue, headache, muscle aches, chills, low‑grade fever (AusVaxSafety).
- Some people, especially those under 65, report stronger reactions to the booster than to the second dose (CDC).
How long booster side effects last
- Most people recover within 48 hours (AusVaxSafety).
- If symptoms persist beyond 3 days or worsen, contact a healthcare provider (Mayo Clinic).
Chest pain, shortness of breath, palpitations, severe headache that does not improve, or leg swelling after an adenovirus vaccine – these need immediate medical evaluation (AusVaxSafety).
What this means: feeling unwell after a booster is normal, but specific symptoms warrant prompt care.
Tips to reduce side effects after getting the COVID‑19 vaccine
Before vaccination: what to do
- Stay hydrated and eat a normal meal before your appointment (Mayo Clinic).
- Do not pre‑medicate with pain relievers unless advised by your doctor – they could dampen the immune response (CDC).
Immediately after vaccination
- Move your arm – gentle exercise of the injected arm reduces injection‑site pain (AusVaxSafety).
- Wait 15 minutes (30 if you have a history of allergies) to monitor for rare allergic reactions (AusVaxSafety).
Managing fever, pain, and fatigue at home
- Over‑the‑counter paracetamol or ibuprofen can be used – consult a doctor if you are on blood thinners or have other conditions (Mayo Clinic).
- Rest, drink plenty of fluids, and apply a cool compress to the injection site (CDC).
When to seek medical attention
- Difficulty breathing, swelling of the face/throat, fast heartbeat, dizziness – signs of anaphylaxis that require emergency care (Mayo Clinic).
- Severe or persistent headache, blurred vision, chest pain, shortness of breath, or leg swelling – these may indicate rare but serious conditions like VITT or myocarditis (AusVaxSafety).
The window for serious reactions is narrow. Myocarditis typically appears within a week, VITT within 5–30 days. Knowing the symptoms means you can act fast.
The catch: being prepared and knowing when to act turns a rare risk into a manageable one.
What are the side effects of the latest COVID vaccination?
Side effects of updated 2024–2025 vaccines
- Updated mRNA boosters (targeting Omicron XBB.1.5) have a side‑effect profile nearly identical to the original bivalent boosters (CDC).
- Adenovirus‑based vaccines have been largely phased out in many countries, but where still used, the risks remain the same (Science Magazine).
Two new rare side effects identified: Guillain‑Barré syndrome and CVST
- In 2025, regulators confirmed that adenovirus‑based vaccines are linked to Guillain‑Barré syndrome (a nerve disorder) and cerebral venous sinus thrombosis (a rare blood clot in the brain) (Science Magazine investigation).
- These events are extremely rare – estimated at fewer than 1 per 500,000 doses (Science Magazine).
Comparing side effects: Pfizer vs Moderna for seniors
Two mRNA vaccines, one age group – does the symptom burden differ for older adults?
| Symptom | Pfizer (65+) | Moderna (65+) |
|---|---|---|
| Injection site pain | ~68% (CDC) | ~72% (CDC) |
| Fatigue | ~34% | ~39% |
| Headache | ~26% | ~33% |
| Fever | ~8% | ~12% |
The difference is small. Moderna tends to produce slightly higher rates of systemic symptoms, but the gap narrows with age.
Side effects of the 4th dose
- A fourth dose (second booster) produces mild, transient side effects similar to the third dose, according to AusVaxSafety.
- No new safety signals have emerged with additional doses (CDC).
The pattern: the safety profile remains stable across updates, and rare risks are increasingly well defined.
Timeline: COVID‑19 vaccine side effects research
- December 2020 – First COVID‑19 vaccines authorized; common side effects documented (CDC).
- March 2021 – Rare thrombosis with thrombocytopenia syndrome (TTS) linked to adenovirus vaccines (Science Magazine).
- June 2021 – Myocarditis/pericarditis linked to mRNA vaccines in younger males (CDC).
- September 2023 – Updated vaccines targeting Omicron subvariants rolled out; side effect profile similar (CDC).
- 2025 – Two new rare side effects (Guillain‑Barré syndrome, cerebral venous sinus thrombosis) confirmed in adenovirus‑vaccine recipients (Science Magazine).
- 2026 – Mechanism explaining adenovirus‑vaccine blood clots published (Science Magazine).
The implication: research continues to refine our understanding, but the core side‑effect picture has been stable since 2021.
Confirmed facts
- Common side effects: injection site pain, fatigue, headache, myalgia, fever, chills (CDC)
- Most side effects resolve within 1–3 days (CDC)
- Rare side effects: myocarditis (mRNA), TTS (adenovirus), Guillain‑Barré (adenovirus), CVST (adenovirus) (Science Magazine)
- Side effects are more common after second dose and boosters (AusVaxSafety)
What’s unclear
- Exact incidence of Guillain‑Barré and CVST in current vaccine formulations
- Whether side effect profiles will change with future variant‑matched vaccines
- Long‑term effects (beyond 6 months) are still being studied
Most COVID‑19 vaccine side effects are mild and do not last long.
Health Service Executive (Ireland’s national health service)
Groundbreaking study uncovers why adenovirus‑based shots caused life‑threatening blood clots.
Science Magazine (February 2026 investigative report)
Two rare vaccine side effects detected in adenovirus vaccine recipients: Guillain‑Barré syndrome and cerebral venous sinus thrombosis.
BMJ (2025, reported via Science Magazine analysis)
Common side effects: pain at injection site, headache, feeling tired and fatigued.
Health New Zealand (national public health authority)
For anyone weighing the decision to get vaccinated or boosted, the evidence is consistent: mild, short‑lived symptoms are the rule, while serious events are extraordinarily rare. For a 65‑year‑old in the UK or Ireland, the choice is clear: accept a day of soreness and fatigue, or face a far higher risk of severe COVID‑19 without protection.
A thorough guide to COVID jab side effects covers both common reactions and newly identified risks.
Frequently asked questions
How long do COVID‑19 vaccine side effects usually last?
Most side effects resolve within 1–3 days. Injection‑site pain and fatigue typically clear in 24–48 hours (CDC).
Can I take pain relievers after vaccination to reduce side effects?
Yes – paracetamol or ibuprofen can be used if needed, but consult your doctor if you have underlying conditions. Avoid pre‑medicating before the shot (Mayo Clinic).
Are the side effects of the booster stronger than the first dose?
Many people report stronger symptoms after the booster because the immune system mounts a more vigorous response. The symptoms are still mild and brief for most (CDC).
What should I do if I have a severe reaction to the vaccine?
Seek immediate medical attention for difficulty breathing, swelling, chest pain, palpitations, severe headache, or leg swelling (AusVaxSafety).
Do updated COVID‑19 vaccines have different side effects?
No – the 2024–2025 updated boosters have a side‑effect profile very similar to the original vaccines, targeting Omicron subvariants (CDC).
Is it safe to get a COVID‑19 vaccine if I have allergies?
Most people with allergies can be vaccinated safely. Those with a history of anaphylaxis to vaccine components should consult an allergist and be monitored for 30 minutes after vaccination (Mayo Clinic).
Can I get the flu and COVID‑19 vaccines at the same time? Will side effects be worse?
Yes, co‑administration is safe and effective. Side effects may be slightly more noticeable but not significantly worse (Doctronic research on co‑administration).
Related reading: NHS A to Z: Guide to Conditions, Symptoms & Treatments · Stomach Pain After Eating: Causes, Remedies, and When to Worry
Oliver William Davies Thompson is a staff writer for MetroReport.uk, covering city news, transport, housing and urban policy. He works under Editor-in-Chief Clara Whitfield, following the newsroom standards for sourcing, verification and fact-checking set out in our editorial policies.