Why Did I Get the Flu After Receiving the Flu Shot?
Why Did I Get the Flu After Receiving the Flu Shot?
TL;DR (Quick answer): Yes, you can still catch the flu after a flu shot. Common reasons include exposure before the vaccine takes full effect (about two weeks), catching a different strain than the vaccine targets, normal year-to-year variation in vaccine effectiveness, and illnesses that look like flu but aren’t. Vaccination still reduces your risk of getting the flu and makes illness milder if you do get sick.
While flu vaccination is a proven way to lower your risk of influenza and serious complications, it isn’t a shield against all respiratory infections. “The flu vaccine trains your immune system for influenza — it doesn’t block every cold, cough, or virus going around the office,” says Aitor Aspiazu, Corporate Care Founder & Lead Nurse Consultant (RN).
1) The vaccine hasn’t kicked in yet
It takes around two weeks after vaccination for protective antibodies to build. If you’re exposed to influenza just before or within those two weeks, you can still become unwell, which is why some people feel their “flu shot gave them the flu”. Inactivated vaccines cannot cause influenza. Evidence consistently shows this (NEJM; Vaccine 2018).
2) It may be another illness that looks like the flu
Many infections mimic influenza: COVID-19, the common cold, pneumonia, bronchitis, or “stomach flu” (usually gastro). The flu shot doesn’t protect against these other pathogens — which explains many “post-vaccine” colds.
3) You might have caught a different flu strain
Each year’s vaccine targets the most likely circulating strains, but influenza viruses change (antigenic drift), and occasionally a mismatch occurs. That can reduce vaccine effectiveness in a given season (Treanor, NEJM; Krammer, Nat Rev Immunol). Typical effectiveness in many seasons sits around 40–60% in the general population, often lower for older adults (Ng et al., JID).
4) Protection isn’t absolute, and it differs by person
Flu shots mainly generate a strong systemic (IgG) response, with less mucosal (IgA) protection in the nose and throat, so upper-airway infection can still occur (Krammer). Your past infections and vaccination history also shape how you respond (“imprinting”/pre-existing immunity), which can narrow or dampen new responses (Nature Medicine 2022; Nat Communications 2021; CSH Perspect 2021; JID 2019; JID 2023).
5) Older adults need extra protection
People aged 65 and above are at a higher risk of severe influenza and hospitalisation. Enhanced/adjuvanted vaccines are recommended where available (NSW Health). Even with lower effectiveness in this age group, vaccination reduces pneumonia/flu-related hospitalisations by an estimated 25–53% in some studies (reviewed evidence).
“For older adults and those with chronic conditions, a flu shot is like a seatbelt — it won’t stop every crash, but it dramatically reduces the damage,” says Aitor Aspiazu (RN).
Common side effects after a flu shot (and what’s normal)
- Pain, redness, or swelling at the injection site
- Mild fever, tiredness, headache, or muscle aches
- A small, temporary lump at the injection site (usually needs no treatment)
- Rest, hydrate, and avoid strenuous activity for 24 hours.
- Don’t overdress if you feel feverish; keep cool and comfortable.
- Consider paracetamol for fever/pain if suitable for you — speak to a pharmacist about dosing.
- Seek medical advice urgently if you experience difficulty breathing, chest pain, a rash with swelling, or worsening symptoms.
How to lower your risk for the rest of the season
- Get your annual flu shot (for everyone over 6 months of age, unless contraindicated).
- Stay home when you are unwell; avoid close contact with people who are sick.
- Cover coughs/sneezes; clean shared surfaces and wash hands often.
- Improve ventilation in meeting rooms and break areas.
- Workplaces: Set a clear “if you’re sick, stay home” policy and run an on-site vaccination program to reduce absenteeism.
Workplace tip: Protect teams and reduce downtime — book a Corporate Care workplace flu vaccination program. We vaccinate across Australia & New Zealand, on-site and with minimal disruption.
FAQs
Does the flu vaccine give you the flu?
No. Inactivated vaccines cannot cause flu. Post-shot colds are often caused by other viruses (Rikin et al., Vaccine).
Why did I still get sick after my flu shot?
Timing (before immunity builds), different strains, and normal variation in vaccine effectiveness (NEJM).
Does vaccination still help if I get breakthrough flu?
Yes — it generally shortens illness and reduces complications by boosting immune responses (CID).
Do older adults need a different vaccine?
Enhanced/adjuvanted options are recommended for individuals aged 65 and above, where available (NSW Health).
Sources (evidence-based)
- Treanor JJ. Influenza Vaccination. NEJM 2016.
- Krammer F. Human antibody response to influenza infection & vaccination. Nat Rev Immunol 2019.
- Moritzky SA et al. Pre-existing immunity & vaccine responses. J Infect Dis 2023.
- Sanyal M et al. Diminished B-cell response after repeat vaccination. J Infect Dis 2019.
- Ng TWY et al. Vaccine history & HI titres in older adults. J Infect Dis 2020.
- Auladell M et al. Infection history shapes responses. Nature Medicine 2022.
- Wild K et al. Pre-existing immunity & CD4/IgG responses. Nat Communications 2021.
- Linderman SL et al. Immunisation in context of pre-existing immunity. CSH Perspect Med 2021.
- Liu W et al. Breakthrough illness characteristics. Clin Infect Dis 2010.
- Rikin S et al. Acute respiratory illness after vaccination. Vaccine 2018.
General advice only. For personalised care, speak with your GP, practice nurse, or pharmacist. If you need urgent help, call triple zero (000).
Have questions about workplace clinics? Contact Corporate Care or book your on-site flu vaccination program.
- Behind the Syringe
- Flu Insights
Introducing Aitor Aspiazu, your go-to flu expert and a Registered Nurse accredited by AHPRA (NMW0001159845). Boasting over two decades of experience in healthcare, Aitor has navigated everything from Emergency Departments to Corporate Wellness Programs—and yes, he’s even battled the flu himself. As the Founder and Lead Nurse Consultant at Corporate Care, he’s the driving force behind workplace health and well-being. No longer in scrubs, he’s now at the forefront of healthcare innovation, leveraging Artificial Intelligence and automation to revolutionise workplace health and well-being. As the Founder and Lead Nurse Consultant at Corporate Care, he’s the visionary behind cutting-edge flu vaccination programs that keep businesses running smoothly. Off-duty, he’s a proud dad and a Padel addict who represented Australia in the World Padel Championship. Trust Aitor; he’s got your back when it comes to flu prevention and modern healthcare solutions.