1. Mild Immune-System Activation That Mimics Flu Symptoms
When you get a flu shot, your body’s immune system is essentially being primed to recognise the virus. That can sometimes feel like you’re slightly under the weather—even though you’re not developing influenza. For many folks, this reaction is so subtle they don’t even notice. But in some cases, it’s more pronounced.
What it looks like
- Low-grade fever, fatigue, muscle aches or chills that start within a day or two of vaccination.
- Mild cough or feeling “off” even though you didn’t contract the flu.
- Slight swelling or redness at the injection site — standard, but perhaps more than expected.
Why it happens
The flu vaccines for 2025-2026 are updated to match the circulating strains better. According to the Centers for Disease Control and Prevention (CDC), the seasonal vaccine for 2025-26 uses a trivalent formulation and recommends that everyone six months or older should get vaccinated.
As your body builds antibodies, it may cause transient symptoms.
What to do
These effects usually resolve within 24-48 hours.
If you’re covered by an insurance plan that includes preventive care, you’ll likely face no out-of-pocket cost for the vaccination. Consider logging the vaccination in your HSA ledger.
If symptoms last more than a couple of days or you feel seriously ill, use your plan’s virtual doctor consultation option or contact your healthcare provider.
