How long can you feel under the weather from having Covid and flu jabs

The short version: most people feel rough for a day or two, but the studies here don't give you a clean single number, and the "under the weather" feeling can last longer for some.

The clearest data comes from a 2022 study of children with juvenile arthritis: after their mRNA shots, tiredness, muscle aches, headache, and fever were common, but all resolved within 1–2 days [3]. That matches the typical picture. A 2025 Sri Lankan study of working-age adults found symptoms were most common after the third dose, with about 6 in 10 people reporting something, but it doesn't state how long symptoms lasted [2]. A 2021 survey across three vaccines (Pfizer, AstraZeneca, Sinopharm) found mostly mild-to-moderate reactions, with a few unusual events lasting up to 13 days, like hand weakness or chest pain [1]. So the usual answer is 1–3 days, but outliers exist.

For flu shots, the only relevant study here is from 1983, testing an older subunit flu B vaccine. It found only mild local reactions and no systemic symptoms at all [4]. That's dated and doesn't reflect current flu vaccines, so treat it as weak evidence.

What's missing: no study here directly compares COVID and flu shots given together, and none tracks how long "under the weather" lasts in a general population with modern vaccines. The evidence is mostly short-term surveys, not rigorous trials with hard endpoints. The 2025 review linking spike protein persistence to long-term symptoms is speculative, not proof of duration [5].

My call: expect 1–3 days of feeling off after either shot, possibly longer after a booster, but the evidence for anything beyond that is thin. Confidence: moderate.