How do covid symptoms differ from flu?

The short version: COVID-19 and flu share many symptoms, but the single most distinguishing feature is sudden loss of smell and taste, which is rare in flu and common in COVID-19.

The evidence here comes from small, early-pandemic studies, mostly from 2020 and 2021, and none were funded by a manufacturer. That is the good part. The bad part is that most of these studies are small, retrospective, or based on subjective self-report, and none compare COVID-19 to flu in the same population using the same methods. A 2025 study of kidney transplant patients in Iran did compare the two directly [6]. In that group, myalgia (muscle pain) was far more common in COVID-19 (67%) than in flu (7.7%), with a p-value of 0.001. Cough was also more common in COVID-19 in that study.

The most reliable difference across the broader evidence is loss of smell and taste. A systematic review of 17 studies found that loss of smell typically starts 4 to 5 days after other symptoms and lasts 7 to 14 days [3]. Reported rates vary wildly, from 4.5% in one Indian study [4] to 68% in a US cohort of 1,480 patients [1]. The wide range tells you the data is noisy, but the signal is real: anosmia and ageusia are a hallmark of COVID-19 in a way they are not for flu. The mechanism involves the virus infecting supporting cells in the olfactory epithelium via the ACE2 receptor, not the sensory neurons themselves, which explains why most people recover within weeks [1].

Flu, by contrast, hits harder and faster with fever, cough, and body aches. A 2010 Korean study of pandemic H1N1 found that cough was the strongest predictor of flu, with a sensitivity of 90% [5]. Fever and myalgia were also strong predictors. The classic flu presentation is sudden onset of fever, cough, and muscle aches, often within hours. COVID-19 more often has a gradual onset, and the loss of smell and taste can be the first or only symptom.

The evidence here is moderate for the anosmia/ageusia difference, but low for most other symptom comparisons because the studies are small, use different populations, and rarely compare the two diseases head-to-head. The 2025 Iranian study [6] is the only one that does, and it is limited to kidney transplant recipients, a very specific group.

My call: loss of smell or taste is the single most useful symptom to distinguish COVID-19 from flu, but the evidence for other symptom differences is thin and inconsistent. Confidence: moderate for anosmia/ageusia as a distinguishing feature, low for all other symptom comparisons.