# Relative probability of COVID-19 versus flu, in patients with symptoms typical of both **URL:** \ **Category:** The Quarantine Zone\ **Tags:** covid-quarantine\ **Created:** [December 19, 2020, 8:46pm UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416 "2020-12-19T20:46:29Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [December 19, 2020, 8:46pm UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416/1 "2020-12-19T20:46:29Z")
Of all the people currently who have symptoms that are ambiguously consistent with flu and with COVID-19, who go and get a COVID test, what’s the likelihood of a positive test result? There’s no anosmia, which I think would influence the odds. I don’t think it’d be right to just look at the prevalence of COVID with the typical prevalence of flu, because masks and social distancing and working from home should be greatly reducing both, right? I’m passing time while waiting for my friends in this situation to get their test result. ---
**Author:** ![FigNorton](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fignorton/32/5619_2.png) [@FigNorton](https://boards.straightdope.com/u/FigNorton)\ **Post date:** [December 19, 2020, 10:39pm UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416/2 "2020-12-19T22:39:27Z")
There was a quick “truth” that came into existence that anosmia was _the_ covid symptom. It’s actually common in many respiratory infections. From pre-covid times: > **[What Is Anosmia?](https://www.healthline.com/health/anosmia)** > > Anosmia is an absent or decreased sense of smell. Read more on what causes it and how to treat it. > [@](#): > > Below are the main causes of anosmia: > > ### Irritation to the mucus membranes lining the nose > > This can result from: > > - sinus infections > - [common colds](https://www.healthline.com/health/cold-flu/head-cold) > - smoking > - the [flu](https://www.healthline.com/health/flu-causes), or influenza > - allergies ([allergic rhinitis](https://www.healthline.com/health/allergic-rhinitis)) > - chronic congestion not related to allergies (nonallergic rhinitis) ---
**Author:** ![Aspidistra](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aspidistra/32/3894_2.png) [@Aspidistra](https://boards.straightdope.com/u/Aspidistra)\ **Post date:** [December 20, 2020, 10:55pm UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416/3 "2020-12-20T22:55:29Z")
If their symptoms are consistent with COVID and the flu, then presumably they are consistent with the common cold too, I would think. In which case, a comparison with the common cold would be the most relevant, since that’s much more prevalent than actual flu. You’d basically calculate it from the prevalence of cold (or flu) compared to Covid, if you really can’t tell from the symptoms. So - the[CDC](https://www.cdc.gov/features/rhinoviruses/index.html#:~:text=Each%20year%20in%20the%20United,any%20time%20of%20the%20year.) reckons that most people get 2 or 3 colds a year, “mostly in winter or spring” - say that means the average person gets one cold over the winter months, you’d expect in the US that this would lead to around 3.7 million new colds appearing each winter day. Adjust that somewhat downward for the fact that They Say there are a lot less colds around this year, with all the handwashing and masking everyone’s doing. Flu: supposedly somewhere between [10 and 50 million in the US each year](https://www.healthline.com/health/influenza/facts-and-statistics#:~:text=The%20flu%20has%20resulted%20in,than%20200%2C000%20hospitalizations%20each%20year.) - for argument’s sake lets say they’re all in winter but that we’re on the low end of the scale this year, that would make about 0.1 million new cases of flu per day be a likely estimate round about now - though if it were a heavy flu season you’d expect up to 0.5 million per day Diagnosable Covid is currently appearing at a rate of around 0.2 million per day. So for someone experiencing cold/flu symptoms at the moment, a priori I’d estimate it’s about 10 times more likely to be a cold than either flu or Covid - probabilities of the last two fairly similar to each other. If they have symptoms that _can_ exist with either but are _more characteristic_ of one than another, then those probabilities shift up or down somewhat, but usually not really drastically unless it’s a symptom or factor overwhelmingly more common in one illness than another (or there are other factors like ‘we were at a party with Uncle Bob last Tuesday and now he’s in hospital on a ventilator’) ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [December 20, 2020, 11:16pm UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416/4 "2020-12-20T23:16:16Z")
> [@FigNorton](#): > > There was a quick “truth” that came into existence that anosmia was _the_ covid symptom. It’s actually common in many respiratory infections. My impression from the following link is that it’s loss of taste/smell without a runny or stuffy nose that is specifically a covid symptom. > **[COVID-19, cold, allergies and the flu: What are the differences?](https://www.mayoclinic.org/diseases-conditions/coronavirus/in-depth/covid-19-cold-flu-and-allergies-differences/art-20503981)** > > Knowing if you have COVID-19, a cold, the flu or seasonal allergies isn't easy. Find out about the differences between these illnesses. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [December 21, 2020, 10:34am UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416/5 "2020-12-21T10:34:06Z")
The probability of a positive test result became 1, late yesterday. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [December 21, 2020, 2:19pm UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416/6 "2020-12-21T14:19:42Z")
Sorry to hear that. Are these people in your “bubble”, do you have reason to be concerned for yourself too? ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [December 21, 2020, 10:13pm UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416/7 "2020-12-21T22:13:43Z")
> [@Riemann](#): > > Are these people in your “bubble” […] Hey, good thought! Thanks for asking! No, I last saw one of them four weeks earlier, and we distance, wear masks, and stay outdoors. I still think it’d be interesting to know this probability, though. I have to think colds and flu would be somewhat less common this year due to COVID practices, especially among people who are diligent. For my part I haven’t caught a cold since isolation started in March. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [December 25, 2020, 5:23am UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416/8 "2020-12-25T05:23:33Z")
> [@Aspidistra](#): > > You’d basically calculate it from the prevalence of cold (or flu) compared to Covid… So for someone experiencing cold/flu symptoms at the moment, a priori I’d estimate it’s about 10 times more likely to be a cold than either flu or Covid - probabilities of the last two fairly similar to each other. > [@Napier](#): > > I don’t think it’d be right to just look at the prevalence of COVID with the typical prevalence of flu I just came across this paper. It indicates a _massive_ reduction in in flu cases this year, down by 90% or more. Presumably something similar is likely to happen for colds, unless there’s some subtle difference in the way colds are transmitted? So if you do get ambiguous symptoms, unfortunately the probability that it’s COVID is quite a lot higher that @Aspidistra’s estimates. > **[Decreased Influenza Activity During the COVID-19 Pandemic ...](https://www.cdc.gov/mmwr/volumes/69/wr/mm6937a6.htm)** > > After recognition of widespread community transmission of SARS-CoV-2, the virus that causes coronavirus disease 2019 (COVID-19), by mid- to late February 2020, indicators of influenza activity ... ---
**Author:** ![Tamerlane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tamerlane/32/7602_2.png) [@Tamerlane](https://boards.straightdope.com/u/Tamerlane)\ **Post date:** [December 25, 2020, 1:45pm UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416/9 "2020-12-25T13:45:08Z")
> [@Riemann](#): > > It indicates a _massive_ reduction in in flu cases this year, down by 90% or more Yeah one of the most important points is, as that paper notes, influenza is substantially less contagious than COVID-19. All this social isolating and mask-wearing is having the relatively happy side effect of being considerably more impactful on holding down flu infections. A minor silver-lining to an otherwise grim season. ---
**Author:** ![puzzlegal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/puzzlegal/32/3827_2.png) [@puzzlegal](https://boards.straightdope.com/u/puzzlegal)\ **Post date:** [December 25, 2020, 1:56pm UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416/10 "2020-12-25T13:56:31Z")
> [@Thudlow\_Boink](#): > > My impression from the following link is that it’s loss of taste/smell without a runny or stuffy nose that is specifically a covid symptom. Maybe with no history of other symptoms? I had anosmia for a few days following a cold several years ago, with no other symptoms at that time. Actually, it’s happened twice. I normally have an excellent sense of smell, and it was really disconcerting when a hamburger didn’t taste like beef but only had a vaguely umani flavor, and every apple in the taste taste tasted the same, just maybe a little more or less sweet. ---
**Author:** ![FigNorton](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fignorton/32/5619_2.png) [@FigNorton](https://boards.straightdope.com/u/FigNorton)\ **Post date:** [December 25, 2020, 2:36pm UTC](https://boards.straightdope.com/t/relative-probability-of-covid-19-versus-flu-in-patients-with-symptoms-typical-of-both/928416/11 "2020-12-25T14:36:16Z")
An acquaintance who works at a children’s hospital told lots of diseases are down, not just respiratory. And she said some of these were definitely diseases that people would come in for not just ride out at home.