Flu or COVID-19? Why Symptoms Alone Cannot Tell Them Apart

Hello. I am Shunichi Gamo, MD, PhD, a board-certified pulmonologist and the director of Sendai Airport North Clinic.

As winter approaches, I hear more and more from people who say, “I have a fever and a sore throat, but I can’t tell whether it’s influenza or COVID-19.”

You don’t want to pass it to your family, you are concerned about how many days you will miss from work or school, and you wonder by when you need to start medicine. There is a lot to be unsure about.

To give you the conclusion first: influenza and COVID-19 cannot be reliably told apart by symptoms alone, even by doctors. To be sure, we judge by combining a test with information on what is going around and who nearby is infected.

In this article, as a lung specialist, I explain how the symptoms overlap, what it means to test for both at the same time with an antigen test, how the treatment drugs differ, and who should be seen early and when to call 119.

Before that, a note about our fever clinic on Sundays and holidays. PCR tests for influenza and COVID-19 (a test that multiplies the virus’s genetic material to detect it) give you a result the same day.

Since October 4, 2026, our fever clinic is also open on Sundays and holidays
10:00 to 15:00 (no lunch break). No appointment is needed; we accept patients by same-day check-in. We give you your PCR test result the same day. On Sundays and holidays we run the fever clinic only, with no general outpatient care and no IV drips. We are closed on Sunday, October 25, and from November we are generally open every Sunday and holiday. Announcement: fever clinic on Sundays and holidays

The short answer: can influenza and COVID-19 be told apart by symptoms alone?

No, they cannot. Judging by symptoms alone makes a mistake quite likely.

There is a review (a paper that summarizes earlier studies) pooling 67 studies of how accurately influenza can be diagnosed [1]. It concludes that the diagnostic accuracy of symptoms and combinations of symptoms for influenza is “very limited” [1].

The same is true of COVID-19. A Cochrane review (from an international medical review organization) pooling 42 prospective studies (52,608 people) says that the presence or absence of symptoms alone can neither confirm nor rule out COVID-19 [2].

A statement from the Japanese Association for Infectious Diseases cites a study in which, of the cases clinically diagnosed as “influenza-like illness” from symptoms at the visit, a little under 60% were confirmed as influenza by testing or other means [3].

In short, the basic approach is this: if you feel “this is different from my usual cold,” do not decide by symptoms; confirm with a test.

How are the symptoms similar, and how are they different?

Both mainly cause fever, cough, sore throat, and fatigue, and the overlap is very large. There are, however, differences in tendency.

One report analyzed 100 papers comparing influenza and COVID-19 with a meta-analysis (a method that pools and analyzes several studies). Symptoms of the upper airway, such as the throat and nose, stood out more in influenza [4]. The study included 226,913 people with COVID-19 and 201,617 people with influenza [4].

What to look at	Influenza	COVID-19	Useful for telling them apart?
How symptoms appear	Fever of 38°C or higher, headache, joint pain, muscle aches, and fatigue appear relatively suddenly [5]	The way symptoms appear varies widely between studies [2]	There are differences in how they appear, but they are not decisive [1][2]
Throat and nose symptoms	Upper airway symptoms tend to stand out [4]	Having a sore throat does not raise the likelihood of COVID-19 [2]	A tendency only. Cannot be judged on its own
Loss of smell or taste	–	Loss of smell or taste is a clue pointing to COVID-19. However, many people do not have it [2]	Helpful if present. Its absence does not rule out COVID-19

Loss of smell or taste is a clue that raises suspicion of COVID-19. In this review, the specificity of loss of smell or taste (the proportion of people without the disease who are correctly ruled out) was 92.1% [2]. On the other hand, the sensitivity (the proportion of people with the disease who are detected) was only 39.2% [2].

For that reason, having no loss of smell or taste does not mean it is not COVID-19. Note that this review covers studies up to June 2021, and the frequency of symptoms may have changed with later changes in the virus [2].

When both are circulating, how should I choose a test?

An antigen test that checks for influenza and COVID-19 at the same time (a test that finds viral proteins) is a strong option. The statement from the Japanese Association for Infectious Diseases also asks medical facilities to make effective use of the combined rapid antigen test (combo kit) [3].

Because a single test checks for both, we can decide quickly “which medicine is needed.” It also says to be ready to run a genetic test for COVID-19 in case the result is hard to interpret [3].

There are other reasons to test for both at once. The review of influenza diagnostic accuracy states that we need to rely less on symptoms alone and make more use of tests that can be done on site (point-of-care testing) [1].

Both can also be positive, and the statement from the Japanese Association for Infectious Diseases lays out the flow for when both are positive [3]. Because the symptoms are similar, even when one is found it is worth checking for the other.

If a test is done right after symptoms begin, it may not turn positive even when you are infected. If the test is negative but symptoms are strong, or you have an underlying condition, the doctor judges by combining symptoms and what is going around.

The statement from the Japanese Association for Infectious Diseases says that if COVID-19 is negative but symptoms are present, influenza and other illnesses should also be considered [3]. Please show us your home test result when you are seen.

How do the treatment drugs differ between influenza and COVID-19?

The big difference is that for influenza, medicine is started “within 48 hours of onset,” while for COVID-19 it is started “early, in people at risk of becoming seriously ill” [5][6].

When an antiviral drug for influenza (a drug that stops the virus from multiplying) is started within 48 hours of onset, the fever period is usually shortened by 1 to 2 days [5]. The Ministry of Health, Labour and Welfare explains that virus shedding is also reduced [5]. If it is started after 48 hours, a sufficient effect becomes harder to expect [5]. That is exactly why I recommend consulting early.

For COVID-19, the clinical practice guide says that giving an antiviral drug early to people at high risk of becoming seriously ill is expected to reduce hospitalization and death [6].

	Influenza	COVID-19
Guide to when to start the drug	Within 48 hours of onset [5]	Within 5 days of onset, for people at risk of becoming seriously ill (oral drugs and others) [6]
Main drugs	Oseltamivir (Tamiflu and others), zanamivir, laninamivir, peramivir, baloxavir (Xofluza) [5]	Nirmatrelvir/ritonavir, molnupiravir, remdesivir (IV drip) [6]
Whether to use a drug	Not essential for everyone; the doctor decides [5]	Even without risk factors, if within 3 days of onset and symptoms are strong, ensitrelvir may be considered [6]
Handling fever and discomfort	Lower the fever with a fever reducer (acetaminophen, for example Calonal). Take fluids and rest [5]	Likewise, symptom relief with fever reducers and pain relievers as needed [6]

COVID-19 drugs require checking for drug interactions (contraindicated combinations) and kidney function [6]. Be sure to bring your medication notebook.

In children and teenagers with influenza, abnormal behavior such as suddenly running off can occur whether or not they have taken the medicine [5]. After diagnosis and the start of treatment, do not leave them alone for at least 2 days [5].

[Signs to be seen quickly] Who should be seen early, and when to call 119?

Older adults, people with underlying conditions, pregnant women, and children of elementary school age or younger should be seen early if they develop a fever. The statement from the Japanese Association for Infectious Diseases also says that, when visiting a medical facility, priority should go to these people at high risk of becoming seriously ill [3].

People who should be seen early

The clinical practice guide states that older age is the most important risk factor for severe illness, and that risk is especially high in older adults with underlying conditions [6]. People at high risk can worsen between several days and the second week after onset, even if the illness is mild at diagnosis [6].

Even with influenza, severe illness can occur, with acute encephalopathy (brain dysfunction) in children, and with bacterial pneumonia in older adults and people with weak immunity [5].

Call 119 right away

If even one of the following applies, call 119 without hesitation, however high or low the fever is [7]. This is the same at night and on holidays.

The Fire and Disaster Management Agency materials point to emergency advice lines (such as #7119, Japan’s emergency medical advice line) as a place to ask when you are unsure about calling 119 [7]. However, when any of the signs above is present, call 119 without stopping to ask for advice.

Go to an emergency department the same day, even at night or on a holiday

The Ministry of Health, Labour and Welfare advises seeing a medical facility early if you feel quite unwell, for example with a persistent high fever, trouble breathing, or altered consciousness [5]. In the following cases, go to an emergency department without waiting for morning. If the person is in distress and cannot be moved, call 119.

The guide of an SpO2 of 93% or lower corresponds, in the clinical practice guide, to a condition that needs oxygen (moderate illness II) [6]. Also, a person with low oxygen may not complain of breathlessness, so look at both the number and how the person looks [6].

Be seen the same day (the next morning if it is night)

Even if breathing, consciousness, and fluids are holding up, when a high fever continues, or when the illness is going around your family or those nearby, do not wait too long to be seen.

Related reading: what to tell the doctor when you have a fever from influenza. For how to spend your time after a positive COVID-19 result, see when you can return to work, and for influenza, see when you can return to school or daycare.

Frequently asked questions

I have a fever and a sore throat. Can I tell at home whether it is influenza or COVID-19?

Symptoms alone cannot reliably tell them apart, even for doctors [1][2]. “A high fever that comes on suddenly, with aches all over the body” is a course typical of influenza, but it is only a rough guide. We confirm with a test, so I recommend being seen. If the person does not answer when called or has trouble breathing, call 119.

Do I need separate tests for influenza and COVID-19?

Medical facilities can use an antigen test that checks for both at the same time (a combo kit) [3]. One test gives both results, so decisions about medicine come sooner. When the result is hard to interpret, a genetic test is also considered [3].

By when do I need to start taking influenza medicine?

The guide is within 48 hours of onset [5]. Starting well after onset makes a sufficient effect harder to expect, so please be seen early once a fever appears [5]. The doctor decides whether to use a drug [5].

Can anyone take COVID-19 medicine?

In the clinical practice guide, oral drugs are considered for people at risk of becoming seriously ill within 5 days of onset [6]. Even for people without risk factors, ensitrelvir is considered if symptoms are strong within 3 days of onset [6]. There are interactions with other medicines, so please bring your medication notebook [6].

The fever is high and hard to bear. Can I use a fever reducer?

Yes. Acetaminophen (for example Calonal) can bring the fever down. The clinical practice guide also says that symptom relief with fever reducers and pain relievers should be given as needed [6]. Please tell us the time you took it when you are seen.

Summary [Don’t decide by symptoms; get tested and be seen early]

Telling “influenza or COVID-19” apart is our job. If you find yourself spending time unsure, consult us first. Please leave the medical care to us.

That’s all.

Disclaimer

This article is intended to provide general medical information and does not replace an examination, diagnosis, or treatment by a doctor. The content is based on medical knowledge at the time of publication, but the appropriate response differs from person to person. If you are worried about your symptoms, please consult a medical institution.

References

1. Ebell MH, Rahmatullah I, Hulme C, Buhr M, Kotnik JH, Geyer R, et al. Accuracy of individual signs and symptoms and case definitions for the diagnosis of influenza in different age groups: a systematic review with meta-analysis. BMJ Open. 2025 Mar;15(3):e067574. doi:10.1136/bmjopen-2022-067574 PMID: 40032401
2. Struyf T, Deeks JJ, Dinnes J, Takwoingi Y, Davenport C, Leeflang MM, et al. Signs and symptoms to determine if a patient presenting in primary care or hospital outpatient settings has COVID-19. Cochrane Database Syst Rev. 2022 May;5(5):CD013665. doi:10.1002/14651858.CD013665.pub3 PMID: 35593186
3. The Japanese Association for Infectious Diseases. Outpatient care flowchart for a co-epidemic of COVID-19 and influenza (in Japanese). Ministry of Health, Labour and Welfare website. Accessed October 2026. https://www.mhlw.go.jp/content/10900000/001002376.pdf
4. Han Y, Guo J, Li X, Zhong Z. Differences in clinical characteristics between coronavirus disease 2019 (COVID-19) and influenza: a systematic review and meta-analysis. NPJ Prim Care Respir Med. 2025 Jan;35(1):8. doi:10.1038/s41533-025-00414-0 PMID: 39875405
5. Ministry of Health, Labour and Welfare. Influenza Q&A (in Japanese). Ministry of Health, Labour and Welfare website. Accessed October 2026. https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/kenkou/kekkaku-kansenshou/infulenza/QA2024.html
6. Clinical Practice Guide Editorial Committee (Health, Labour and Welfare Administration Promotion Research Project, research group). Clinical Practice Guide for COVID-19, version 10.1 (in Japanese). Ministry of Health, Labour and Welfare website. 2024. Accessed October 2026. https://www.mhlw.go.jp/content/001248424.pdf
7. Fire and Disaster Management Agency, Ministry of Internal Affairs and Communications. Ambulance use leaflet (for older adults) (in Japanese). Fire and Disaster Management Agency website. Accessed October 2026. https://www.fdma.go.jp/publication/portal/items/portal009_kourei.pdf

▶ Overview of this topic: Fever clinic on Sundays and holidays (from October 4, 2026, 10:00 to 15:00)

Related articles on fever

For the fever clinic on Sundays and holidays, see the announcement about the fever clinic on Sundays and holidays; for how to visit, see the page on the fever clinic pre-visit questionnaire and same-day check-in.