If you are in a hurry
- The onset date is the day symptoms first appeared. That day is day 0, and the next day is day 1. It is not the day of the test or the doctor’s visit[1][2].
- If the person does not respond when called, a seizure does not stop, or breathing is so hard that they cannot speak, call 119 (Japan’s emergency number) without hesitation.
- A test can miss infection within 12 hours of onset, and the guide for starting an anti-influenza drug is within 48 hours[3][4]. Please see a doctor even if that time has passed. Fever can be brought down with Calonal and similar medicines.
Hello. I am Shunichi Gamo, MD, PhD, a board-certified pulmonologist and the director of Sendai Airport North Clinic.
“The fever started Monday night, but I went to the doctor on Tuesday. Which day is the onset date?” This is a question we are asked very often in the examination room.
The “onset date” comes up everywhere: when notifying school, reporting to your workplace, or checking how long you can take a medicine. Yet surprisingly few people know how to count it correctly.
To state the conclusion first, the onset date is the day symptoms first appeared; that day is “day 0,” and the next day is “day 1”[1]. It is not the day you were tested, or the day you learned you were positive.
In this article, as a pulmonologist, I explain the official way of counting in Japan, when to get tested, how to think about the 48 hours for anti-influenza drugs, and how to decide when symptoms are unclear.
First, a note about our Sunday and holiday fever clinic. For influenza and COVID-19, PCR testing (a test that copies the virus’s genetic material to detect it) gives results the same day.
From October 4, 2026, we also run the fever clinic on Sundays and holidays
10:00 to 15:00 (no lunch break). No reservation is needed; you check in on the day. We give you the PCR result the same day. On Sundays and holidays we see fever patients only, and we do not offer regular outpatient care or IV drips. We are closed on Sunday, October 25, and from November we are open, in principle, every Sunday and holiday. Announcement: Sunday and holiday fever clinic
Shunichi Gamo, MD, PhD
Director, Sendai International Airport North Clinic
Born and raised locally, he attended Yuriage Elementary School and Yuriage Junior High School in Natori City. After graduating from Tohoku University School of Medicine, he worked at Sendai Open Hospital and Kesennuma City Hospital, and then at Tohoku University Hospital. In April 2021, in the middle of the COVID-19 pandemic, he opened Sendai International Airport North Clinic. Holding to a fever clinic that never turns patients away, he cared for well over 20,000 patients with fever in the two years before COVID-19 was moved to Class 5 under Japan’s infectious disease law.
Society memberships and board certifications
Certified Member and Board Certified Fellow, Japanese Society of Internal Medicine
Board Certified Pulmonologist, Japanese Respiratory Society
Board Certified Allergist, Japanese Society of Allergology
Board Certified Specialist, Japanese Society of Anti-Aging Medicine
PhD in Medicine (Tohoku University)
[The short answer] What is the “onset date”? How do you count day 0?
The onset date is the day the symptoms typical of the illness first appeared, such as fever, sore throat, cough or fatigue. That day is counted as day 0[1].
For influenza, “onset” means the appearance of fever; the day of onset is not included in the count, and the next day is counted as day 1[2]. For COVID-19 as well, the Ministry of Health, Labour and Welfare counts the onset date as day 0 and gives a period of 5 days[1].
- [Day 0]
The day symptoms first appeared. Not the day of the visit or the test - [Day 1]
The day after day 0. From here you count “how many days have passed” - [Positive without symptoms]
If there are no symptoms, the day the sample (the fluid from the nose or throat used for the test) was taken is day 0[1]
Dr. GamoIt is easy to remember this way: if the fever starts in the evening, before the date changes, that day is day 0.
How are the days counted for school and work? [Influenza and COVID-19]
Exclusion from school is set by the Enforcement Regulations of the School Health and Safety Act (the national rules that set out infectious disease measures in schools).
For influenza, it is “until 5 days have passed after onset and 2 days have passed after the fever went down (3 days for preschool children)”[5].
For COVID-19, it is “until 5 days have passed after onset and 1 day has passed after symptoms improved”[1].
For COVID-19, the Ministry of Education, Culture, Sports, Science and Technology (MEXT) indicates that phrases such as “5 days have passed after onset” are counted from the day after the day of onset or the day after symptoms improved[6].
| Influenza | COVID-19 | |
|---|---|---|
| Starting point | The day of onset is day 0[2] | The onset date is day 0[1] |
| Guide for exclusion from school | Until 5 days have passed after onset and 2 days have passed after the fever went down (3 days for preschool children)[5] | Until 5 days have passed after onset and 1 day has passed after symptoms improved[1] |
| How to count | The next day is day 1[2] | Counted from the next day[6] |
For example, suppose a fever starts on a Monday. Monday is day 0 and Saturday is day 5. Five days have then passed, so Sunday is the earliest day to return.
If the fever goes down on Wednesday, 2 days pass with Thursday and Friday, so the calculation allows a return from Sunday. If the fever goes down on Friday, 2 days pass with Saturday and Sunday, so the return is from Monday.
Dr. GamoIf you write “Monday night, fever = day 0” on your home calendar, contacting the school goes more smoothly.
Note that there is no need for another test before returning, and a certificate of recovery or a negative test certificate is not required[4]. For the final decision, please follow the instructions of your school or workplace.
How do you decide the onset date when symptoms are unclear?
When symptoms are unclear, please tell your doctor the day you clearly felt “different from usual” as the onset date.
For influenza, the day the fever began is the standard[2]. If a mild fever or fatigue came first, it is easier to decide if you sort it out as follows.
- Look back at your temperature records, phone notes and conversations with family, and find “the first day I thought something was off”
- If you felt tired or had a throat discomfort before the fever, say so as it was: “the tiredness started Monday, the fever Tuesday”
- If symptoms started at night, note the approximate time as well as the date (to work out the 48 hours for medicine)
- Even if a fever reducer brings the fever down, the onset date does not change
- If you really cannot remember, do not force a decision; just say, “probably from around this time”
Once we have an idea of the date and time, we can move on to the timing of tests and a discussion of medicine. Even if you cannot remember exactly, you do not need to hesitate to see a doctor. At the examination, the doctor makes an overall judgment from the course of illness and the examination findings together.
Dr. GamoI often hear people say, “I find it hard to come because I am not sure of the onset date.” Please do not worry; we will sort it out together on the spot.
How many hours after onset is it best to be tested?
A rapid influenza test (a simple test using fluid from the nose or throat) can miss infection within 12 hours of onset[3][7].
In a Japanese study, the sensitivity (the proportion of sick people the test detects) of the rapid test within 12 hours of fever onset was 44.4%[3]. A study in children also reported 37.5% at 6 hours and 60.9% at 12 hours after fever onset[7].
On the other hand, within 48 hours of onset is considered the appropriate time to start an anti-influenza drug[4]. While there is a period when the test can miss infection, the medicine has a guide of 48 hours from onset, so if you are unsure, please see a doctor early.
| Time since onset | How to think about it |
|---|---|
| Before 12 hours | The test can miss infection[3][7]. If you are suffering, see a doctor and talk to us |
| 12 to 48 hours | There are reports that infection is easier to detect than within 12 hours[7]. You are also still in time for the 48-hour guide for starting medicine[4] |
| After 48 hours | It is still fine to be examined. There is still much we can do: diagnosis, fluids, fever reducers and checking for complications |
Even if the first test is negative, it may simply have been too early. If symptoms continue, we recommend talking to us again. Whether to test is decided by the doctor, also taking into account your symptoms and the local outbreak.
From when is the antiviral “48 hours” counted?
The basic rule is that the 48 hours are counted from the time symptoms began. The Ministry of Health, Labour and Welfare explains that starting an anti-influenza virus drug within 48 hours of onset usually shortens the fever by 1 to 2 days[4].
For example, if the fever started on Monday at 6 p.m., the guide is up to Wednesday at 6 p.m. Counting by hours rather than by dates is the difference from how day 0 is counted.
The main anti-influenza drugs available in Japan are as follows[4]. The doctor decides which drug to use after looking at age and symptoms.
| Generic name | Example brand name | How it is taken |
|---|---|---|
| Oseltamivir | Tamiflu | Oral |
| Zanamivir | Relenza | Inhaled |
| Laninamivir | Inavir | Inhaled |
| Baloxavir | Xofluza | Oral |
| Peramivir | Rapiacta | IV drip |
For baloxavir, recommendations on how to use it differ by age. The doctor decides based on the recommendations of the Japanese Association for Infectious Diseases and the Japan Pediatric Society[4].
COVID-19 medicines also have a deadline for starting. Lagevrio and Paxlovid must be taken within 5 days of onset, and Xocova tablets within 3 days[4]. Here too, the way of counting day 0 is useful.
Even after 48 hours, there is a reason to see a doctor
Please see a doctor even if more than 48 hours have passed since onset. We can make the diagnosis and check for complications such as pneumonia or dehydration.
Fever and sore throat can be eased with acetaminophen (such as Calonal). We will also guide you on how to use the medicine at the examination. Older people, people with chronic conditions, and pregnant women should talk to us especially early.
Anti-influenza drugs are not essential for everyone, and whether to use one is the doctor’s decision[4]. If you are unsure whether 48 hours have passed, please think of it as “talk to us first.”
Dr. GamoPlease do not think, “It has been 2 days already, so there is no point in going.” There are other ways to ease the hard symptoms.
[Signs to see a doctor quickly] Call 119, go to an emergency department, or be seen the same day
Call 119 right away
If even one of the following applies, call 119 without hesitation, regardless of the onset date or how high the fever is. This is the same at night and on holidays.
- Does not respond when called, or is dazed
- Breathing is so hard that the person cannot speak, is breathing with the shoulders, or the lips look purplish
- A seizure that does not stop
- Sudden severe chest pain, or a feeling of fainting
Go to an emergency department the same day, even at night or on a holiday
- Can hardly drink, or has passed no urine for half a day or more
- Even at rest, breathing is clearly faster than usual, or the person looks to be in distress
- Cold hands and feet, a pale or ashen face, and limpness
- A child or teenager suddenly runs off, tries to rush out of the room, or shows other abnormal behavior
When a child or teenager with influenza is recovering at home, care is needed so that they are not left alone for at least 2 days after treatment starts[4].
Otherwise, go to a fever clinic or similar the same day (the next morning if it is night)
- More than 12 hours have passed since the fever began and you want to be tested
- The fever does not go down, or cough or breathlessness is getting stronger
- Influenza or COVID-19 is going around in the family, workplace or school
- An older person, a person with a chronic condition, or a pregnant woman has fever or cough
If the person’s condition changes during the night, go back to the “119” and “emergency department” guides above at that moment and judge again.
Frequently asked questions
- Is the onset date the day I saw the doctor?
-
No. The onset date is the day symptoms first appeared, and that day is day 0[1]. Even if the visit or test happens on a later day, the onset date does not change.
- What should I do if I do not know the onset date?
-
Use “the day you felt different from usual” as a guide, and tell the doctor just as it was, for example “the tiredness started Monday, the fever Tuesday.” For influenza, the day the fever began is the standard[2].
- Is there no point in seeing a doctor once more than 48 hours have passed since onset?
-
That is not so. We can make the diagnosis, check for complications, and advise on how to drink fluids. Fever can be eased with acetaminophen (such as Calonal). Whether to use an anti-influenza drug is the doctor’s decision[4].
- Should I be tested as soon as the fever starts?
-
A rapid influenza test can miss infection within 12 hours of onset[3]. Still, if you are suffering, it is fine to see a doctor without waiting. If needed, we will consider testing after some time has passed.
- Can my child go back to school as soon as the influenza fever goes down?
-
The fever going down alone is not enough. Exclusion from school lasts until 5 days have passed after onset and 2 days have passed after the fever went down (3 days for preschool children)[5]. The exception is when a doctor finds there is no risk of infection[5].
Summary: The onset date is the day symptoms appeared, which is day 0
- The onset date is the day symptoms first appeared. That day is day 0, and the next day is day 1. It is not the day of the test or the doctor’s visit[1][2].
- If the person does not respond when called, a seizure does not stop, or breathing is so hard that they cannot speak, call 119 without hesitation.
- A test can miss infection within 12 hours of onset, and the guide for starting an anti-influenza drug is within 48 hours[3][4]. Please see a doctor even if that time has passed. Fever can be brought down with Calonal and similar medicines.
- Exclusion from school is counted from the next day. For influenza, it is 5 days after onset and 2 days after the fever goes down (3 days for preschool children)[5][2].
- If the onset date is unclear, tell the doctor “the day it felt different” and the course of illness just as it was.
- If the person cannot drink, breathes fast, or is limp, go to an emergency department even at night.
The onset date is an important starting point that decides treatment, school and work. If you are unsure, please tell us just as it is. Even if 48 hours have passed, please do not hesitate to see a doctor. Please leave the medical side 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
- Ministry of Health, Labour and Welfare. Q&A on recuperation from COVID-19 (in Japanese). Ministry of Health, Labour and Welfare website. Accessed October 2026. https://www.mhlw.go.jp/content/10900000/001093929.pdf
- Ministry of Health, Labour and Welfare. Guidelines for infectious disease measures in nursery schools, 2012 revised edition (in Japanese). Ministry of Health, Labour and Welfare website. Accessed October 2026. https://www.mhlw.go.jp/file/05-Shingikai-11921000-Kodomokateikyoku-Soumuka/sankou.pdf
- Boku S, Naito T, Murai K, Tanei M, Inui A, Nisimura H, et al. Near point-of-care administration by the attending physician of the rapid influenza antigen detection immunochromatography test and the fully automated respiratory virus nucleic acid test: contribution to patient management. Diagn Microbiol Infect Dis. 2013 Aug;76(4):445-9. doi:10.1016/j.diagmicrobio.2013.04.029 PMID: 23743175
- Ministry of Health, Labour and Welfare. Q&A on comprehensive measures against acute respiratory infections (ARI), fiscal year 2025 (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/QA2025.html
- Enforcement Regulations of the School Health and Safety Act (Ministry of Education Ordinance No. 18 of 1958), Article 19 (in Japanese). e-Gov Law Search. Accessed October 2026. https://laws.e-gov.go.jp/law/333M50000080018
- Ministry of Education, Culture, Sports, Science and Technology. On the enforcement of the ministerial ordinance partially amending the Enforcement Regulations of the School Health and Safety Act (Notice No. 345 of fiscal year 2023, April 28, 2023) (in Japanese). MEXT website. Accessed October 2026. https://www.mext.go.jp/content/20230427-mxt_ope01-000004520_2.pdf
- Nakao A, Hisata K, Matsunaga N, Fujimori M, Yoshikawa N, Komatsu M, et al. The clinical utility of a near patient care rapid microarray-based diagnostic test for influenza and respiratory syncytial virus infections in the pediatric setting. Diagn Microbiol Infect Dis. 2014 Apr;78(4):363-7. doi:10.1016/j.diagmicrobio.2013.11.005 PMID: 24582577
▶ Overview of this topic: Sunday and holiday fever clinic (from October 4, 2026, 10:00 to 15:00)
More articles to read when you have a fever
For our Sunday and holiday fever clinic, please see the announcement about the Sunday and holiday fever clinic. For how to come in, please see about the pre-visit questionnaire and same-day check-in for the fever clinic.
