1. What Causes Influenza: Type A and Type B Viruses That Cause Epidemics
Influenza is a disease caused when the influenza virus enters the body. Viruses are much smaller than bacteria and multiply inside the cells of other living things [1][3]. Influenza viruses are broadly divided into four types: A, B, C, and D. Among these, Types A and B are the ones that cause large epidemics [1]. The surface of the virus has two types of proteins called HA and NA. The body’s defense system, known as immunity, uses these two proteins as markers to attack the virus [3].
According to the Ministry of Health, Labour and Welfare, the strains currently circulating in Japan are Type A H1N1 and H3N2, and Type B Victoria lineage [2]. The numbers in "H" and "N" represent the specific types of HA and NA proteins on the surface [3]. So, what happens inside the body when the virus enters?
2. Why the Body Gets a Fever When Fighting the Influenza Virus
When the virus enters the body, the immune system starts working and releases substances called "cytokines." This triggers the production of a substance called "prostaglandin E2" inside the blood vessels of the brain [4]. This substance acts on the part of the brain that sets the body temperature, issuing a command to "raise the temperature." The body shivers to create heat and narrows blood vessels in the skin to prevent heat loss. As a result, body temperature rises [4].
It is thought that fever has a purpose. When body temperature rises, the speed at which pathogens like viruses multiply decreases, and the attacking power of immune cells like white blood cells is believed to increase. Fever is a defensive reaction using the body’s temperature regulation system [4]. When the infection stops and the effect of prostaglandin E2 disappears, the temperature set point returns to normal, and the body releases heat. This is why you sweat when your fever breaks [4].
The symptoms of influenza are characterized by the sudden onset of fever above 38°C (100.4°F), headache, joint pain, muscle pain, and general fatigue [1]. How does the virus spread from a person with a fever to others? And what are the rules for staying home from school?
3. How Influenza Spreads and When Students Must Stay Home from School
Transmission occurs through "droplet infection," where people inhale viruses mixed in droplets from coughs or sneezes, and "contact infection," where people touch their mouth or nose with hands that have viruses on them. The virus is released from the nose and throat from the day before symptoms appear until about 3 to 7 days after [1].
To prevent infection from spreading at school, there are rules for how long students must stay home. Under the School Health and Safety Act, students must stay home until 5 days have passed since the onset of symptoms, and 2 days have passed since the fever has gone down (3 days for young children) [2]. The way symptoms appear and how to count the days for staying home depend on the instructions of doctors and schools. Influenza repeats its epidemics almost every year. What happens to the virus each year? What are the differences between Type A and Type B?
Safety Note: According to the School Health and Safety Act, the period for being absent from school is defined as "until 5 days have passed since the onset, and 2 days (3 days for young children) have passed since the fever has gone down" [2]. Always follow the specific instructions from your school and doctor.
4. How Type A and Type B Viruses Differ, and Why Type A Can Cause Pandemics
The materials clearly show three differences between Type A and Type B: Type A is distinguished by "subtypes," while Type B is distinguished by "lineages"; Type A is also found in pigs and birds; and the major changes that lead to pandemics are explained for Type A.
Influenza viruses change their appearance slightly every year. Even within the same subtype, genetic mutations accumulate, causing HA and NA to change gradually. This is called "antigenic drift." Because this happens frequently, epidemics repeat every year [3].
Type A can change its appearance significantly. This is called "antigenic shift," where a virus of a different subtype appears. This mechanism is what causes a pandemic (a global epidemic) [3]. Type A is found in humans, as well as pigs and birds [3]. Type B is distinguished by "lineages" rather than HA and NA subtypes. The Victoria lineage is currently circulating [2]. So, when and how did vaccines for preventing influenza come to be used?
5. The History of Influenza Vaccines in Japan and How Well They Work
We trace the history of how vaccines were used in Japan and how vaccination rules changed, year by year, after the virus was discovered. The influenza virus in humans was first isolated in 1933. In Japan, the first "inactivated vaccine," which renders the virus inactive, was sold in 1951 [5].
In 1962, group vaccinations for elementary, junior high, and high school students began. In 1994, influenza was removed from the list of diseases covered by the Vaccination Law. In November 2001, a law revision added elderly people to the list of those eligible for regular vaccinations [6].
The Ministry of Health, Labour and Welfare explains that vaccines have the effect of lowering the chance of developing symptoms if infected and preventing severe illness [1]. The Q&A section states that in residents of elderly welfare facilities aged 65 and older, the "effect of preventing illness" is 34 to 55%, and the "effect of preventing death" is 82%. For children under 6 in the 2015/16 season, the effectiveness rate was 60%. These are figures from studies with limited conditions. It is also recommended to finish vaccination by mid-December [2]. So, how have medicines for after catching the flu progressed?
Safety Note: In Japan, between 1919 and 1920, a "Pfeiffer's bacillus vaccine" was administered to over 200,000 people, but the cause of the illness was misidentified [5].
6. How Influenza Medicines Developed: From Stopping Spread to Stopping Multiplication
Influenza medicines can be organized by where they stop the virus's movement. We look at two types in the order they became available in Japan.
In February 2001, Tamiflu (oseltamivir) was released in Japan. This medicine interferes with the action of an enzyme called "neuraminidase," which is necessary for the virus to leave infected cells and spread to other cells [8]. Relenza, Rapiacta, and Inavir are in the same family [7]. Rapiacta is an intravenous drug approved in January 2010 [9]. Inavir is an inhalation drug approved in September 2010 [10].
Xofluza (baloxavir), approved in February 2018, is a new type of oral medicine that interferes with the enzyme needed for the virus to multiply [11].
The Ministry of Health, Labour and Welfare explains that starting anti-influenza drugs within 48 hours of symptom onset usually shortens the fever period by 1 to 2 days [1]. Also, although the causal relationship with the medicine is not known, abnormal behaviors such as falling have been reported in children regardless of whether they took medicine or which type. For 2 days after the fever starts, take measures to prevent accidents like falling [2]. Whether to use medicine depends on the judgment of a doctor or pharmacist.
7. How to Investigate Influenza Further Using Sources and Searches
You can verify the content of this notebook on the listed sources yourself. Today, you can search for "prostaglandin E2" to find diagrams explaining the mechanism of fever. Explanations about the brain issuing commands to raise the temperature set point are found in specialized commentaries [4]. You can also investigate what the name "H3N2" represents. You can find out which number of HA and NA it refers to in the commentary by the National Institute of Infectious Diseases [3].
If you write down just one term that interests you, it can become an entry point for later research. Terms like "cytokine" or "pandemic" are enough.