Basic Knowledge of Measles

🔹 Latest Trend Status (as of April 17, 2026)

・In 2026, there has been an increase in measles reports. National cumulative cases from weeks 1-16 are 362. is reached (preliminary figures).
This is the highest level in recent years, and in addition to cases brought in from overseas, secondary infections within the country have also been confirmed.
・According to official Tokyo Metropolitan Government data (updated April 23, 2026), The majority of patients are concentrated in the late teens to early thirties.and further A certain number of patients in their 30s and 40s(Reported by the Tokyo Metropolitan Infectious Disease Surveillance Center.)
📣 The Japan Pediatric Society issues a warning effective April 1, 2026. and announced, "Ensure two vaccinations for those aged 1 and pre-schoolers" and called for "prompt vaccination for adults without immunity."

🔹 ① What is measles?

Measles (hives) is an infectious disease caused by the measles virus,Airborne transmissionis a highly contagious disease. If unvaccinated people are in the same space,Approximately 901 TP3T were infectedIt is said that.

🔹 Transmission route

・Airborne transmission (strongest) ・Droplet transmission ・Contact transmission
Viruses that spread through coughing or sneezingCan remain in the air for up to 2 hoursI will.

Progress of symptoms

Incubation period: 10-14 days (7-21 days)
・Prodromal phase (2-4 days) ・High fever ・Cough ・Runny nose ・Conjunctivitis ・Koplik spots
・Rash phase ・ Rash appears 2-4 days after the onset of fever ・ Spreads from face to head, then to the body, and finally to the limbs ・ Rash lasts for 4-7 days
Fever is usually 5 to 8 days To be continued
・Other symptoms ・Diarrhea (may appear early and persist)

Infectious period

4 days before rash onset to 4 days after rash onset

🔹 Suspension from Attendance (Japanese Standards)

・Until 3 days after fever subsides School Health and Safety Act

Complications

・Complications occur in approximately 30% of measles patients. ・Pneumonia ・Otitis media ・Diarrhea ・Encephalitis (1 in 1,000)

🔹 ② Measles Immunity (Vaccination)

Vaccine efficacy

・1 dose: 93–961 TP3T 2 doses: 97–991 TP3T

🔹 No additional dose is needed after the second vaccination.

Even if antibody levels appear low, Memory B cells remain, allowing for rapid antibody production upon re-exposure.CDC, WHO, and all Japanese manuals state that "additional doses are not necessary."

📘 Measles: Understanding "Age-Specific Risks" Seen in the 2026 Epidemic

In Japan, the vaccination system has changed, so there are differences in the number of vaccine doses received depending on the age group.
Born October 1972 - April 1990 (Ages 36-53) → Only one dose. No catch-up.
Born between April 1990 and April 2000 (ages 26-36) One dose is recommended by the system. Many people have not received the catch-up vaccination.
Born between April 2000 and April 2006 (ages 20-26) One dose is standard. Some people received a second dose voluntarily, but individual differences are significant.
Since April 2006 (under 19 years old) Immunity is stable with a two-dose system.

“The ”actual risk structure' that cannot be explained by vaccination rates alone"

🔹 Why is the 2026 epidemic concentrated among people in their teens and twenties?
In 2026 trends, even with age-specific data from Tokyo The majority of patients are concentrated in the late teens to early thirties. Despite this generation often being “fully vaccinated” by the system, a certain number of people exist who are "insufficiently immune" for the following reasons.

① Immune exhaustion

Even after two doses, antibody titers decrease annually.
Antibody titer 4.8–9.71 TP3T per year Report of decline
After vaccination 10 to 20 years There are people who fall below the defense level
Even among young adults in Japan, even though they’ve already received two doses, There are many cases that do not meet the IgG reference values.い
Especially Born between 2001 and 2011 (currently ages 15-25) This is the period when immunity tends to decrease.

② An era where natural boosters are unavailable

In Japan, where measles has been eliminated, there is almost no opportunity to come into contact with wild strains.
As a result, Immunity enhancement (booster) from natural infection does not occur Antibodies continue to decrease, leading to weakened immunity even in younger generations.

③ Young people have many opportunities for exposure (school, university, workplace).

Measles is airborne and highly contagious. School, university, dormitory, workplace Group environments like these are prone to the spread of infection.
The 10s to 20s are involved in classes, club activities, part-time jobs, circles, dormitory life, etc.A lifestyle with frequent close contact Because it is there, infection tends to spread rapidly.

④ The input examples are biased towards young people.

In Tokyo, multiple imported cases of B3, D8, and others have been confirmed in genetic data.
The median age in the input example is Late teens to early twenties。
The reasons are that opportunities to travel are concentrated among young people, such as for study abroad, overseas travel, international exchange, and working holidays.

Input examples occurring in younger age groups tend to spread to schools and universities.

Institutionally, it tends to be explained as "two doses = safe," but This current trend cannot be explained by that.

The reason why patients are often in their teens and twenties is ① Immunosuppression, ② Lack of natural boosters, ③ Population exposure, ④ Bias in imported cases It is due to these complex factors.

🔹Why there are fewer actual patients aged 36-53 (born 1972-1990)
Institutionally, it is considered an immunity gap for those who have only received one dose, but the actual number of patients is lower than in younger age groups. The reasons for this are as follows:
① Even with a single dose, protection ranges from 93 to 941 TP3T; this applies not only to completely unvaccinated individuals but also to susceptible individuals, with protection levels ranging from approximately 6 to 71 TP3T.
Younger people are less likely to be exposed in groups because they are not in environments like schools, universities, or dormitories, which reduces their opportunities to encounter places where infections spread.
③ Touchpoints for international travel are concentrated among young people. The majority of import cases involve young people, while middle-aged people tend to travel less frequently.

🔹 Revised Edition: Age-Specific “Actual Risk Classification” (2026)

In the 2026 trends, the institutional vaccination history does not match the actual number of patients. The following is a breakdown by age group based on “actual risks.”

Age (2026)	Institutional vaccination record	Actual risk	Main reason
15-25 years old	Two doses	The most patients	Immune senescence, population exposure, overseas travel, lack of natural boosters
26-36 years old	One time plus part of a second time	High risk	Sensitivity 6–71 TP3T, Immune Suppression, Social Activities
36-53 years old	Once	Institutionally high risk, but with a moderate number of patients.	Immunocompromised but with low exposure risk
53 or older	Natural infection	Low risk	Lifelong immunity

6. Who should consider a booster shot and why

Who should consider a measles booster shot and why

Medical, childcare, and education professionals. Reason: Frequent close contact with patients, children, and students necessitates preventing secondary infections within groups. It is the international standard for healthcare professionals to receive two vaccinations plus immunity confirmation, regardless of birth year.
📖 Born between 1972 and 2006. Reason: For those born between 1972 and 1990, only one dose was administered, resulting in susceptibility levels of 6–71 TP3T. For those born between 1990 and 2006, there is a possibility they did not receive a second dose. Furthermore, even if they received two doses, immunity may have waned more than 10 years after vaccination, making this the age group that requires the most verification.
Women hoping to become pregnant. Reason: MR vaccination is not allowed during pregnancy, so immunity confirmation is necessary before pregnancy. This is also important from the perspective of rubella prevention.
🌍 International Travelers Reason: The majority of import cases are among young people, increasing their risk of exposure at their destination. International standards require two doses.
🔍 Individuals with unknown vaccination history. Reason: Verbal declarations do not serve as proof of immunity, and if verification through documentation is not possible, the principle is to treat them as unvaccinated.
University and vocational school students. Reason: High risk of exposure in group living situations such as dormitories, lecture halls, and club activities, which aligns with the trends popular among people in their teens and twenties.
Family members/cohabitants of immunocompromised individuals. Reason: The individual may have a contraindication to vaccination, and it is necessary to indirectly protect them by ensuring those around them are immune.

The number of vaccine doses alone cannot be used to assess risk.
The prevailing trends among those in their teens and twenties are primarily driven by waning immunity and skewed exposure opportunities.
The one-dose vaccination generation tends to have high institutional risks but a low number of actual patients.
Who should consider a booster shot is determined by factors such as exposure risk, weakened immunity, pregnancy plans, and travel history.

Post-exposure emergency response

・What to do if you come into contact with a measles patient: ① Pregnancy status ② Age (in months) ③ Vaccination History ④ Immune status (presence or absence of immunodeficiency) It changes greatly.

🔹Adults not pregnant (including breastfeeding)

2 doses of MR vaccine (with records)

1. Booster shots are not necessary.
   ・No antibody testing is required due to immunological memory.
   ・Observation only

1 dose only / Vaccination history unknown

• Within 72 hours: MR vaccination is the first choice
  Can be vaccinated while breastfeeding
  ・If more than 72 hours have passed, the vaccine's effectiveness becomes uncertain, so Immunoglobulins (IG) considerations

3) Immunodeficiency (e.g., high-dose steroids, chemotherapy)

• MR vaccine is contraindicated
  Immunoglobulin (IVIg) within 6 days
  Immunocompromised individuals are at high risk of severe illness.

Pregnant woman

MR vaccine is contraindicated during pregnancy.

• If exposed to measlesImmunoglobulin (IVIg) within 6 days is the only option
  Considering the potential impact on the fetus, prompt action is required.
  Postpartum MR vaccination is possible (also possible while breastfeeding).

🔹 Infants (0-11 months)

0-5 months (under 6 months)

• Vaccination not possible
  Immunoglobulin (IGIM) within 6 days
  Infants are prone to severe illness and require the highest priority for protection.

2) 6-11 months

• Within 72 hours: Emergency MMR vaccination possible
  However, this is a "booster shot," andAfter 12 months, it is necessary to repeat two regular vaccinations.
  If it has been over 72 hours Immunoglobulin (IGIM)

Children 1 year and older

2 doses of MR vaccine received

• No additional dose needed
  Observation only

2) Single dose only

• 2nd MMR vaccine within 72 hours
  If more than 72 hours have passed, the vaccine's effectiveness is uncertain.Investigating Immunoglobulin

3) Vaccination history unknown

• Within 72 hours: MR vaccination
  The harm from excessive vaccination is almost nil.

🔹If there are multiple immunocompromised individuals in the household

Since the household infection rate is very high, Check vaccination history for all cohabiting family members.And do the following:
No immunity MR vaccine within 72 hours
Pregnant women and infants 免疫グロブリン
2回接種済み → 経過観察のみ

Literature

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