Increased risk of pneumonia, influenza, whooping cough! Measles has a terrible “secondary damage” called “immune amnesia

The spread of measles is a growing concern in Japan and abroad.

For many people, measles may be thought of as “an infectious disease that causes high fever and rash in children. However, this perception is not sufficient. In recent years, it has come to the attention of the world that measles infection in adults causes “immune amnesia,” a condition in which the immune system is reset.

An elementary school in Shinjuku, Tokyo, was closed for the school year after a mass outbreak of measles infection. The spread of the disease is not limited to one region, and as of April 21, 2026, the cumulative number of cases reported in Japan was 299; as of mid-April, this had already exceeded the number of cases reported during the previous year and is approaching the 744 cases reported in 2019.

Why is measles spreading again? And what should we do? We asked Dr. Yu Masumori, a general practitioner based in Hokkaido (” ” denotes Dr. Masumori).

Increase in inbound travel and spread of infection

There are three major factors behind the recent spread of infection.

The first is cases imported from overseas. Measles is still prevalent in Southeast Asia, the Middle East, and Europe. After the COVID-19 crisis, the movement of people returned, and the number of cases brought in increased.

The second is the decline in the domestic vaccination rate: in FY2024, the MR vaccination rate was 92.7% in the first period and 91.0% in the second period. The rate remains below the World Health Organization’s guideline of 95% for maintaining population immunity.

The third is a generational difference due to the history of system changes. In Japan, the vaccination system began in 1978 with a single dose and was changed to a two-dose system in 2006. It is no coincidence that 76% of the notifications in 2026 are concentrated among people in their teens to 40s.

It is undeniable that the recent inbound travel is a contributing factor to the spread of infection. Measles is often thought of as a children’s disease, but it can easily become serious in adults.

There are reports that when adults are infected, pneumonia and lung damage are seen in about 40% of hospitalized cases, and liver damage in about 60%. Encephalitis occurs in about 1 in 1,000 people, but can lead to sequelae and death. Infection during pregnancy also increases the risk of miscarriage, premature birth, and low birth weight. In immunosuppressed individuals, it can be life-threatening.”

What cannot be overlooked is the weakening of the immune system after infection.

Immune amnesia is a phenomenon in which, after contracting measles, the body’s memory of the immunity it has acquired from previous infections and vaccines is weakened, making it more susceptible to other infectious diseases. This is caused by damage to the cells in the body that store the immune system. For some time after recovery, the risk of other infections such as pneumonia, influenza, and whooping cough increases. Measles does not end with recovery.”

Particularly susceptible are people undergoing anticancer treatment, after organ transplants, HIV-infected patients, and those on immunosuppressive drugs for autoimmune diseases. Pregnant women are also restricted from vaccination and need to be protected. This is the “cocoon strategy.

Cocoon means cocoon. The idea is to protect people who cannot be vaccinated themselves, such as babies, pregnant women, people undergoing anticancer treatment, and immunocompromised people, by surrounding them with immunizations from family members, people who live with them, and medical personnel who can vaccinate them. If a person is unable to vaccinate himself/herself, he/she can only be protected by the immunity of those around him/her.

Society also needs to take prompt measures to prevent the spread of infection.

Protect babies under one year old, cancer patients, and pregnant women.

The closing of the Shinjuku school year is a “swift and appropriate decision,” says Dr. Masumori.

Measles is contagious even a few days before the rash appears, and it can be too late to give them a day off after symptoms appear.” It is too late to give students a day off after symptoms appear, and the infectiousness is too strong to be prevented by hand washing and masks in general, so it is important to take measures to reduce the chance of contact itself.

In addition to schools, companies are also required to be prepared.

Companies should take the perspective of a measles BCP (business continuity plan), such as checking the vaccination history of employees, providing support for those who have not been vaccinated, and establishing rules for telecommuting and dealing with contacts in the event of an outbreak. Prevention during normal times is far less expensive than post-epidemic response.”

A little over a year has passed since the Tokyo Metropolitan Government announced an outbreak of measles cases in Tokyo on January 22, 2025. In today’s world of increased international traffic, infectious diseases are no longer a local problem, but a risk directly linked to the rest of the world.

Looking back, it is undeniable that society as a whole did not have a sufficient sense of crisis at that point in time, accepting the outbreak as a “single imported case” or a “limited outbreak.

Now is the time to rethink measles as a crisis for society as a whole, not just an individual illness. There is no possibility of a local outbreak if countermeasures are not taken.

Measles is much more contagious than COVID-19 or influenza in its early stages. It is airborne and can be transmitted even before the onset of illness.
Rather than a nationwide pandemic, localized outbreaks are more likely to occur at schools, daycare facilities, dormitories, workplaces, and other places where unvaccinated or immunocompromised people congregate. Measles is not a disease of the past, and even today, an immune deficiency can cause an explosive outbreak of infection.

The generation that has been vaccinated against measles includes those in their 30s to 50s and some in their 20s, who have only received one dose of the vaccine.

The way to check is simple. If they have received two doses, it is a good reassurance. If there is no record, only one dose was given, or you are unsure, you have the option of taking an antibody test or receiving an additional dose of MR vaccine. Early confirmation is especially important for those who wish to become pregnant, those in the medical profession, and those who have family members with compromised immunity.”

Finally, Dr. Masumori emphasized the importance of consideration for infants.

The MR vaccine cannot be given to babies under one year of age. After six months of age, antibodies from the mother decrease and the baby is unprotected until the age of one.

Pregnant women, those undergoing cancer treatment, and those undergoing immunosuppressive treatment also cannot be vaccinated. Those in their 20s and 30s are the generation that raises children and is closest to such people. Your own immunization status directly affects the lives of the baby in front of you and your family members undergoing treatment.

Check your mother-child handbook, and if you do not have two records, consult your family doctor or local government. The one you are going to have to re-cut is not just for yourself. It is also one for your baby.”

Even if you are unsure of the vaccination history, please consult your health care provider.

Measles is a disease that is too late to regret after you have been exposed to it. Don’t assume that you are fine, but checking once is the first step to protect yourself and your loved ones.