Immune amnesia - when your immune system forgets

What is immune amnesia?

Immune amnesia is a phenomenon that many of us have never heard of despite it being observed for more than 100 years.

Immune amnesia is caused by the measles virus (rubeola), which can partially wipe out your immune system memory. In the aftermath of a measles infection you can be left vulnerable to other infections and may need to re-learn how to fight off many of the pathogens or germs that your body once recognized.

Immune amnesia is thought to leave you susceptible to catching other infections for about two to three years, during which time your immunity has to be slowly rebuilt as you encounter different pathogens.

Measles is a highly infectious airborne viral illness that is thought to be eight times more contagious than the original SARs-CoV-2 virus, which causes COVID-19.

The symptoms of measles include a fever, rash, runny nose, cough and red watery eyes. It can also lead to ear infections, diarrhea and serious complications such as pneumonia and brain damage, as well as death.

How was immune amnesia discovered?

For a long time we’ve known that many of the deaths linked to measles aren’t caused by the virus itself, but by the subsequent infections that follow. It took years of investigation by different researchers to uncover the reason for this and finally give immune amnesia its name.

In 1908, pediatrician Clemens von Pirquet published a report linking measles infection with subsequent suppression of the immune system. von Pirquet observed that children who had previously produced a positive cutaneous tuberculin reaction - indicating they’d been infected with tuberculosis (TB) - didn’t react to the test while infected with measles. Measles was also frequently associated with worsening TB infection in such children.

Many years later in 2002, researchers discovered that measles did not bind to cells in the respiratory tract, as you might expect of a respiratory virus, but instead bound to CD150 - a receptor found on immune system cells.

In 2012, immune amnesia was finally given a name. Researchers studying measles in macaque monkeys found that the virus infects the immune system cells that remember what you’ve been infected with before. When your body fights off the measles virus these cells are also killed - leading to immune amnesia.

Measles attacks memory B-cells and T-cells (also called lymphocytes), which are the part of the immune system that remembers previous infections and helps protect against future attacks. B-cells produce antibodies to ward off bacteria and viruses, while two different types of T cells - helper T-cells and killer T-cells - help stimulate B-cells, help killer T cells develop and kill cells that have become infected.

Despite causing suppression of the immune system, infection with the measles virus results in life-long immunity against measles itself.

Measles wipes out pre-existing antibodies in children and makes them vulnerable to subsequent infections

Researchers have continued to study measles in children to learn more about immune amnesia.

One study measured antibody levels in 77 children 2 months after infection with measles. The study found that 11-73 percent of their antibodies against other pathogens - bacteria and viruses - had been eliminated. Subsequent exposure to the pathogens resulted in antibody recovery.

Another study compared the medical records of 2228 children who’d caught measles with the records of 19,930 children who had not. The study found that children who had had measles were significantly more likely to pick up other infections during the 5 years that they were followed up.

Upper respiratory tract infections were the most common type of subsequent infection. In the first month after infection with measles the children were 43 percent more likely to get another infection compared with children who hadn’t contracted the virus. Even 2.5-5 years after infection the children who had had measles were still 15 percent more likely to get another infection.

Children who had had measles also required more prescriptions for anti-infective drug treatments compared with those who hadn’t contracted measles.

Measles vaccination likely protects against immune amnesia

Vaccinating children against measles has the unexpected benefit of reducing childhood death, more so than would be expected from preventing measles alone. A study conducted in children in developing countries found that immunization with a measles vaccine reduced the risk of death from all causes in the months or years that followed by between 30 to 86 percent.

Before the discovery of immune amnesia it was thought that the measles vaccine protected against measles and probably gave a general boost to the immune system - it was thought that this accounted for the reduction in childhood deaths. Now it appears likely that the additional benefit the vaccine provides stems from its ability to prevent immune amnesia. Antibody elimination is not seen in children who have been vaccinated against measles, unlike in children who become infected with the virus.

Declining measles vaccination rates may contribute to an increase in immune amnesia

Before the global COVID-19 pandemic arrived there were already concerns about declining measles vaccination rates among children. With an estimated 22 million infants missing their first dose of a measles-containing vaccine in 2020 - up 3 million on the year before - concern has only continued to grow.

The possibility of a measles outbreak and subsequent increase in immune amnesia among children is expected to increase as mitigation measures are removed for COVID-19, which is also an airborne virus.