Measles—one of the world’s most contagious diseases—induces immune amnesia, an anomaly of the immune system that results in increased susceptibility to infections by other pathogens. Not surprisingly, there are concerns globally about populations that have recently experienced measles outbreaks. These populations could be at elevated risk of developing more severe COVID-19 illness, resulting in increased mortality.
The emergence of the current COVID-19 pandemic in the first months of 2020 occurred closely after a global resurgence of measles. According to the World Health Organization (WHO), by November 2019, measles case numbers had tripled compared with the same period in the previous year. Measles spreads easily when an infected person coughs or sneezes and, before a vaccine became widely available in the 1960s, it caused millions of deaths worldwide annually. It is estimated that between the years 2000-2018, vaccination against measles alone saved around 23 million lives. However, measles outbreaks occurred in the past few years in all regions of the world and in many countries—from the Democratic Republic of Congo (DRC), to Samoa and Ukraine. These outbreaks, along with inadequate immunization rates, threaten to derail the crucial gains obtained in the two past decades.
Last year, in Samoa—a small island country in the central South Pacific Ocean—an unprecedented measles outbreak overwhelmed the nation’s health infrastructure, with hospitals at about 300% past capacity. Before the outbreak began, vaccination rates—already lower than the approximately 95 percent required for herd immunity—had plunged to 31%.
The first case of measles was confirmed in September 2019 and, on November 15, the government declared a state of emergency. Up to 3% of the population became infected, and 87 people died, mostly very young.
The state of emergency was lifted in December 2019. However, measles may have long-term consequences. Even after recovery, immune amnesia can persist for over 2 years.
Results from a study published a few days ago indicate that, in Samoa, immune amnesia may result in more severe cases of COVID-19. To model the morbidity and mortality impact of a potential COVID-19 epidemic in the country, researchers used data from the WHO situation reports of the measles epidemic in Samoa, and assumed that immune amnesia would be present in children infected from November 2019 onward for at least 12 months and up to 36 months.
They found that immune amnesia could increase the total number of COVID-19 cases by 8% and deaths by more than 2%. The highest rate of death would be in people older than 60 years, but a smaller peak in death could occur in younger people, with more than 15% of total deaths in the age group under 20 years old. Therefore, the researchers recommended that Samoa should rapidly achieve high rates of measles vaccination and enhance surveillance for COVID-19, as the impact may be more severe due to measles-induced immune amnesia. The researchers extended the recommendation to other severely measles-affected countries in the Pacific, Europe and elsewhere.
So, what is exactly immune amnesia? The measles virus targets cells of the immune system. It starts by invading those found in the upper respiratory tract, and it rapidly replicates inside them. Then, it spreads to organs of the immune system—bone marrow, thymus, spleen, tonsils and lymph nodes. The virus gains entry into the immune system cells by attaching to a protein called CD150, which is present on their surface. Once inside the cells, it destroys them and causes immunosuppression—in other words, it suppresses the body’s innate ability to ward off infection and disease. Children who overcome the first measles infection are protected against future measles infections, but may be more vulnerable to infections caused by other pathogens.
Indeed, the virus directly infects, among other cells of the immune system, B and T memory cells, thus dismantling the immune memory earned by fighting previous infections. Memory B and T cells remember pathogens the body has already eliminated, allowing the immune system to spring into action quickly if those pathogens are encountered again. Notably, the ability to induce immune amnesia is unique to the measles virus.
Results from a recent study show that the measles virus wipes out up to three-quarters of the protective antibodies generated in response to past microbial invaders and vaccinations. For the study, researchers analyzed blood samples using a tool called VirScan, which detects the pool of antibodies produced by a person after immunization or encounters with viruses, bacteria, or other pathogens. Therefore, the tool provides a comprehensive snapshot of acquired immunity at a particular moment.
In simple terms, the study results could be explained as follows: “If a person had 100 different antibodies against chicken pox before contracting measles, they might emerge from having measles with only 50, cutting their chicken pox protection in half. That protection could dip even lower if some of the antibodies lost are potent defenses known as neutralizing antibodies.”
According to the World Health Organization, even though a safe and cost-effective vaccine is available, there were more than 140,000 measles deaths globally in 2018, mostly among children under the age of five—the result of insufficient vaccine coverage. Previous epidemiological research into immune amnesia suggests that death rates attributed to measles could be even higher—accounting for as much as 50 percent of all childhood mortality—if researchers factored in deaths caused by infections resulting from measles’ ravaging effects on immunity.
Michael Mina, lead author of the study, said: “Imagine that your immunity against pathogens is like carrying around a book of photographs of criminals, and someone punched a bunch of holes in it. It would then be much harder to recognize that criminal if you saw them, especially if the holes are punched over important features for recognition, like the eyes or mouth.”
A previous study found that children experienced increased risk of infection and required more anti-infective prescriptions for up to 5 years following measles infection. However, “It’s important to note that, unlike measles infection, the MMR (measles, mumps, rubella) vaccine does NOT compromise previously acquired immunity.”
Therefore, widespread vaccination against measles can also potentially prevent hundreds of thousands of additional deaths caused by the lasting damage to the immune system.