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Why Measles Is So Contagious

An outbreak on a plane back in 2010 illustrates why this virus spreads so easily

Even if he had been aware of what was happening inside the girl’s body, the man likely would not have been worried. She was, after all, sitting quite a long ways away.

The girl was 11 years old. She was a refugee from Malawi, a narrow African country surrounded by Zambia, Tanzania, and Mozambique. She had flown from Malawi to South Africa earlier that day and was now on a 12-hour flight to Australia. The day before the trip began, she had started to feel a bit sick.

She would be hospitalized in Victoria the day after landing. Her doctor would notice an infection of the middle ear and a reduced appetite. Laboratory tests would come back positive for measles, but hospital staff would assume it was because of the vaccine. Five days before leaving Malawi, the girl had received the measles vaccine. A weakened version of the virus was thus in her system. But when the laboratory performed a more advanced test, they saw that the measles virus she had tested positive for was not the one found in the vaccine; it was the one found in the wild, able to rapidly infect anyone who is not immune to it.

Ten days after that plane landed in Australia, the man started developing symptoms. He was a 29-year-old resident of the United Kingdom with no documented history of receiving the measles vaccine. As the three Australian state health departments in charge of the investigation tried to figure out what had happened, they first believed the man had been sitting in the row in front of the Malawian girl, whose seat had been 47E. They then realized the man’s seat number had been flipped during the investigation. He hadn’t sat in 46J but in 64J… a whole sixteen rows away from her. From her seat facing the toilets, you had to walk past nine rows, a galley, an emergency exit, and seven more rows before reaching the man’s seat.

A few other passengers from this 2010 flight contracted measles, all within a handful of rows from the girl. The British man looks like an anomaly. It is possible he breathed in the virus when going to the bathroom or while waiting in line to board the plane. Still, this episode and many more like it showcase the power of the measles virus: it is one of the most contagious viruses to infect humans.

Even before COVID-19 made us realize that many more infections can be transmitted via airborne aerosols, measles was widely known to stay in the air for hours. These aerosols are particularly stable when the relative humidity is either low or quite high. The dry air of a plane cabin certainly helps the virus.

In people who are not immune to it, measles causes flu-like symptoms, pinkeye, and a distinct red skin rash. While many recover from it, nearly one in three will develop complications, which can mean breathing problems and inflammation of the brain. It is, to this day, a leading cause of death for young children worldwide, as vaccination rates in many countries are far too low.

Part of its lethality can be explained by how efficiently it can jump from one person to another. Research conducted in the last fifteen years has exposed the peculiarities of the measles virus and how it interacts with the human body, its only known host.

By shrinking ourselves down to the size of this vicious virus, we can understand what makes it so contagious and what might have happened on that flight from South Africa to Australia.

Measles lingers in the air

The measles virus is a little ball, too small to be seen under a regular microscope. Inside the ball, coiled like a snake, is a molecule of RNA which codes for the proteins needed to make more copies of the virus. Its cousins include the mumps virus and canine distemper virus, which gives dogs a distinct jaw movement known as “chewing gum fits.”

Prior to her trip, the girl became infected by the measles virus, probably breathing it in from air that had been contaminated with it. The fact that measles can be breathed in already gives it an advantage over other viruses like HIV, which require the mingling of bodily fluids. A person sick with measles can cough inside a room, walk out, and two hours later claim a victim when a new person walks into the room and breathes in the invisible viral particles.

Inside the girl, the virus does something notable. While influenza and SARS-CoV-2 would stick to the epithelial cells lining the inside of her airway—infecting them first—the measles virus has a different target in mind. Its attachment proteins, like specialized hooks at the surface of its viral ball, bind to a molecule called SLAM, which is present at the surface of active immune cells. Unlike airway epithelial cells, immune cells move around.

The virus thus enters inside of macrophages and dendritic cells and hijacks their molecular construction zone to make many copies of itself. These infected cells, like Trojan horses, move to places where immune cells congregate: the bone marrow, the thymus, the spleen, tonsils, and lymph nodes. They pass the virus on to small white blood cells known as lymphocytes. Some of these lymphocytes hold the memory of past infections: as they tear asunder, releasing an army of measles viral particles, their immune memory dies with them. This is why the young girl’s immune system is now experiencing a temporary bout of amnesia. It forgets how to fight many other disease-causing microorganisms, and this forgetfulness could last three years. Many of the children who died of infections at a time when measles was very common got infected because measles had made them more vulnerable: scientists believe that up to half of these deaths could be tied to measles decreasing their immune response.

Infected lymphocytes travel the girl’s body, eventually reaching the epithelium lining her respiratory tract, the target it had initially ignored. The virus is engulfed by these cells before its newly formed viral pups burst out of them in a cellular massacre. If the girl is unlucky, the virus makes its way into her brain, possibly as infected lymphocytes penetrate the blood brain barrier and spread the disease to her brain tissue.

Thankfully, the strain of the virus she was vaccinated with is a little bit different than the virus that is making her sick. The vaccine virus, like a clumsy brother of the real deal, is able to infect another set of cells, those that express a protein called CD46, which alerts the immune system to its presence. Inside the girl’s body, there are now a stealthy thief and a bumbling one all at once. The alarm has been tripped.

The day before boarding her flight, she started developing symptoms, but four days before that she was already contagious, and this is the insidiousness of a disease like measles. As with COVID-19, people who have yet to feel sick are actually already infecting others. How can you quarantine a person when they do not even know they are sick?

On the plane, though, the girl is most likely coughing. Her cough is full of measles viral particles and infected cells, some of which land in front of her while others, much lighter in mass, hang around in the cabin air. A recent study conducted using cells in the laboratory demonstrated that people with measles might not simply be coughing individual viruses; rather, entire rafts of infected cells get dislodged from the airway, peeling away on their own and shooting out of the mouth when the person coughs. And these big masses of infected cells can go on to infect someone else’s cells. Thus, infectious material ends up contaminating the surfaces around the girl as well as the air in her vicinity, which gets moved around by the currents until it hits a HEPA filter.

In a study done in a single hospital room in which a woman with measles was confined, scientists detected the virus on a rarely touched table three metres away from the patient and on a handful of respirator masks the hospital personnel wore when entering her room as her cough was worsening. Measles gets around and it only takes a very small dose of the virus to become sick.

The Malawian girl ended up being the index case—meaning the first one identified—in an outbreak that counts eight additional people. If nobody on the plane had been vaccinated against measles, the outcome would have been much bleaker. During the COVID-19 pandemic, people became familiar with the concept of R0 (pronounced “R naught”). It is the average number of people who will catch the infection from a typical case when the population has no immunity against the disease. The R0 for measles is usually reported as hovering between 12 and 18, meaning that if nobody is vaccinated against it or has had it in the past, someone with measles would go on to infect 12 to 18 other people. But a review of the evidence showed that the R0 for measles was much more variable than this, ranging from 1.5 to 770. It is not an intrinsic characteristic of a virus. Instead, it varies based on the number of contacts people have in a society. By comparison, Ebola has a much harder time spreading from people to people, with a typical R0 between 1.5 and 2.5.

Measles is incredibly contagious. The dose needed to get infected is small; it lingers in the air and sneaks out of a person before they’ve even developed symptoms; and because it has been eliminated from many countries, doctors may not think of it as a possible diagnosis, allowing the infected person to spread the virus even longer. Measles had indeed been eliminated from Australia when the Malawian girl landed in the country, which is why it took doctors 12 days to diagnose her with the illness after she presented to hospital.

Contact tracing at the time was limited to two rows on either side of the index case. Health authorities figured that most of the population was immune to the disease and that air circulation inside the cabin is really good, at least when the plane is in the air. But this case alerted them to the fact that measles spreads far and wide. Only one of the four people who caught measles on the flight was seated within two rows of the girl.

We are now seeing a resurgence of measles in parts of the world where it had been eliminated. We can blame both apathy and a well-funded anti-vaccine movement that has spent decades demonizing the measles-mumps-rubella vaccine. At the time of writing, there are 44 active cases of measles in Canada and 121 in the United States.

It may not seem like much but given the knowledge and technology we have developed with regards to this dangerous and highly contagious infection, those numbers should be zeroes.

Take-home message:
- Measles is one of the most contagious viruses to infect humans
- It spreads very easily because the dose needed to infect someone is very small; the virus lingers in the air for hours; infected individuals are contagious before they even develop symptoms; and doctors in countries where measles has been eliminated may not think their patient has measles, allowing the virus to spread some more
- By infecting specific immune cells, the measles virus causes a temporary type of immune amnesia, making the person more susceptible to other infections


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