Few things set public health departments scrambling like a measles outbreak. The virus is a bear to deal with, even in highly vaccinated populations. That’s because the virus is exquisitely adept at finding people who are susceptible to it, and infecting them.
A combination of factors makes this virus a veritable infecting machine. Its mode of transmission and the period during which it is infectious mean one infected person can sicken many others, if they haven’t already had measles or didn’t receive the vaccine.
Let’s take a look at the tricks measles has up its sleeve:
Back in the days before measles vaccine was widely used, it was estimated that one child with measles would infect between 12 and 18 other children, on average. We are talking about children here deliberately. Measles spreads so efficiently that before measles vaccine was introduced in the 1960s, few people made it to adulthood without having contracted the disease.
That rate of spread is very high. Pertussis, which is also known as whooping cough, is considered quite contagious, too, but one case of pertussis would infect only about five other people who are vulnerable.
What is it about measles that makes it such an effective spreader? For starters, you don’t have to be exposed to a lot of the virus to become infected. The so-called infectious dose for measles is small. And people who are infected emit a lot of viruses as they cough or even exhale. From a containment point of view, that’s a bad combination.
“Measles makes an infected child into a viral Uzi,” explained Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota.
Measles is an infection of the respiratory tract, which also enhances its ability to spread. Pathogens that are emitted in blood or stool may be limited in how far they can be transmitted because people are hard-wired to avoid contact with someone else’s blood or stool if they can. But a virus that wafts through the air is much harder to evade.
Osterholm was involved in an outbreak investigation that illustrates this point clearly. Back in 1991, Minneapolis-St. Paul hosted the Special Olympics. A competitor from Argentina who was infected with measles arrived in the Twin Cities to compete. That one case led to an outbreak of more than 25 other cases in multiple states over three generations of spread.
Many of the people who contracted measles in this outbreak had no known contact with the child. The investigation concluded some most likely were infected during the event’s opening ceremonies — even though they were seated in the upper deck of the domed stadium, more than 100 feet above the point where the teams paraded into the event.
That’s one of the things about measles. After someone breathes or coughs out the viruses, they can remain in the air for significant periods of time. In fact, it’s estimated that a susceptible child who entered a room — say a doctor’s waiting room — two hours after an infected child was there can become infected. That gives the virus a huge advantage, transmission-wise.
Another thing that measles has working for it is the fact that people who contract the virus are contagious for about eight days. Most importantly from a transmission point of view, half of that time is before they have developed the rash that typically lets people know: This child has measles.
When children are at their sickest with measles, they will be in bed, which limits their opportunities to spread the virus. But being infectious before they reach that point means they can be exposing other people and no one is the wiser.
Still another problem with containing the spread of measles: The introduction of measles vaccine drove down the number of cases to the point that now some doctors don’t recognize measles when confronted with it. In the pre-vaccine era, there were an estimated 3-4 million cases a year in the United States. Now a year with over 600 cases — there were 667 in 2014 — is considered a very bad year for measles.
“So if you don’t get a diagnosis of measles right away, you can continue to expose people,” said Dr. Tom Clark, deputy director of the division of viral diseases at the Centers for Disease Prevention and Control.
The good news is the measles vaccine is one of the best around. Two doses of vaccine in childhood protect about 97 percent of the children who receive it. The bad news, of course, is that lingering fears about the vaccine sparked by since-discredited research linking it to the development of autism have fueled vaccine resistance and vaccine hesitancy.
Clark said most children in the United States are vaccinated and the rate of vaccine refusal remains low. Still “there are pockets of folks who are vaccine hesitant and are often clustered geographically,” he said.
With measles outbreaks reported in a number of countries and the frequency of international travel — a child infected in Israel started an ongoing outbreak in New York City — measles will remain a potent threat.