January 14 – The Spottiest Place On Earth

Today’s factismal: One person went to Disneyland with the measles; by the end of her day, she had infected at least 26 other people.

Every day, some 41,000 people stream through the gates of Disneyland looking to have some fun. ANd, not surprisingly, a few of those folks are sick. Usually, they have a cold or the flu but sometimes they have something worse. That was the case this last December when a young woman who had just contracted measles (she didn’t know about it) went to the park. By the end of the day, she had spread the disease to at least 26 other people who have since become ill. Her case is a perfect way to explore why we vaccinate against measles and what happens when we don’t.

Why do we vaccinate?
Put as simply as possible, measles is a deadly disease. Sure, you probably heard tales from your grandparents about getting measles and getting over it. Back in the day, getting measles was easy both because so many kids had it and because it was so virulent; approximately 90% of the people exposed to measles would come down with it. But what you grandparents probably didn’t tell you is about all of their friends and neighbors who didn’t get over the disease. Measles is considered to be one of the deadliest of childhood diseases; back in the days before wide-spread vaccination, measles would kill 500 children in the US alone. Even today, measles kills hundreds of thouseands of people each year. In 2013 (the last year with complete data), measles killed an estimated 145,700 people. That number would have been much higher without vaccination; rates have increased to 84% world-wide (and nearly 100% in the US), which has reduced measles deaths by some 75% in the past decade. Put another way, thanks to the MMR vaccine, there are 437,100 people walking around who would have died from the measles in 2013; for comparison, there were some 19 deaths associated with the MMR vaccine.

A comparison of the deaths caused by measles and those caused by vaccines; the vaccine deaths were exaggerated for clarity. Each face represents 1,000 deaths.

A comparison of the deaths caused by measles and those caused by vaccines; the vaccine deaths were exaggerated for clarity. Each face represents 1,000 deaths.

How effective are vaccines?
Vaccines have come a long way since Jenner took the pus from a cow’s sores and used it to prevent smallpox. Today’s vaccines are clean, effective, and safe. How safe are they? Last year, more than a billion vaccinations were given in the US alone; there were fewer than a million adverse reactions, the vast majority of which were mild fevers and aches – no deaths from vaccines were reported last year. And vaccines are generally effective. Based on current data, the MMR vaccine is about 99% effective in preventing the disease; that is, if you give the vaccine to 41,000 people and expose each them to the virus, you would only see about 370 cases of the disease. But vaccines in the US are even more effective thanks to herd immunity.

What is herd immunity?
Put simply, herd immunity is the idea that a disease can’t spread if it can’t find someone to spread to. If you catch a cold and go to work, odds are that there will be a couple of people hating you next week because they’ll have caught the cold from you. And then their kids will curse you because they caught the disease from their folks who caught it from you. And then their teachers will curse you because they caught the disease from the kids who caught it from the parents who caught it from you. You would have started an outbreak because there simply isn’t an effective vaccine against the common cold (which is actually a whole bunch of related viruses and not a single disease).
But there is a vaccine that works against the flu. If you catch the flu and go to work and everyone there is vaccinated, you won’t be able to pass along the virus, so the outbreak will stop with you. Even if their kids haven’t had the flu vaccine, they can’t catch it because their parents can’t transmit it to them. They have herd immunity.

A simplified view of herd immunity

A simplified view of herd immunity

The Disneyland outbreak provides a great example of herd immunity. If all 41,000 people at Disneyland that day were vaccinated against the measles, then 1% of them would have had vaccinations that didn’t work. That means that there would have been 410 people who could have gotten ill; if they were all exposed to the virus, then we would have seen some 370 cases (because only 90% of the folks would have caught the measles). Instead, we’ve only seen 26 cases. That’s because the person who brought the disease to Disneyland didn’t cough in the face of everyone at the park; she only interacted with a few thousand of them. If those people hadn’t been vaccinated, then they would have been able to pass the disease to other people. But htey were vaccinated which made them immune which prevented them from carrying the disease. As a result, only the 30 or so people whose vaccinations didn’t take that she interacted with directly could get the disease.

Of course, measles isn’t the only disease that can be prevented with a vaccine. Hepatitus, influenza, smallpox (now thankfully extinct), rubella, mumps, HPV, tuberculosis, and dozens of other diseases can be prevented by vaccination saving hundreds of thousands of lives every year. And if you’d like to help doctors improve the vaccines we have and develop new ones, then why not GoViral? This project asks participants to send them a sample of saliva whenever they feel ill; the samples will be analyzed to detect emerging viruses and to help develop new techniques to tackle them:

3 thoughts on “January 14 – The Spottiest Place On Earth

  1. Pingback: August 1 – Pokey Dokey | Little facts about science

  2. Pingback: August 26 – Cough, Cough | Little facts about science

  3. Pingback: August 1 – A Real Shot In The Arm | Little facts about science

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