The 2019 Measles Outbreak

We are not even half way through the year, but the number of confirmed cases of measles has in the United States reached the highest number in over two decades. A count which will continue to rise as we progress through the year. Here, we will explore the reason behind the reemergence of a disease that had been locally eradicated since the start of the millennium.

The 2019 Measles Outbreak of the United States of America

2019 is the year marked with the largest outbreak of measles in the U.S. since the elimination of the disease. Last time an outbreak of this caliber was recorded was in 1994, with 963 cases [10]. According to the CDC, 839 cases of measles have been reported in 2019 so far [2]. A surge which has made President Trump, who has previously expressed his concerns in vaccines [3], urge Americans to get vaccinated for measles [4]. A couple of weeks ago on May 3rd, Saint Lucia reportedly quarantined a Cruise Ship following the diagnosis of a female crew member with the disease [5]. Due to the nature of the disease and the outbreak in the U.S., officials found it prudent to quarantine the vessel. Although the disease was declared eliminated in the United States in 2000 [6}, it isn’t the first time we’ve seen such an outbreak since. With the cases going up and down once every several years, 2014 had 667 cases reported and 2018 having 372 cases reported [2]


How does an eradicated disease come back to life? – Let’s break it down!

As mentioned in a previous article, an important factor in population wide immunization is the concept of herd immunity. Herd immunity is, by Merriam-Webster, defined as “a reduction in the probability of infection that is held to apply to susceptible members of a population in which significant proportion of the individuals are immune because the chance of coming in contact with an infected individual is less”.  Although the concept of herd immunity is widely known and referred to, the exact meaning  of it is interpreted in different ways [7]. Some refer to it as the complete resistance of a population to a disease when a threshold is reached, whereas others have described it as a reduction in frequency of infection in susceptible individuals. In general, one can consider herd immunity as a concept describing the ability of a population of mainly immune individuals to protect susceptible ones, by limiting the ability of a pathogen to spread.

The theory has multiple aspects to it, defining the calculus behind disease transmissions. In 1906, Hamer introduced the theoretical basis of herd immunity in the context of measles dynamics [7]. He defined the ability to infect for each case of measles as a function of the number of susceptible individuals in a population.

The law of mass action is [7]:

where C is the number of cases of measles, t is the first period and t + 1 the following period. S is the number of susceptible individuals in the population, and r is the transmission parameter describing the proportion of all contacts between infected and susceptible individuals that lead to new cases. One that will be different for different diseases.

Thus,            is the number of new cases of measles per case of measles, defined as

The transmission parameter can therefore be defined as a ratio between the total number of cases at a given time and the product of the number of susceptible individuals and cases in the previous period. The period is the serial interval, aka the time between successive cases in a chain of transmission.

This led to the threshold theorem: Incidence increases when                                     , and thus

only when              . Thus, the epidemic threshold          is defined by     , and thus if the number

of susceptible individuals is kept below this value, incidents will drop.

The herd immunity threshold, aka the proportion of a population that needs to be immune for herd immunity to take place can thus be described as [7]:

Although this is a very simple model, not taking into consideration many factors that may affect the ability to infect, it does show the kind of model that is used to describe transmission models for diseases. I urge you to read more about the different models in the review article [7].


But how does this explain it?

Considering this model, we can now try and understand why a disease that has been eliminated for nearly two decades in the United States suddenly reappears. If we consider the susceptible individuals to be unvaccinated people, then the herd immunity is defined by the proportion of people who are vaccinated and thus immune. Bearing in mind that an elimination/local eradication means that a particular area is no longer affected by diseases, traveling to and from places that have the disease may lead to the reintroduction of the disease. In these cases, if the vaccination rate (and thus the proportion of immune individuals) is above the herd immunity thresholds, the disease cannot spread and will remain contained. However, if the rate is below that, the disease may be cause an outbreak, as the chance of it coming in contact with an unvaccinated individual is high enough.

According to the CDC, 6 of the 13 outbreaks in the U.S. are associated with communities of lower vaccination rates. And these six outbreaks account for 88 % of the total number of cases. Forty-nine percent of the measles patients have been aged less than six months to four years [1]. This is not surprising as the vaccination rate of MMR in children between the age 19-35 months in the U.S. nationally has not exceeded 92 % since 2008 [8]. The herd immunity threshold for measles is generally assumed to be 92 % [9}.

If in your opinion, this does not link the lack of vaccinations with increase disease spread, scroll down to the bottom of this page on CDC’s website to read about another more extreme example in Japan [10}


So what do we do now?

One way of stopping the spread is by a physical barrier. In other words, imposing quarantines as was seen with the cruise ship [5] and two universities in California [11]. Other ways include the use of software to look through electronic medial records to try and identify and reach out to unvaccinated individuals [12]. And obviously the urge to get children vaccinated early is important. Although it seems that a certain disease in unlikely to occur, it should not demotivate people from getting the vaccine. On the contrary, the decreased likelihood should be taken as evidence that the vaccine is successful and encourage more people to opt-in. Let me leave you with this analogy before you go: When it starts raining and you open up an umbrella to prevent yourself from getting drenched, you don’t close it if it works, you keep it over your head. Similarly, if you don’t bring you umbrella with you, for the sake of your friends having one, and they had the same thought, you all get wet.

As always, join the conversation either here or with comments on Facebook. Stay tuned for more!




[1] Patel M, Lee AD, Redd SB, Clemmons NS, McNall RJ, Cohn AC, Gastañaduy PA (2019). Increases in Measles Cases – United States, January 1-April 26, 2019; Morbidity and Mortality Weekly Report 68, pp 402-404.
[2] Centers for Disease Control and Prevention (2019). Measles: Cases and Outbreaks.
[3] The Washington Post (2015). The origins of Donal Trump’s autism/vaccine theory and how it was completely debunked eons ago.
[4] Reuters (2019). Trump tells Americans: Go get your measles vaccination.
[5] Reuters (2019). Caribbean nation of St. Lucia quarantines cruise ship over measles case.
[6] Centers for Disease Control and Prevention (2018). History of  Measles.
[7] Fine PEM (1993). Herd Immunity: History, Theory, Practice; Epidemiologic Reviews 15 (2), pp 265-302.
[8] U.S. Departments of health and human services (2017). Vaccination coverage for selected diseases among children aged 19–35 months; Health, United States, 2017.
[9] Bednarczyk RA, Orenstein WA, Omer SB (2016. Estimating the Number of Measles-Susceptible Children and Adolescents in the United States Using Data From the National Immunization Survey – Teen (NIS-Teen); American Journal of Epidemiology 184 (2), pp 148-156.
[10] Centers for Disease Control and Prevention (2018). Vaccines & Immunizations: What Would Happen If We Stopped Vaccinations?
[11] CNN (2019). Measles quarantine issued at two California universities.
[12] Reuters (2019). U.S. doctors use medical records to fight measles outbreak.

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