Measles: Defeating a Debilitating Virus

Measles is one of the most contagious viruses in the world. This means that measles can spread rapidly in a population with low or no immunity, such as when Europeans first introduced measles to Native Americans around 1492 AD, leading to centuries of devastating outbreaks in Native American populations. Before a vaccine for measles became available in 1963, nearly everyone caught measles during childhood.

How do scientists measure how contagious a virus is? Scientists use a mathematical term called R₀ (pronounced “R naught”). R₀ is the basic reproduction number of an infectious disease; R₀ quantifies how many people, on average, one infected person could infect if people have not been vaccinated or do not have any prior exposure, and therefore no immunity to the disease. For example, for influenza (i.e., the seasonal flu), the R₀ is around 1 to 2, meaning that a person who has the flu can, on average, infect one to two other people. For COVID-19, the R₀ is around 2 to 3; for an example of a COVID-19 outbreak, see the graph in Figure 1. For measles, the R₀ is around 12 to 18, which arguably makes it the virus with the highest known R₀ value. Without immunity, a measles outbreak can spread so quickly that health officials do not have time to mount a proper response.

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Figure 1. This is a screenshot from Simpandemic: Comparing COVID-19 Variants showing a simulated outbreak of the original COVID-19 variant. How does this outbreak -- including the number of infected individuals over time -- compare to the modeled measles outbreak shown below?

How does measles spread so quickly? Measles can be easily transmitted, or passed from one person to another, in many ways, including through touching a contaminated surface (i.e., one with contaminated saliva or mucus) or through the air (such as from sneezing, coughing, or even just talking). While measles includes a distinctive rash, people are contagious for four days before developing this distinct symptom, and then for four days afterwards. While most people recover, having measles can be serious and even deadly.

In this science project, we simulate a measles outbreak in a population of 100,000 people with no immunity to measles, starting with ten infected school age children and using an R₀ of 18. The resulting graphs and table (below) show that nearly everyone (>95%) is quickly infected (blue line), with a rapid exponential growth pattern, and this impacts economic output and overwhelms hospitals (see the table). As people recover from the infection, the number of immune individuals (orange line) increases, following a similar rapid exponential growth pattern. How does this type of outbreak compare to COVID-19 outbreaks, such as the one shown in Figure 1, above? For another comparison to COVID-19, check out Simpandemic: Comparing COVID-19 Variants.

To better understand the assumptions and parameters involved in SimPandemic, read the FAQ.

A vaccine can prevent measles infection. Being fully vaccinated for measles is considered the safest way to prevent contracting and spreading measles. As an example, in the early 2020s in the United States, the adult population had high immunity to measles, around 95% to 97% immunity on average. This high immunity is due to common childhood exposure (before the vaccine became available) and then widespread adoption of measles vaccination starting in the 1960s. Measles vaccines are typically given as two childhood doses, with the first dose given between 12 and 15 months old, and the second dose between four to six years old. If a child does not receive both doses, they do not gain full immunity to measles. Receiving both doses offers lifelong protection and decreases a person’s lifelong risk of being infected by measles to 3%.

Community immunity refers to the fact that if enough people in a population are immune to a disease, then they protect others, who are not immune, from contracting the disease. Community immunity, sometimes called herd immunity, works because someone who is immune to a disease cannot catch it, nor can they transmit it (pass it along). Populations or pockets of people with lower measles vaccination rates are at a higher risk of having a measles outbreak.

In the United States, the percentage of kindergartners fully vaccinated for measles has been decreasing. In 2019-2020, 95.2% were fully vaccinated, and by 2023-2024, this had decreased to 92.7%. If adults still have high immunity to measles, how does lower immunity in children affect the occurrence of outbreaks? If lower vaccination rates among children are combined with lower rates in adults, as is seen in some populations, how does this affect the course of outbreaks?

In this science project, we simulate a measles outbreak in a population of 100,000 people with some decreased immunity to measles. This simulation starts with ten infected school age children (using an R₀ of 18) where immunity among children (age <18 years old) is 90%, immunity among adults (age 18-64 years old) is 92%, and immunity among the elderly (≥65 years old) is 97%. Use the green “Zoom In” button (you can click it several times to zoom in farther) to see the number of people infected over time (blue line). Deaths caused by the outbreak are shown in the table. Note the days on the horizontal axis (x-axis) and how, while this outbreak is less severe than the one above, this outbreak takes longer to resolve.

How does this type of outbreak compare to COVID-19 herd immunity simulations? For one such comparison, check out Simpandemic: Preventing Outbreaks with Herd Immunity.

In the previous simulation, we saw that a smaller outbreak occurs when the population has some immunity compared to when there is no immunity to measles (as shown in the first simulation). But an outbreak still occurred, even with some immunity in the population. How much immunity is needed to prevent an outbreak from happening? To achieve this, we need to reach the herd immunity threshold. The herd immunity threshold is defined as the minimum percentage of the population that must be immune to a disease to prevent that disease from spreading.

The easier a disease is to transmit, the higher the herd immunity threshold has to be. What is the herd immunity threshold for measles? To try and answer this question, you can use this SimPandemic Sandbox (below). To get started, press the “Customize Settings” button below and test the following:

• In the “Population Statistics” section, increase the “Percentage of immune individuals” for children from 90% to 91%.
• Click the “Done” button.
• Change the “Name for simulation” and “Description for simulation” to reflect the new scenario you are testing.
• Click the “Run Simulation!” button at the bottom.

Look at the new graph and table, being sure to “Zoom In” all the way. Does an outbreak still occur? How does the total outbreak size compare to no starting immunity (the first simulation, above) and some decreased immunity (the second simulation)?

Continue to explore different starting percentages of immune individuals, including increasing the “Adult (18-64 years old)” demographic group. Try continuing to increase the percentages in the different demographic groups by 1% or 2% at a time. If the infected line (blue line) includes a hill or mountain shape, that indicates an outbreak occurred. Can you identify the herd immunity threshold?

Read the FAQ (above) if you have questions about how to use the Sandbox, how to save and share your work, or what the different settings mean. There are many more related questions you can explore about measles outbreaks. Here are a few to get you started:

• How close to the herd immunity threshold for measles is your community? To answer this, search for data about local herd immunity and vaccination rates.
• How do measles outbreaks compare to COVID-19 outbreaks? For example, how does the speed at which a measles outbreak spreads compare to the speed at which a COVID-19 outbreak spreads? How might this difference affect how health officials can successfully respond to these different types of outbreaks? Check out the COVID-19 SimPandemic simulations discussed above to explore all of this more.
• If a child misses one of the two doses of the measles vaccine, how does this increase their risk of contracting measles? Learn more about how measles immunity changes for receiving one dose compared to the full dose. See if you can use SimPandemic to model this difference.
• How can efforts to fight an outbreak affect a measles outbreak? Explore the “Interventions to fight pandemic” options in SimPandemic for modeling this with a measles outbreak.
• Different countries have different demographics. For example, some countries, such as Japan, have an aging population; it currently has the highest percentage of people age 65 and older. How does having different demographics affect a measles outbreak? Explore this by changing the “Population Statistics” options in SimPandemic for modeling different populations.