Measles Information

Why Is Measles So Contagious?

An infected person who coughs or sneezes can spray the virus into the air in small droplets, which can remain suspended for up to two hours. People with measles are infectious for about four days before and four days after the appearance of the rash. Before there was a vaccine, nearly everyone in the United States became infected with measles before the age of 15. The disease kills about 1 in every 1,000 people infected, and those who survive have lifelong immunity.

What Are the Advantages of Having Everyone Vaccinated?

Behind the push to vaccinate as many people as possible is a concept called herd immunity. If the vaccination rate within a community is very high, vaccinated people act as a barrier and reduce the risk of infection for people who cannot be immunized, like the very young or those with compromised immune systems.

Drawing diagram showing an person standing in center of ring of vaccinated people


In a hypothetical community where nobody has immunity from the measles virus, one infected person might infect 12 to 18 people, who might each infect another 12 to 18 people. At this rate, a small outbreak would quickly grow out of control.

Diagram showing how one person can infect many more


Every person who is successfully vaccinated reduces the potential sources of infection, thus reducing the risk to unvaccinated people. This reduction in risk is sometimes called the herd effect. The presence of vaccinated people helps slow the spread of the virus.

Diagram showing one person infecting only one other in group

Measles infographic - it isn't just a little rashHERD IMMUNITY

For an outbreak to end quickly, each infected person must infect, on average, fewer than one other person. In this example, at least 17 of every 18 people (more than 94 percent) would need immunity. This threshold is sometimes called the herd immunity threshold.

To maintain herd immunity within a community over time, children need to be vaccinated at a high rate. The measles vaccine is ineffective in about 5 percent of people who have had only one dose, so two doses are recommended to ensure that nearly all children will get immunity before entering school.

Sources: Paul Fine, London School of Hygiene and Tropical Medicine; Clinical Infectious Diseases; Journal of Infectious Diseases; C.D.C.

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