The United States has seen over 800 measles cases this year.  The majority of these cases are in West Texas, where an outbreak has been ongoing since January.  Most people who catch measles have not been vaccinated.  However, a small number of cases, about 3%, are in people who have been fully vaccinated with the MMR (measles, mumps, and rubella) shot.  This is known as a breakthrough infection.  These infections happen when someone gets sick despite being vaccinated.  Doctors say the MMR vaccine is very effective.  One dose is about 93% effective, and two doses boost that to 97%.  But that means 3 out of 100 people might still be at risk.

    People who are fully vaccinated but still get measles might have a milder version.  This is often called modified measles.  Symptoms like rash, fever, cough, and runny nose might be less severe.  These breakthrough cases are also less contagious, but they can still spread the virus.  People are contagious four days before and four days after the rash appears.  The rash usually shows up 14 days after exposure.

    There are several reasons why breakthrough infections happen.  Some people might not have realized they weren't fully vaccinated.  Others might have a single dose, which offers good protection but not as much as two doses.  The timing of vaccination can also make a difference.  A child who just got the vaccine might still be vulnerable for a couple of weeks.  Sometimes, the body's immune system might not respond well to the vaccine.  This can happen due to genetic factors or certain health conditions.

    The measles virus itself is very stable and hasn't changed much over time.  This means the vaccine should still work well.  However, some people might lose their protection over time.  This is more likely to happen in older age groups.  Researchers are studying these rare cases to better understand how to prevent outbreaks.  They are also looking at how the body's immune system responds to the vaccine.  Some people might have a stronger response, while others might not respond as well.  This can depend on genetic factors and how many different parts of the virus the immune system recognizes.

    Before the measles vaccine was developed, almost everyone got measles as a child.  This provided lifelong immunity.  Some people born between 1957 and 1968 got a first-generation vaccine that wasn't as effective.  Others might have only gotten one dose of the MMR vaccine.  These people might be more at risk for breakthrough infections.  Doctors recommend that anyone born in this time frame should get at least one MMR dose.  If someone who got only one dose is near an outbreak or traveling to a country where measles is common, they might want to consider getting another dose.  Even if they are already immune, there's no harm in getting another dose.

Measles On The Rise: Why Some Vaccinated People Still Get Sick

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