Measles Season in the Post-Elimination Era

Measles Basics

Sick girl in bed with mother
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People are used to there being a specific season for different childhood infections.

For example, we expect cold and flu season to run through the fall and winter and are getting used to the idea that West Nile season runs in late summer and early fall.

Measles Season

Classically, in the pre-vaccine era and in parts of the world that still have endemic measles, rates of this vaccine-preventable disease are highest:

  • during the late winter and early spring (temperate climates, like the United States)
  • after the rainy season (tropical climates)

In the post-vaccine era, measles season seemed to shift a little later, to the spring and early summer. In 1994, for example, when we had 963 cases of measles in the United States, 79% of those cases occurred between April and July.

Similarly, in 2011, we had only seen 15% of the year’s total measles cases by April 1. By August 1, that was up to about 70%.

We do see measles cases year round though, they just seem to be more of them in the spring and early summer months.

Modern Era Measles Season

Since most measles outbreaks in the United States are imported from other parts of the world, why do we continue to have a measles season?

Is it because people are more likely to travel out of the country during the spring and early summer months, so there are more opportunities to get measles and bring it back home?

Or is it simply because that is when measles cases are peaking in other parts of the world?

While that would seem to make a lot of sense, measles season occurs at different times in different parts of the world. For example, in recent years, many cases were imported from:

  • Africa - Kenya, Nigeria
  • Eastern Mediterranean - Pakistan
  • Europe - France
  • South-East Asia - India, Indonesia
  • Western Pacific - Philippines, China

In the Philippines, the dry season often starts in mid-March, so that may be when measles cases begin to rise again. After a huge measles season in 2014, with at least 58,010 cases and 110 deaths, another big year for measles in the Philippines could mean more measles in the United States too unless travelers get fully vaccinated.

In contrast, the rainy season in India typically doesn’t end until about September, so you might not expect measles cases to peak there until the late fall.

In recent years, the largest numbers of cases in the United States have coincided with importations associated with big outbreaks in other countries and regions and then an even “greater viral transmission after importation into the United States, leading to a greater number of importation-associated case,” including:

  • measles outbreaks in Europe in 2008
  • measles outbreaks in Europe and India in 2011
  • measles outbreaks in Europe in 2013
  • measles outbreaks in the Philippines in 2014

But it should be clear that we are not importing a lot more measles. It is rather that each imported measles case is spreading to a lot more people in recent years because of a rise in unvaccinated people, often because they are intentionally unvaccinated because of non-medical vaccine exemptions.

And it should be even clearer that we need more people vaccinated and protected against measles to stop these outbreaks.

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