Traditionally, the time when measles case counts are the highest occurs:
during the late winter and early spring (temperate climates, like the United States)
after the rainy season (tropical climates)
when kids are in school
So just like flu season, it’s always measles season somewhere…
And in areas of the world where measles is still highly endemic, you can expect cycles of larger measles epidemics every 1 to 4 years.
Can you guess why?
“As higher uniform population immunity is achieved the scale of epidemics, both their duration and absolute number of cases, progressively decreases. Epidemic frequency simultaneously decreases with increasing time intervals between epidemics. Another uniform feature as elimination is approached is the loss of epidemic seasonality.”
Durrheim et al on Measles – The epidemiology of elimination
I’ll give you a hint – there is nothing different about the measles virus during those years.
Eventually though, as the number of people susceptible to measles builds up, there is the opportunity for bigger outbreaks. Of course, that doesn’t happen if most people are vaccinated and protected.
“Source countries included Philippines (14 cases), Ukraine (8), Israel (5), Thailand (3), Vietnam (2), Germany (2), and one importation each from Algeria, France, India, Lithuania, Russia, and the United Kingdom.”
Increase in Measles Cases — United States, January 1–April 26, 2019
In the early part of 2019, we saw a lot of cases because unvaccinated travelers were returning from Philippines, Ukraine, and Israel, countries in peak measles season.
As cases in those countries hopefully slow down over the summer, unfortunately, we might see a rise in other parts of the world.
Of course, there is an easy way to end our measles seasons once and for all.
Get vaccinated and protected, especially before traveling out of the country.
Do you remember when measles was eliminated in the United States?
You probably should.
It wasn’t that long ago…
When Measles Was Eliminated in the United States
After several years of declining numbers of measles outbreaks in the United States, the endemic spread of measles was declared eliminated in March 2000.
“The data indicated that, during 1997–1999, measles incidence has remained low (<0.5 cases per 1,000,000 population) and that most states and 99% of counties reported no measles cases. In addition, measles surveillance was sensitive enough to consistently detect imported cases, isolated cases, and small outbreaks. Evidence of high population immunity included coverage of >90% with the first dose of measles vaccine in children aged 19–35 months since 1996 and 98% coverage among children entering school. In 48 states and the District of Columbia, a second dose of measles vaccine is required for school entry. A national serosurvey indicated that 93% of persons aged >6 years have antibody to measles.”
So, measles wasn’t gone yet. It was just that most cases were imported from outside the country. Only imported strains of the measles virus were causing outbreaks.
“The end of endemic measles transmission in the United States has both domestic and international importance. Domestically, the absence of endemic exposure to measles means few persons in the United States will be infected and risk complications of measles. This decreased risk of exposure provides protection to groups not protected directly by vaccination: children too young for routine vaccination; the few persons who, although vaccinated, are not protected, primarily from failure to mount an adequate response to vaccine; persons for whom vaccine is contraindicated (e.g., those with immunodeficiency); and persons who choose not to be vaccinated.”
Wharton on Measles Elimination in the United States
Unfortunately, the “choose not to be vaccinated” group might be endagering our claim to have eliminated measles.
“Endemic measles transmission is the existence of any continuous indigenous chain of transmission of measles virus that persists for >1 year in any defined geographic area (e.g., the United States).”
Orenstein on Defining and Assessing Measles Elimination Goals
Still, when you look at the numbers, technically, measles isn’t yet spreading endemically in the United States.
We are getting close though.
The outbreaks in Rockland County and Brooklyn started in September and October 2018. If they aren’t stopped soon, will it be easier to make a case that measles is no longer eliminated in the United States?
“Outbreaks in New York City and Rockland County, New York have continued for nearly 8 months. If these outbreaks continue through summer and fall, the United States may lose its measles elimination status.”
U.S. measles cases in first five months of 2019 surpass total cases per year for past 25 years
It sounds like it.
One thing to consider though, after an unvaccinated visitor introduced measles to Brooklyn from Israel back in October, measles has been reintroduced into the community at least seven other times! This includes travelers from Israel, UK, and Ukraine.
So the outbreak isn’t necessarily a “continuous indigenous chain of transmission of measles virus.”
It is multiple chains in the same community.
“There have been additional cases of measles from international travelers to Rockland, exposing more people to measles.”
2018 – 2019 Measles Outbreak in Rockland County
The same thing has happened in Rockland County after the initial importation from Israel in September.
Measles Elimination Criteria
Does that matter?
It likely should, but let’s also look at how we do with other criteria that are often used to assess the absence of the endemic spread of measles:
Few measles cases/low measles incidence – we will have the most cases in 2019 since 1992, but most cases are clustered in just a few big outbreaks
The duration of outbreaks is short – outbreaks are getting longer and harder to contain, but part of the problem is the global rise in measles and the reintroduction of measles into existing outbreaks
Most cases are associated with international importation – still very true
No endemic measles virus strain – outbreaks this year have been associated with strains that are commonly seen in Ukraine, the Philippines, and a few other areas
Don’t think that this year’s measles outbreaks will break any records?
Which Measles Records Will We Break This Year?
Unfortunately, as measles cases rise, we keep breaking new records.
The Rockland County, New York measles outbreak, which has been going on since September 2018, has the record for being the longest outbreak since the endemic spread of measles was eliminated in the United States.
What other records will we break?
What other records are there?
37 cases – the record low for the number of measles cases in a year, set in 2004
667 cases – the recent record high set in 2014 and the most cases since 1994, when there were 963 cases.
383 cases – the largest single outbreak since the endemic spread of measles was eliminated, set in Ohio in 2014