2018 is turning out to be a big year for measles.
25 states, along with Washington DC have reported Measles outbreaks this year. These outbreaks are spread out across the country.
By definition, a measles outbreak is not just a case of measles, its at least 3 cases that are linked together. This indicates that measles is being transmitted from patient to patient in those outbreaks.
Measles in the United States typically comes from individuals who have travelled abroad. Measles has been nearly eradicated in the United States thanks to extensive vaccination, but it remains endemic in many other countries. When individuals travel abroad and return home to the United States, they can infect other members of their families and communities with measles.
Fortunately, thanks to herd immunity from extensive vaccination, these outbreaks have been relatively isolated this year. Measles is transmitted from person to person. If a community is entirely vaccinated, measles will likely not spread.
There have been 220 measles cases so far in 2018, according to the CDC, which is more than 2015, 2016, and 2017. Additionally, the 2018 data only covers about 5/6ths of the year, up until November 3rd, 2018. It is therefore possible that this number will rise before the end of 2018 as we see more cases.
However, 2018 is not the year with the most measles cases this decade.
That honor goes to 2014, which had the most measles cases in the past decade partly due to a large outbreak in Amish country in Ohio. 393 individuals were affected by the outbreak. According to a study in the New England Journal of Medicine, 99% of the cases in this outbreak were confined to the Amish community where only 14% were vaccinated.
This just goes to show how much of an impact a single case can have in an under vaccinated community.
Measles is extremely infectious. In an unvaccinated community, measles can spread like a wildfire.
Epidemiologists estimate the contagiousness of a disease by the basic reproduction number (R0), which represents the number of people one case can infect in their community.