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~700 cases in 2019 is the highest annual measles cases since the US declared it completely eliminated in 2000.
Short answer is that measles easily spreads in community pockets where its vaccination levels have dropped below a certain level. A New York Times podcast states that the current outbreak began among two unvaccinated Orthodox Jewish communities in New York (1) after some there came back infected from Israel, having caught it from unvaccinated individuals who got recently infected in Ukraine. However, this is merely the latest in many such outbreaks since 2000. Careful examination shows most of the infected in such outbreaks are unvaccinated. To understand why requires understanding
- Measles infectiousness.
- Dynamics of measles vaccination and infection: clusters of susceptible unvaccinated people lead to measles outbreaks within a community.
Measles infectiousness
Measles is considered the most infectious human virus.
- Highly airborne as well as hardy, being capable of sticking to a surface for at least as long as 2 hours (2).
- Stays lodged in the trachea (windpipe), poised for extremely efficient spread when an infected person coughs. As lifelong measles researcher Roberto Cattaneo memorably puts it (3),
“The measles virus uses the trachea as a trampoline.”
- An infected person is highly infectious even during the initial asymptomatic (outwardly healthy) phase of up to 4 days.
- Capable of spreading to 12 to 18 persons from just one infected person during the infectious phase. The CDC states that 90% of those not immune to it will catch it from one infected person (2).
- The WHO estimates measles is 6 times more infectious compared to the flu (4).
The Guardian’s animated infographic fleshes this infectiousness out across vaccination rates ranging from as low as 10% to ~100% (below from 5).
Dynamics of measles vaccination and infection: clusters of susceptible people lead to measles outbreaks within a community
Two sets of retrospective observational data offer insight into how this dynamic plays out.
I. Examine how measles spreads within a community with inadequate measles-specific herd immunity.
II. Examine how measles spreads within a community where the vaccinated and unvaccinated mix freely with each other.
I. Evidence that measles easily spreads among the unvaccinated
2005 saw a measles outbreak in Tippecanoe and Clinton counties in Indiana (below from 6).
- Although measles coverage in Indiana was quite high at ~98% at that time, pockets of home-schooling church goers there had unvaccinated children.
- A group that included an unvaccinated 17 year old visited a Romanian orphanage. Back then, Romania had epidemic levels of measles. The girl got infected and brought measles back to Indiana, spreading it at a church meeting that discussed a report of the group’s missionary work.
- 32 of 34 individuals who caught measles from her were unvaccinated.
- 30 were <20 years of age, 24 belonged to just 4 families and 3 required hospitalization.
- It cost >$167000 to contain this outbreak.
- Genetic profiling identified the virus strain as originating in Romania.
Between 2001 and 2008, the US had a total of 557 confirmed cases of measles. Majority were among the unvaccinated (below from 7).
II. Evidence that measles selectively spreads among the unvaccinated within a community where they mixed freely with the vaccinated
After the live attenuated measles vaccine gained licensure in the US in 1963, Rhode Island began a measles eradication program in January 1966. The intense surveillance that followed helped bring to light a compelling example of how measles vaccination could selectively protect vaccinated school children even when they intermingled freely with unvaccinated peers among whom epidemic measles had broken out.
- Back then Fox Point in the city of Providence (est. pop. ~5300) and East Providence (est. pop. ~50000) were hubs of Portuguese immigrants recently arrived from mainly the Azores and less so from the Cape Verde Islands.
- Such immigrant populations were largely unvaccinated against measles at the time this epidemic broke out in September 1968, after the index case, a 3 year old Fox Point resident, returned from Portugal early in the phase of his infection.
- Over the next 3 and a half months, 91 children fell ill from measles across 7 elementary schools.
- Though 1300 Portuguese origin and 1000 non-Portuguese origin had the opportunity to intermingle during the course of this outbreak, only 3 of the infected were of non-Portuguese origin from the general population (below from 8).
“Equally impressive as the localization of cases within the Portuguese community is the lack of spread to the general population; only three of 91 cases were in non-Portuguese in spite of extensive opportunity for the virus to escape this ethnic boundary. Table 4 shows that nearly 1000 non-Portuguese school children were almost entirely spared. Even further it suggests that, when the more adequately immunized non-Portuguese made up about 50 per cent of the student body, their presence sufficiently diluted the susceptible pool of Portuguese to reduce significantly the attack rate among the Portuguese in the public schools (6.9 per cent) compared with the attack rate in the parochial school (17.0 per cent) [p < 0.001).”
Given measles is so highly infectious, it’s estimated to require 92 to 95% vaccination levels to assure herd immunity within a given community. Thus, measles easily spreads within a community that has inadequate measles-specific herd immunity as a result of inadequate vaccination coverage.
Bibliography
1. How the Measles Outbreak Started
2. Measles | Transmission | CDC
3. Why Measles Is the Most Contagious Virus
4. Measles
5. Watch how the measles outbreak spreads when kids get vaccinated – and when they don’t
6. Parker, Amy A., et al. “Implications of a 2005 measles outbreak in Indiana for sustained elimination of measles in the United States.” New England Journal of Medicine 355.5 (2006): 447-455. Implications of a 2005 Measles Outbreak in Indiana for Sustained Elimination of Measles in the United States | NEJM
7. Parker Fiebelkorn, Amy, et al. “Measles in the United States during the postelimination era.” The Journal of infectious diseases 202.10 (2010): 1520-1528. Measles in the United States during the Postelimination Era
8. SCOTT, H. DENMAN. “The elusiveness of measles eradication: insights gained from three years of intensive surveillance in Rhode Island.” American journal of epidemiology 94.1 (1971): 37-42.
https://www.quora.com/Why-are-we-having-a-measles-epidemic/answer/Tirumalai-Kamala