Lancaster Physician Summer 2019 - 25

SUMMER 2019

First some perspective; in the decade
prior to the 1963 availability of a measles
vaccine, an average of 549,000 cases were
reported annually. These resulted in 48,000
hospitalizations, 1,000 cases of encephalitis,
and 495 deaths-all at a time when the U.S.
population was one third the present size.
In reality, measles cases were likely grossly
under-reported, with a more accurate estimate being 3-4 million cases annually in the
U.S. prior to vaccine availability.
Humans are the only known host for
the measles virus. It is efficiently spread
through airborne droplets that are produced
when an infected person coughs or sneezes.
Those droplets remain suspended in air for
about two hours. Measles is so contagious
that 90 percent of non-immune persons
exposed, even briefly, to a measles patient
will become infected. The virus incubates for
7-14 days prior to symptoms. Measles can
be transmitted four days before the onset
of the characteristic rash, during the phase
where only the non-specific 3-C's (cough,
coryza, and conjunctivitis) are present.
Transmission risk continues for four days
after the appearance of the rash.
Measles is a major problem worldwide,
with an annual estimated death toll of over
100,000. And unfortunately the incidence of
new cases has been increasing. As such, measles introduction into the U.S. from countries
endemic for the disease is a perennial issue.
In fact, in the U.S. there are multiple small
clusters of measles cases every year, often
initiated from an exogenous introduction
either by a returning U.S. traveler or a visitor
from a country where measles is endemic.
Venezuela, Turkey, Israel, Madagascar, the
Philippines, and many countries in Europe
have active measles transmission.
The R-0 (R- naught), the basic reproduction number, or contagion ratio, for measles
is 18, meaning that a single infected person
will infect on average 18 exposed persons
in a fully susceptible population. So one
case yields 18, then 18x18 or 324...etc. For
context, the R-0 for the 1918 devastating
pandemic of influenza was calculated at 2.8,
and for Ebola the R-0 is 2.0.

As an illustrative example, in 2014 a
missionary worker who had traveled to
the Philippines returned to an Amish
community in Ohio that had previously
shunned immunization. This resulted in an
explosion of 383 cases of measles in their
local community. Aggressive education,
infection control measures, and subsequent
acceptance of MMR vaccination eventually
quelled the outbreak.
As another example, CDC reported that
in 2015 a single measles-infected person
attending a theme park in California resulted
in 147 cases of measles, which then spread
to multiple other states, Canada, and Mexico as exposed people incubating the virus
returned home.
Measles in otherwise healthy children can
be self-limiting, although most children are
quite miserable during the illness. Complications can be severe, however, and can include
pneumonia, blindness, encephalitis, and
in 1 in 1,000, death. In about 1 in 10,000
cases, measles can cause subacute sclerosing
pan-encephalitis, a fatal late neurologic
manifestation of measles that can occur a
decade after the initial infection.
Infants less than one year of age, prior to
their first dose of MMR, are at particular
risk of contracting measles. Other groups at
risk for complications include children less
than 5 years of age, adults over 20 years of
age, and pregnant women. Compromised or
waning immunity related to HIV infection,
leukemia, lymphoma, high-dose steroids,
and immunomodulator therapy are risk
factors at any age.
Nosocomial transmission of measles to
such compromised patients is of particular
concern, because the disease may have an
atypical presentation (including lack of
the characteristic rash) and the complications can be devastating. In 2015 in a
Shanghai oncology center, for example, a
single measles-infected child infected 23
immune-compromised children. Half of
those children developed severe measles disease, and 21 percent died. This highlights a
particularly consequential threat of endemic
measles in the modern era.

LANCASTER

PHYSICIAN

What has changed to facilitate the current
outbreak is a waning of immunity in the U. S.
population. Measles occurs in people who are
either unvaccinated or whose immunity has
waned. Control of this highly contagious virus
requires high levels of herd immunity (95 percent of eligible persons) to prevent sustained
transmission. Currently, the United States'
overall measles immunity rate is estimated to
be about 92 percent, but it is important to
realize that in many sub-populations the rate
is much lower. Vaccine hesitancy, fueled by
the propagation of misinformation, as well
as lax school enforcement of state requirements and loose granting of exemptions, are
themselves major contributors to the current
measles resurgence.
To be clear, there is absolutely no scientific
link between the MMR vaccine and autism
spectrum disorder. The now infamous study
purporting such a link from 1998 was subsequently thoroughly debunked and the paper
withdrawn by the journal that originally
published it. Since then, there have been
many studies looking at MMR safety and
finding no link to autism whatsoever. The
public should remain confident in the safety
of MMR vaccination, especially relative to
the consequences of measles infection. MMR
is a highly effective vaccine. For just the
years 2000-2015, the CDC estimates that
MMR prevented over 20 million measles
deaths worldwide.
MMR is, of note, a live attenuated vaccine.
It is contraindicated in immunocompromised hosts, pregnancy, and those with
prior allergy or severe adverse reaction to
the vaccine or its components (the vaccine
may contain trace amounts of neomycin and
gelatin). These restrictions, in addition to
those risk groups outlined above, comprise an
at-risk population that can only be protected
by maintaining broad community (herd)
immunity through vaccination. One dose
of MMR confers immunity in 93 percent
of recipients, with a second dose increasing
the efficacy to 97 percent.

Continued on page 26

Lancaster Physician Summer 2019

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