[conspire] San Mateo County elevated measles risk

[Rick Moen]

Quoting Paul Zander (paulz at ieee.org):

> I grew up before the vaccine. I can tell you that none of those
> diseases are any fun.

Around 1981, I heard the now-late science fiction/fantasy author Ray
Bradbury give a talk in San Francisco entitled 'Why I'm an Optimist'.
According to Bradbury, the subject hadn't been his choice, but he'd
thought about it and liked the idea, and said on reflection  he agreed
with it because he remembered as a boy in the 1920s visiting his
immediate family on Sundays -- in the graveyard.  As he reminded us,
that used to be what families did:  You got to visit the graves of your
brothers and sisters killed by childhood diseases that in those days
killed about 50% of children.

There he stood in 1981, most families no longer needed to observe that
sad Sunday ritual, and he considered that cause for optimism.


> I did get a polio shot and then the oral version a few years later. 

I remember, one Saturday afternoon in 1963, going with my mother and
sister to Woodside High School, Redwood City, to get the oral (sugar
cube) polio vaccine, OPV for short, invented by medical researcher
Albert Sabin and first available commercially in 1961.  Vaccine day was
a Big Deal.  Nobody was talking about it as we stood in line, but in
later years I realised just what a horrific threat polio had been all
the way through the mid-1950s, especially against children.

Albert Sabin and Jonas Salk (developer of the inactivated polio vaccine,
IPV for short, in 1955) were -deservedly- treated like gods for the rest
of their lives.  If anyone ever merited public admiration, it was the
both of those great Americans, especially since they both refused to
patent their creations, considering them gifts to humanity.[0]  (Wholesale
cost per dose of the oral vaccine at of 2014 was 25 cents.)  

I cannot recall my return visit for a second dose, but probably had one.
Apparently (per Wikpedia) this was part of the second, post-Salk wave of
USA mass polio vaccinations in 1962-1965, that covered about 56% of
Americans -- about 100 million of us marching down for slightly
funny-tasting sugar cubes.  And it was Sabin's vaccine, not Salk's, that
truly conquered polio in the USA:
https://www.technologyreview.com/s/404390/the-myth-of-jonas-salk/


By the way, one measure of progress:  It's no longer necessary to
include in the oral vaccine one of the three original polio strains,
because it was confirmed totally eradicated around the world in 1999.
New Sabin vaccine doses since then have been 'bivalent' rather than
'trivalent'.  (Unlike, say, malaria, it's possible to completely
eradicate a wild polovirus through vaccination.  Malaria cannot be
beaten that way because it persists in non-human hosts.)  The Salk 
IPV vaccine (killed samples of three wild poliovirus strains) is still
used but is much more expensive (wholesale $25-$50/dose), requires
injection, and has stringent transport/storage requirements, making it
problematic for hot or remote areas.  Wikipedia claims that a variant
form of Salk's IPV has since 1987 been the _preferred form_ in the USA
(https://en.wikipedia.org/wiki/Polio_vaccine#1987), and Sabin's OPV has
been discontinued entirely in the USA since 2000, which surprised me, as
I'd assumed IPV'd been obsoleted by Sabin's OPV (which also helps cover
non-vaccinated people because we OPV-vaccinated people shed the
attenuated Sabin virus, causing 'passive immunisation').


The reason I was skimming Wikipedia articles on the subject was to
guesstimate percent effectiveness, which is also important for measles
and ties to the related concept of 'herd immunity'.  

Effectiveness rates (percent of vaccinated individuals who develop
protective antibodies), per Wikpedia:
IPV is 60–70% effective against PV1 (poliovirus type 1).
IPV is 90+% effective against PV2 and PV3.
IPV is 94% effective against development of bulbar polio.

OPV immunity is thought to be longer-lasting than that of IPV, as it
provides both antibodies resident in the bodily fluids ('humoral
immunity') and immune response internal to cells ('cell-mediated
immunity').  Humoral immunity prevents a wild virus that arrives in the
human gut from spreading to the nervous system.  Salk's IPV works
through humoral immunity only.

OPV one dose is 50% effective against all three strains.
OPV three doses is 95-98% effective against all three strains.


Getting back to measles, the main reason it's a huge threat is its
unusually high infectiveness (especially airborne spread).   The rule of
thumb is that you need 90-95% of a population[1] to be MMR-vaccinated (the
measles, mump, and rubella vaccine) to have enough herd immunity to
prevent epidemic spread if an outsider brings a case in -- a higher
percentage than for any other vaccine-preventable diseases.  The
epidemiological effect is analogous to the dampening effect of pushing
control rods into an atomic pile:  Affected individual get through their
days of misery before they're able to infect enough others to keep the
chain reaction going.  The conventional 'reproduction number' (R) for
measles, the average number of additional cases a single infected
individual will cause to be infected, is 10-20.  For polio and smallpox,
R's about 5.  For influenza and Ebola, R's about 2.  So, measles is
_conventionally_ considered 5-10 times as infectious as influenza and
Ebola.
http://theconversation.com/herd-immunity-and-measles-why-we-should-aim-for-100-vaccination-coverage-36868
(In short, the aim of a herd immunity strategy is to reduce a disease's
R below 1, by reducing the number of available targets.)

But the _Lancet_ researchers' point is that presence of a major
international airport in a county alters the dynamic a bit.  Someone in
the first few days of an infection shows no symptoms at all but is still
breathing out the virus.  Then, 10-12 days after exposure, the victim is
sneezing and coughing, but we don't kick people off airplanes for just
that.  The characteristic red rash doesn't start until around day 3, and
symptoms fade after 7-10 days of symptoms, as the immune system mops up
the virus.  The point is that an infected traveler shares airspace with
an unusually high number of people, hence inflating the value of R way
beyond even 10-20.

Which, enfin, is why San Mateo County is #18 among that list of 25
counties most at risk, not on account of being a hotbed of antivax
beliefs, but rather a place with artificially increased R value for
arriving measles victims.


[0] Salk was embittered by this and by his permanent loss of privacy.
https://en.wikipedia.org/wiki/Jonas_Salk#Becoming_a_public_figure

[1] Despite its greater expense, storage/transport problems, and shorter 
effectiveness, Salk's IPV has one great advantage over Sabin's OPV:  It
cannot cause polio, full stop.  In a very small number of OPV
recipients, about one in a million, the weakened Sabin OPV virus reverts
to an active form and causes polio.  In an even tinier number, its
mutated form is not only paralytic but also infectious.

[2] This assumes random distribution of vaccinated individuals. But if,
say, there are subcommunities of antivaxers, epidemics can be expected
to occur starting in those because of local dips well below 90-95%
MMR coverage -- because, within those subcommunities, R remains well
_above_ 1, thus potential for chain-reaction infection there.