-an example of vaccines oversimplification, bad science
-this "explorable" is full of mistakes and errors:
This explorable illustrates the mechanism of herd immunity. When an
infectious disease spreads in a population, an individual can be
protected by a vaccine that delivers immunity. But there's a greater
good. Immunization not only projects the individual directly. The
immunized person will also never transmit the disease to others,
effectively reducing the likelihood that the disease can proliferate in
the population. Because of this, a disease can be eradicated even if
not the entire population is immunized. This population wide effect is
known as herd immunity.
-1/ "The system is initially fully susceptible with a few infected
individuals randomly scattered into the population". Usually, many
individuals without the vaccination are not susceptible, but immunized
because the natural infection.
-2/ "An infected individual remains infectious for some time, recover
subsequently, and become susceptible again". In many cases, an infected
individual recover subsequently, and become protected for the
whole life.
-3/ "The immunized person will also never transmit the disease to others"
In some case yes, in others no. See the diphtheria vaccine. Vaccinated
individuals can carrier toxigenic diphtheria germs and transmit the
disease to others.
-4/
In most cases (except perhaps for the vaccine of yellow fever)
"inmunity" is not for ever. Most vaccines simply delaye the "window" for
infections (they are time bombs). This is the case of measles vaccine and why in New Zealdand
will add a supplementary (3rd shot)
-the author could help readers with a "note" as:
"This a theoretical situation. In practice the situation of the naive
American indians when the Spaniards arrived with measles, for example (a
new disease in America). [but at that time we have no measles vaccine]
Or, when developing a vaccine against gonorrhea, where a protective
immune response is absent (by contrast syphilis infection produces a
partially protective immunity) "
-it is not so simple. Herd immunity. Measles. The 95% coverage mantra.
Measles outbreak with more than 97% coverage (Navarra, Spain; Porto,
Portugal; Gothenburg, Sweden
And many vaccines have no heard inmunity at all: tetanus, rabies, diphtheria,...