Myth: polio was already declining before the introduction of the polio vaccine in 1955

Fact: the epidemics were actually getting worse
1952: peak year in US with 57,879 polio cases
1954: vaccine trials, over 420k children vaccinated; 38k cases = 3rd highest year
1955: vaccine widely available; 29k cases = 6th highest
1957: 5,485 cases, lowest in 14 yrs
1962: 910 cases, lowest since start of epidemics in 1916
Even more obvious and dramatic on a log-scale chart:

Over 1000 cases/year every year before 1962
Fewer than 1000 cases/year since then
Fewer than 100 cases/year since 1967
Fewer than 10 cases/year since 1984
Now, many other disease *were* on the decline before their vaccine was available: tuberculosis, influenza, measles.

Why? A major reason was sanitation. Improved water filtration and chlorination, pasteurization of milk and other food safety, etc.
But improved sanitation *didn't* decrease paralytic polio—in fact, it made it *worse*. Much worse: paralytic polio was rare before the 19th century; the epidemics began only in the late 1800s (early 1900s in US).

Why, when polio is waterborne and carried in sewage?
Not all polio *infections* cause paralysis. Like covid, some infections are asymptomatic or only mildly symptomatic—just a mild cold.

Infants seem to escape paralysis from polio. (A little unclear to me whether this is because of age or leftover immunity from the mother)
So when the water was dirty, almost everyone was exposed to polio early—when it was relatively safe for them to get it. Then they got immunity and were protected for life.

But as we cleaned up the water, there was less infection.
This meant that often *first* exposure came *later* in life. And that was dangerous, much more likely to cause paralysis.

So as polio *infections* declined, epidemics of *paralytic* polio were on the rise.
Every year there was a new generation of children born into a cleaner world, a world that was safer in many ways, but now more dangerous in this one particular way.

Basically there was a cohort effect, with more and more susceptible cohorts each year.
This is why polio epidemics were getting worse over time, and why we absolutely needed the vaccine. And why Salk was hailed as a hero (quote from Enlightenment Now by @sapinker):
It's a good example of how progress (such as sanitation) can be mostly good but also have bad side effects that no one could anticipate. But we solve those new problems by moving forwards with new solutions (a vaccine), rather than by moving backwards.
The Salk vaccine announcement was actually *on this day*, April 12, 1955—65 years ago!

(Thanks to @tommorton for reminding me)
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