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Without A Vaccine, Herd Immunity Won't Save Us
h/t Hot Air
[538] - ...Most people understand immunity to mean that once a person has been exposed to a disease, they can’t get it again. It’s an easy concept to grasp, and some people have hoped that widespread immunity could be the way out of this pandemic: If enough of the population becomes immune to the disease, the spread would be stopped, since the virus would run out of new, susceptible targets. The "herd" of immune people would protect everyone.

But getting to herd immunity without a vaccine isn’t as simple as the idea itself. A number of variables can affect when herd immunity is reached — and what it costs to get there — and they vary depending on the disease. How infectious is the disease? How deadly is it? And how long do people stay immune once they’ve gotten it? Adjusting any of these variables can drastically change the outcome of this equation. You can probably sense where this is heading ...

...If everyone in a population is immune to the infection, it can’t spread. But we can prevent a disease’s spread even without everyone being immune. If enough people are immune, the infection is unlikely to spread to big swaths of vulnerable people because those who are immune, the "herd," protect them. The more people who are immune, the more likely it is that infectious people will only come into contact with people who cannot be infected, ending the spread. This creates a societal barrier between the infectious and the vulnerable.

The moment when herd immunity kicks in depends on how contagious the pathogen is, which is measured by what experts call the basic reproduction number, or R0 (pronounced "R naught"). The R0 is simply the average number of people an infectious person will spread the disease to in a population where no one is immune, so an R0 of 3 would mean an infected person spreads the disease to, on average, three other people while they’re contagious.

...The higher the R0, the higher the proportion of the population that needs to be immune to stop its spread. This is known as the herd immunity threshold, and the formula for finding it is actually pretty straightforward: 1 — 1/R0. For a really contagious disease like measles, which has an R0 between 12 and 18, 93 to 95 percent of the population needs to be immune to stop the spread (this is why the U.S. has had recent measles outbreaks when vaccination rates dropped even slightly).

...For COVID-19, we’re still not certain what the R0 is, so we don’t yet know what the herd immunity threshold is. For now, it’s estimated to be anywhere from 70 to 90 percent. But here’s the problem: To reach even the lower end of that range naturally in the U.S. — imagine giving up on any interventions and just letting the disease run its course — 230 million Americans would eventually become infected and, depending on the fatality rate (more on that later), millions could die.
What most "anti-lockers" refuse to acknowledge is that the numbers on herd immunity (sans vaccine) are just as much bull (pulled from their proponents' ass) as the Ferguson model.
...How long are we immune once we’ve had it?
..."The presence of antibodies, everybody thinks that means immunity, but I study HIV and there’s a huge antibody response to HIV, and it’s never able to neutralize that virus," Bauer said.

Posted by: g(r)omgoru 2020-05-15
http://www.rantburg.com/poparticle.php?ID=571529