Rumpy5897 wrote: ↑
11 months ago
Thing is, we still need to be in that problem-solving mode. Just because you have fewer cases of the virus than you did at its peak does not mean it has gone away, and if people get careless then it will happily return. Thankfully some basic mechanisms demonstrated to slow the spread, e.g. mask wearing, can be quite easily adopted. Coupling that with some hopefully smart restriction lifting and contact tracing feels like a way towards relative normality. This is what the SARS/MERS-trained Asian countries seem to be doing. Here's a lovely write-up about the dynamics of this whole ordeal, which has the decency of starting from the absolute basics: https://ncase.me/covid-19/
Plus, recently there's been an influx of antibody studies, which seem to indicate that this thing's even more widespread than estimated. That's actually not necessarily a bad thing, it means that the virus is milder and we're further along our way to herd immunity than expected. Here's a nice collection of various samples: https://www.biospace.com/article/multip ... expected-/
I'm 100% for continued caution, acting on my educated guesses would be reckless and silly, but in addition to the virus being further spread than expected, it's also going to take less immunity for herd immunity to drop the reproductive rate under 1. The equation for calculating herd immunity in theory is a very simple model, and models are always wrong and sometimes useful, and the simpler the equation the less likely it is to be useful. It's like everyone who's taken high school physics has at some point known a projectile launched in a vacuum in gravity goes the furthest if launched at 45 degrees up from horizontal, but since nobody will ever be in that position, it's not something really worth knowing.
The simple theoretical calculation for herd immunity is: herd immunity level = 1 - 1/initial reproductive rate. That initial rate is being estimated between 2 and 3, so if we call it 2.5, you get 1-1/2.5 = 0.6. That why people think it will take 60% infected to reach herd immunity. But that equation is only valid for a homogeneously mixed population of equal susceptibility. It's like doing physics on earth's surface in a perfect vacuum, it's not a naturally occurring circumstance. People don't mix perfectly, some interact with lots of people and some with few. People aren't equally susceptible to viruses. Take measles: the basic reproductive rate is about 15, so in theory it takes 93% immunity to prevent an outbreak. There isn't 7% of the country unvaccinated against measles (at least not yet, hopefully not ever). But we're having measles outbreaks anyway, because children are more susceptible to it than average, and children interact aggressively with other children, the threshold to prevent outbreaks among children, especially in densely populated areas, is higher than that 93%.
Covid-19 is not like measles. Children are disproportionately unaffected. The populations that are more susceptible to it mix less aggressively with the general population than the average person (especially having now identified the at-risk, you're not gonna see many 80 year olds at football games for a while). And in adult populations, we tend to interact with a small set of people regularly (unlike kids who just get in everyone's personal space constantly). Each person who does interact with a large population regularly, people with public facing jobs like the grocery store clerk, the waiter, the nurse, etc, who becomes immune will be a greater roadblock for the virus than if we all were interacting with an equal randomized sample of people from our area. A single person acting as a disease vector to a large population can cause an outbreak, that same person being immune can prevent an outbreak, and a lot of those jobs never turned off. They're the people getting sick and getting immune while we shelter in place. That's going to make the necessary infection rate to impede outbreaks much lower than the simple model suggests. I've seen papers suggest as low as 20% for average population densities.
Edit: like this one https://www.medrxiv.org/content/10.1101 ... 1.full.pdf