In an ideal world, yes, we would want to know this information, but logistically, it would be extremely difficult to collect and process enough blood samples to obtain useful estimates for the entire population.
Actually knowing all the real cases might be nice academically, but it wouldn't impact the actions we need to take today at all.
This is the core problem with any of the "prior COVID is as good as vaccination" plans, beyond that they required having COVDI at least once. (Which is kind of a big deal too.)
With the vaccine, we know the standardized dose received and it's impact. They've studied at least a few different doses to arrive at the one being given. Right down to the more recent kids under 5 needing the dose adjusted more.
To know if someone is likely to have had a adequate response from prior COVID comparable (or better) to vaccination we would need to know several things not normally known.
First did they really have COVID or just think they did? This one is mostly easy, anyone that had a positive COVID test at least knows they really did have it. Those without, it may or may not be to late to figure it out.
Second, was the infection enough of a dose to have a robust response comparable (or better) than vaccination? With the vaccine this is relatively easy. The entire manufacturing and delivery process is designed to give someone the exact (or better) dose that was studied. In recent 5-12 cases where an insufficient dose was used, they gave all the kids another dose of the correct size. Determining this was the hard part in the study so that now it's easy. With natural infection, nobody knows how much they actually got. Instead, we would need an after the fact test that can figure it out. Likely a blood test. It's hard to imagine this would be cheaper/easier/more likely for people to do than just getting the vaccine anyway. Even if you convinced people to get a more complicated blood test to determine it, some number would still need to get vaccinated because of an insufficient natural infection.
For those that had a robust enough response to prior infection, it would at least let them be "right" about vaccination. Probably still need a booster anyway. Getting COVID again doesn't sound like a fun booster. Still, there's value to being "right".
Here's a thought, tell everyone we have a great new test to determine if a prior infection was as robust, for anyone with a valid positive test. It's simpler than a blood test too with less effort. We give you this shot and it'll leave a bright neon green dot on your arm if your prior response was enough. If not, come back in a month for a second test. The second test will turn bright purple if the response wasn't enough and nothing if you're good. Different response to avoid the first test impacting the second. I'm sure we could do it for the exact same cost as vaccination.
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