I have the same question. Shouldn't vast majority of people be immune by now (either through vaccines or natural infection)? Half of the country is vaccinated. We likely had at least twice as many infected people as our official number of infected (which means we had at least 80 millions infected). How is it that we are not at herd immunity?
I think your estimate of undetected COVID cases is a bit high. Short answer is that places like NY, Mass., RI, CA
are approaching or at herd immunity and despite Delta and its upsurge, are not seeing big leaps in per capita cases.
Overall rate of people who have had COVID in the US is 15%. Keep in mind that in some places, it's lower, in others it's higher.
In order to estimate the numbers of asymptomatic/mildly symptomatic people to add to that figure, we can use the overall infection rates in each locale. Mild and asymptomatic cases are not the majority cases. Increased screening in workplaces (for symptoms) all across the US has resulted in many a person getting rapid tested in order to go back to work or school, if they have the sniffles. The truly asymptomatic are very few; most are actually pre-symptomatic and will go on to test positive. Most studies on this topic estimate their number as 5%, but let's be optimistic and add 10% to our overall numbers.
That increases the total number of Americans who have had COVID to 16.5%. Again, let's be optimistic and round up to 17%.
Then, keep in mind that places like NJ or RI or NY, that had COVID circulating far in advance of places like OK or MS. So using the same percentage to estimate this is tricky. NY, NJ and RI are indeed approaching herd immunity, which is why you don't hear about 10 teachers dying in a brief period of time (today's news out of Miami-Dade).
In California, 66% of people over 12 have had at least 1 shot of a vaccine (and for some, that's all they were required to have, with J/J). Some of those, however, already had COVID so we can't just at our 17% to that. Studies here show that about 70-80% of people who had COVID really don't want it again - so they are vaccinated. We can therefore only add about 3-4% from the "have had COVID" pool to that. So in California, we are getting close to 70% of us having either had COVID or are fully vaccinated. Here's a graph that shows that California, while experiencing an unexpected and significant rise in COVID, is no where near where it was at the peak:
California coronavirus cases: Tracking the outbreak
The cases are almost entirely within the unvaccinated group (30% of California is still 13 million people. Larger than entire states.
So, if one of the most highly vaccinated states (where many counties still have mask mandates) still has 13 million people susceptible to COVID, that's a lot - enough to drive the new curve (which as you can see, is half or less of what it was when very few people were vaccinated).
Since society is sharply divided between vaxxers/anti-vaxxers and maskers/anti-maskers we can look at counties in California where those people are high in numbers - and yep, that's where we're getting our bump-up in cases. It's really very sad. It's mostly rural counties (and Marin County, which is a whole 'nuther story), and Orange/San Diego counties. There are also certain occupations in Los Angeles (inexplicably, firemen and some nurses) refusing to vaccinate, even though many said they would once Pfizer had full approval (I get that people are scared).
Some states are below 50% vaccinated. Add in a couple more percentage points for the "immune via having had COVID" category (so add 7%, being optimistic). That's not herd immunity. With this virus, scientists have been saying "at least 70%" (including children) but no one could know until there was actual data. For some viruses, it needs to be higher. It can even be 90%. Since the new variants are mutating faster than the old ones (and are more contagious), I'm guessing we'll need 90%. So...that's why even Massachusetts and California are seeing upticks. Take a look at this heat map (you'll have to scroll to it):
Coronavirus in the U.S.: Latest Map and Case Count
Notice how pale New England is (not much COVID).
And then...there's the waning immunity issue. From what I can tell in the academic literature, people who had mild or asymptomatic COVID are much more likely to experience a loss of immunity and get COVID again. I know a 20-something who has tested positive 3X over the past 15 months. This person is a student athlete and they had to get tested weekly to play. All three cases were mild/almost asymptomatic. No fever. Latest time had loss of taste/smell.
Sad part is that people who survived a severe case AND are vaccinated likely have organ damage of some kind now, so that even a mild case is dangerous for them. This would be the elderly and our immune-compromised populations. This naturally boosts hospitalization rates.
I think the data from Israel (probably the best overall studies) tell this story. Breakthrough rate is low, and the people who had the breakthrough cases (mostly mild - they studied healthcare workers who had to be tested regularly) had lower titers for the immune factors in the blood. For whatever reason, their immunity decayed more quickly than the average person:
https://www.nejm.org/doi/full/10.1056/NEJMoa2109072
3% of their fully vaccinated healthcare workers got COVID again. Of those, 19% had persistent symptoms. No one died. (So 0.6% of the vaccinated) I'd bet Delta variants were responsible for this (there are now several forms of that variant, all of concern, as well as C.1 and MU - I'm not up on all the Greek letters for each variant).
We'll get there. This was pretty wordy, but if you go to the NYT link above and look at "states where COVID is most on the rise" they include the vaccination rates for that state. Excluding Guam, all of the places have a lower than average vaccination rate. Some of them are also weak on social distancing and mask wearing (worldometers has that data).
TL;DR If you live in one of the states with a lower than 50% vaccination rate, and no one else wants to get vaccinated (state is throwing away vaccines, sending them to other places, etc), then...there's going to be continued COVID (probably at the plateau the state is at right now, perhaps increasing) until the other half gets COVID. There are lots of states in that group.
Sadly, Florida is going to probably end up with mortality rates similar to NY's, despite the fact that COVID hit NY way earlier and that there's now a vaccine...they're inching closer ever day. It's just happening more slowly (many factors).
