May Miscellany – Some Covid-19, Some Otherwise

Doc Gumshoe looks at Herd Immunity, Vaccines and Treatments... plus some updates on Alzheimer's Disease, Dementia and more...

I’m happy to report that despite the huge quantity of COVID-related news items that flood my in-box (as well as all news media), the tsunami has not drowned other health and medicine-related releases. Now, with the pandemic retreating – and, yes, it really is retreating – let’s use this as an opportunity to bring a few other issues to your attention.

But first, some COVID-19 bits.

How many people in the US have been infected with SARS-CoV-2? And what does that tell us about getting to herd immunity?

As of today (May 6th, as I am writing this) the recognized number is 32,559,283. These are confirmed cases, generally by means of one of the tests for the coronavirus. The question is, how many people have been infected with the coronavirus without having developed severe symptoms and without having been tested? That question is particularly important from the perspective of attaining herd immunity. We can assume that the 32-plus million people who have had confirmed cases have at least a degree of immunity. But how many more people are there who can be added to the herd immunity cohort?

A study in May through July 2020, in 11,832 volunteers who had not previously been diagnosed with COVID-19 reported that 4.6% of these individuals had indeed been infected with SARS-CoV-2. There was considerable variability in the seropositivity rate, depending on location and individual characteristics. As one would expect, the higher positivity rates were in regions with early outbreaks, and also in specific populations, such as younger participants (5.9%), females (5.5%), African-Americans (14.2%), Hispanics (6.1%), and urban residents (5.3%). Adjusting the data from the study to reflect the entire US population indicated that for every diagnosed case of COVID-19 by mid-July of 2020, there were 4.8 undiagnosed cases, and a total of 16.8 million undiagnosed cases nationally.

If the same ratio of undiagnosed to diagnosed cases detected in that study persisted to the present day, that would imply that in the US as of today, instead of the 32 million currently confirmed cases, there would be about 150 million people who had been infected with SARS-CoV-2, and would consequently have at least partial immunity. That figure is absurd; since those data were collected, the rate of testing has soared. We can be confident that there have not been 4.8 undiagnosed cases for every diagnosed case in the period from July 2020 to the present. At the same time, we can be equally confident that many mild cases escaped diagnosis, and there is abundant evidence that even mild cases result in a degree of immunity.

The question is, how many undiagnosed cases should we (speculatively) add to the 32 million confirmed cases to arrive at a total number of individuals who now have some immunity due to having had COVID-19. Can we reasonably assume that the number is around 50 million or 75 million? Or even 100 million?

Add that number to the 130 million Americans who have received at least one dose of the vaccine, and we get an idea of where we are in getting to herd immunity. The proportion of the population that must have some degree of immunity in order to achieve herd immunity is generally put at about 70%. In the US, we’re not there yet, but we’re getting closer by the day. If the combined total of the vaccinated population plus the people who have had at least a mild case of COVID-19 is somewhere between 180 and 200 million, which seems possible, then the herd immunity threshold in adults – at least in some locales – may already have been reached. Is that overly optimistic?

Understandably, the authorities (Fauci et al) are not making any announcements to that effect, and Doc Gumshoe bets they won’t any time soon. The news media have been treating the concept of herd immunity as though it was an absolute – we either reach it or we don’t, end of story. But of course, there are lots of variables that have to be considered. One of them is this: which herd are we talking about? If the herd consists of the entire human population on Planet Earth, then we’re a long, long way from attaining herd immunity. If, on the other hand, the herd consists of the population of our town in Western Connecticut, we might be pretty close.

Also, the way herd immunity works is by depriving the pathogen – in this case the coronavirus, SARS-CoV-2 – of hosts in which it can multiply. Remember, viruses are unable to reproduce on their own – they must use the resources of the host to replicate. When the novel coronavirus first emerged from whichever animal it had come from, there was no shortage of human hosts anywhere in the world. But now, in many parts of the world, vaccinations and previous infections have already reduced the number of potential human hosts, and this number will continue to shrink.

Taking these factors into consideration, I will state my opinion: in some regions, both in the US and elsewhere, a degree of herd immunity is well within reach.

The Pfizer-BioNTech vaccine works against the South African variant…

This conclusion is based on a Phase 3 clinical trial in South Africa, where the B.1.351 variant is the predominant strain of the coronavirus. The trial enrolled 800 subjects, and nine of those in the placebo group became infected with the virus, versus none in the group that received the Pfizer-BioNTech vaccine. That would translate to a vaccine efficacy of 100%. The trial confirmed previous in vitro studies that also demonstrated broadly neutralizing antibody responses against the variant.

Also, on March 13, Pfizer-BioNTech released data from a larger Phase 3 trial enrolling 46,307 subjects showing 91.3% efficacy against symptomatic illness seven days