Whatever Happened To Herd Immunity To Covid-19?

As I write this article, we have been struggling through the Covid-19 pandemic going on two years. I remember thinking in March 2020 that this will be a tough slog, but by the end of the summer we should be over the hump. Yes, it would be a trying few months as people inevitably became sick and even died, the shutdown created massive unemployment, and connections with family and friends were put on hold. In fact, by the end of the summer of 2020 the outlook was more optimistic. Documented cases and related hospitalizations and deaths were on a steady decline. Oh, there were warnings about an increase in cases during the upcoming winter, but after six months of stringent restrictions, there was light at the end of the tunnel.

Of course, we know about the multi-car pile-up that engulfed that tunnel. Cases surged in the 2020–21 winter beyond the expectations of many experts, creating critical strains on our healthcare infrastructure, as well as the very fabric of our society. In the midst of this, however, there was hope! Remarkable new vaccines had arrived in record time, with the promise of achieving the epidemiologic holy grail of herd immunity. Yes, if we could only achieve herd immunity, that magical threshold of immunized individuals in the population that, once achieved, would allow us to put the raging pandemic in the rear view mirror. Stock markets surged in expectation of the ensuing “re-opening”, and once again, we could see light at the end of the tunnel.

Except it turned out to be a mirage. The tunnel kept getting longer, and the light grew dimmer as a large portion of the population refused to get vaccinated, and then the SARS-CoV-2 Delta variant mutation emerged to change the equation entirely. We stopped hearing so much about herd immunity, we trusted the experts less and less, and we devolved into a cacophony of blame and defiance.

So, what happened to herd immunity? Haven’t we had a very high number of people become infected or vaccinated and thus achieved a high rate of immunity sufficient to dampen the spread of the virus? Well, yes and no. According to a study that examined antibodies to the SARS-CoV-2 virus from more than 1.5 million samples of blood donated through the American Red Cross, through May 2021 more than 80% of the samples had antibodies from infection or vaccination. Six months later, that percentage is undoubtedly even higher. That’s a lot! Nonetheless, in many parts of the U.S., Covid -19 cases are climbing steadily higher. Didn’t experts like Dr. Fauci, the CDC, WHO, and other epidemiology and public health experts tell us that “herd immunity” would be achieved when approximately 75% of us were immunized through infection or vaccination?

Along came the double whammy of the Delta variant along with declining levels of antibody levels from the vaccines.

Yep, they sure did. Does that mean that they were deceiving us? Nope! What it does mean though, is that they were incorrect, because of two critical factors that they did not include in their original estimates. They did warn us that there were many uncertainties about the virus and the vaccines’ duration of protection, and sure enough, along came the double whammy of the Delta variant and a limited duration of circulating antibody levels from the vaccines.

Acquired immunity to viruses, through infection or vaccination, generally consists of an antibody response that can intercept virus before it can infect us and a cellular response that kills cells that become infected with virus, thereby stopping the replication of that virus. With enough antibody, a person can be exposed to a virus, but not become infected or symptomatic. If one does become infected and symptomatic, the cellular part of the immune response can then attack virus-infected cells and put a stop to rampant viral replication. You may feel really crummy for a few days, but your symptoms will soon subside and you’ll recover.

It’s completely normal for antibody levels to steadily decline after vaccination, even after being vaccinated with the mRNA or recombinant viral vectored Covid vaccines. These novel vaccines replicate actual infection better than any previous vaccines, other than modified live vaccines (such as the measles vaccine), and the immunity provided by these vaccines rivals or exceeds that created by actual infection by SARS-CoV-2.

The problem is the Delta variant, which is substantially more effective in attaching to receptors on our cells than the original SARS-CoV-2 virus. This variant began circulating in earnest in the U.S. in early summer 2021, causing another surge in cases. Then, as children returned to school for the first time in over a year, a new population of mostly immunologically naïve individuals became increasingly exposed to the Delta variant. Superimposed on that was the natural decline in antibodies in vaccinated adults, most of whom were vaccinated during the first 5 months of 2021. Because the Delta variant is so effective in attaching to and infecting our cells, even a small amount of virus sneaking past low levels of antibodies can invade our cells and make us sick. That’s the double whammy!

Fortunately, most vaccinated individuals who become sick from the Delta variant recover after just a few days of illness, because their cellular immunity kicks in to defeat the new infection. However, the double whammy of the more highly infectious Delta variant of SARS-CoV-2 combined with waning antibody levels makes previous estimates of herd immunity a moot point. In fact, the Delta variant has been compared to chickenpox and measles, insofar as its effectiveness in infecting people. For perspective, it’s estimated that herd immunity to measles requires at least 95% of the population to have immunity from infection or vaccination. That’s why the Covid tunnel has gotten longer and the light dimmer. The bar for herd immunity to Covid-19, the point at which a sufficient number of people have developed and sustained active immunity to the predominant circulating virus, has been raised to an unachievably high level, given current vaccination rates among adults and children.

So, if herd immunity has become a mirage, what can you do to protect yourself? Get vaccinated, and if you got vaccinated earlier in 2021, get a booster vaccine. Wear a mask in public, especially indoors, and avoid congregate settings. You know, the boring old public health stuff, because this thing ain’t going away any time soon. Public health policy has failed, because the public as a whole failed to comply with recommendations. You can argue about that all day long, but it doesn’t matter. We have what we have, we know what to do, and it’s each individual’s choice as to how to proceed, with known outcomes for each choice. And herd immunity? Go talk to the animals.

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