When Will We Get to Herd Immunity?

Throw out the tedious calculus. It’s not that complicated. The simple answer to the question on everyone’s mind:

Herd immunity to Covid-19 is an enigma. It’s universally sought after regardless of party affiliation, religious creed, or level of vaccine acceptance. Think about it. Are you pro-herd immunity or anti-? Pro, of course! At the same time, it’s so poorly understood. How many people does it require? Is it permanent or temporary? Will it actually change anything? When can I take this frickin mask off?!?

NFL quarterback Cam Newton once said, “Hindsight is always 50–50.” I don’t bring that up to disparage Mr. Newton. In fact, he was quick to correct his mixed metaphor just seconds later. The point is that Cam’s Newtonian intellect astutely recognized how much easier it can be to understand something after it has happened.

While many health experts have tried to predict what percentage of the population must become immune to Covid-19 in order to reach herd immunity, no one really knows for sure. But, guess what? We will indeed know… after it happens.

Herd immunity is an event that transpires when enough of a population is immune to a disease— either by vaccination or prior infection — that the disease can no longer transmit effectively. We’ll have hit herd immunity when each infected individual, on average, passes the virus to less than one additional person.

Want to know what herd immunity looks like? Here you go:

Yep, it’s that simple. Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, described it succinctly, “Herd immunity is when enough people are immune for whatever reason, natural disease or vaccination, that the number of new cases drops with each successive individual.” At that moment, cases start to decline, and the curve slopes downward. That’s herd immunity.

This concept brings up a few questions. Was the United States at herd immunity at the end of January since cases started to decline? What about in April 2020? Weren’t cases declining then too? Take a look at the figure below showing Covid cases across the country this past year.

The answer is, no, we weren’t at herd immunity in April of 2020, August of 2020, or even January of 2021 because cases started to rise again significantly in many areas across the country.

But what about recently? Since cases are now going down once again nationwide, did we just hit herd immunity sometime last month? It remains to be seen, but we could have. It might sound crazy, but it’s actually possible that as long as individuals continue to become vaccinated and vaccine effectiveness outpaces the influence of viral variants, we may never see another large wave of cases sweeping the country like the previous four.

Does that mean Covid cases will drop to zero? Absolutely not.

Next, the novel coronavirus, SARS-CoV-2, will transition from pandemic to endemic which means it will continue to circulate among the population at less-than-pandemic levels. Once herd immunity has arrived in the U.S., case numbers will look more like this:

Even with the arrival of herd immunity, there will continue to be localized outbreaks in susceptible communities where, for example, close living quarters allow for easier transmission or low vaccine acceptance reduces the level of immunity, but the virus will no longer advance unimpeded across large swaths of the country. Especially once vaccines are approved for, and administered to, young children as anticipated by the end of the year, Covid-19 cases will appear only sporadically— similar to the course of another virus — the measles.

Like Covid-19, measles is a highly contagious and deadly respiratory virus. Below is a timeline of measles cases (red line) in the U.S. and how the numbers relate to vaccine development.

Widespread vaccination against measles began in 1963 (green line), and a sharp downward trend in cases occurred one to two years later — this marked the arrival of herd immunity. Measles remained endemic, however, and as you can see, clusters of cases continued to appear at substantially lower levels than what were seen prior to the vaccine. This continued until the arrival of the measles booster in 1989 (blue line). About a decade later, measles was officially eliminated (not quite the same as eradicated) from the U.S.

While measles outbreaks do appear from time to time, they are localized rather than widespread. Such outbreaks arise in communities where fewer individuals are vaccinated or in locations where immune senescence (a waning of immunity that can happen in the elderly and those with conditions that impair the immune system) has occurred, like in a nursing facility, for example.

Turning back to Covid-19, one might wonder: If we have already hit herd immunity for the nation, why should I bother getting a vaccine? Well, think about the measles. Millions of children are immunized against measles each year in spite of the fact that we have maintained herd immunity to measles for decades. It’s a good thing too because we would lose herd immunity in the absence of such vaccines.

Also remember: If at present the U.S. has indeed, providentially, reached herd immunity, it’s not entirely because of vaccines in arms just yet — it’s also on the backs, lungs, hearts, and arteries of those who suffered from natural infection. Unfortunately though, as is the case with other coronaviruses, natural immunity to SARS-CoV-2 wanes over time.

Scientists have good reason to believe that coronavirus vaccines confer an immunity that is more durable and longer-lasting than immunity gained from natural infection. That means even those who have recovered from Covid-19 will need to be vaccinated if they want to maximize protection for themselves and the population as a whole.

I would be remiss not to mention that the United States isn’t alone in its objective to achieve herd immunity. The entire world yearns for its arrival as well. Yes, the leaders of our nation are right to prioritize the health and well-being of its own citizens first, but a time will come when our focus will need to turn toward others abroad who are suffering. Providing vaccinations and additional forms of aid to countries with fewer resources not only serves to ultimately protect us, but it displays the character of our nation as a magnanimous global leader. If we are who we say we are, the world should expect nothing less.

If my experiences with Covid-19 have taught me anything, it’s that this virus is far from predictable, and much of what we know about it comes through the lens of hindsight. While some doubt if it is even attainable, I’m confident that if herd immunity to SARS-CoV-2 does arrive, it will most likely enter through the back door. In other words, we won’t know if it’s coming until it’s already here. I’d say the chances are at least 50–50.

Dr. Stapler practices adult and pediatric hospital medicine in Billings, Montana, and loves every minute of it (usually). Check out more of his work at bostapler.medium.com.

Health & science writer on Elemental & other pubs. Hospitalist physician in internal medicine & pediatrics. Interpreter of medical jargon. bostapler.medium.com

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