Why Are Covid-19 Cases and Hospitalizations Going Down in Some States?

The same reason views of this story will eventually peter out

A screenshot from my stats page

I have immediate family living in California, Tennessee, and Georgia. These areas of the country, along with many others, are suffering. Cases of Covid-19 are rising. Hospitals and health care providers are overwhelmed. Some patients are unable to receive the care they need. So, for my family’s sake and the sake of others living in hotspot areas, I don’t want to come across as tone-deaf or out of touch, but elsewhere, there is actually some good news about the pandemic.

In mid-November, my home state of Montana was hurting. We converted two additional units in our hospital to negative pressure areas so that patients with Covid-19 could be roomed appropriately. Our overflowing ICU was boarding patients in the post-anesthesia care unit. It was challenging to say the least, and for a detailed description, you are welcome to take a look here:

Since November, though, cases and hospitalizations in Montana have steadily declined and are now less than half of what they were at our peak. Other states in the Northwest and Midwest are experiencing a similar phenomenon. Here’s a look at hospitalization trends in Montana, Idaho, and Iowa (the first row if you viewing on a browser or the first three graphs if you’re viewing on the app) compared to trends in California, Georgia, and Tennessee (the next three graphs).

Hospitalizations in Montana, Idaho, and Iowa, respectively, are trending down. Data from the New York Times, open-source.
Hospitalizations in California, Georgia, and Tennessee, respectively, are trending up. Data from the New York Times, open-source.

As of January 6th, Montana had recorded 84,155 positive tests. The latest estimates suggest that the number of actual cases exceeds the documented positive tests by a factor of eight. Multiply 83,529 by eight, and you get 673,240 as a rough estimate of how many people in Montana have been infected, which is 63% of its population. This portion, along with the growing number of individuals becoming vaccinated, leaves the virus with fewer and fewer people available to be infected, at least for the time being.

It’s analogous to tracking the views of an online article. At first, each story I publish receives an increasing number of views each day. Then, as the number of views approaches the number of available interested readers, the proverbial market becomes saturated, and the daily views start to decline. Because readers aren’t interested in viewing my stories more than once — that would be a perverse act of self-cruelty, I must admit—the views continue to trend toward zero.

Let’s take a look at Idaho, where 145,134 patients have tested positive. Multiply that by the factor of eight described above, and you have an estimated 1.2 million cases, which is 65% of Idaho’s population. For Iowa, an estimated 73% have been infected. In contrast, using the same methodology and data sources, both California and Georgia have had approximately 51% infected. The implication is that their proverbial markets aren’t as saturated. That’s why their hospitalizations continue to rise.

But what about Tennessee? The methods above would estimate 71% of those in Tennessee have been infected. Can this be explained? It can by comparing trends in cases. Hospitalizations, as you may know, lag behind cases by two to three weeks. Here’s a look at cases in California, Georgia, and Tennessee. I apologize that the font is small, but the trend represented by the red line is what’s important.

Cases in California, Georgia, and Tennessee, respectively. Data from the New York Times, open-source.

As you can see, cases in Tennessee (the last graph) have already trended down whereas California and Georgia have yet to do so. This makes sense because there are fewer people available for the virus to infect in Tennessee compared with California and Georgia. As a result, I would expect hospitalizations in Tennessee to show a downward trend sooner than they will in California and Georgia.

So now we understand why cases are going down in some states, but what’s the implication? Have some places reached the mythical herd immunity threshold?

The concept of herd immunity has been promoted, criticized, debated, slandered, and basically talked to death to such a degree that I hardly even want to mention it. In fact, I think I just threw up in my mouth a little bit.

Here’s the thing, though: The term, ‘herd immunity,’ is best used when referring to a pathogen that can be eradicated, or nearly eradicated, by a durable vaccine. At this moment in time, there are too many unknowns about Covid-19 to discuss the pandemic in terms of herd immunity. We don’t know how long natural immunity or immunity to the vaccine will last. We don’t know how many individuals can or will become immunized, and we don’t know if the virus will evolve to become resistant to a vaccine.

A more applicable term is that of ‘herd protection,’ which is a combination of factors that influence how Covid-19 interacts with a population. These factors include the degree to which individuals mix with one another, cross-reactive immunity to other coronaviruses, variation in susceptibility, naturally acquired immunity, and immunity acquired by vaccination.

Many have wondered how herd immunity can occur if people can become reinfected and if we don’t know how long natural immunity lasts. These are important questions and are a big reason why thinking about the issue within the framework of herd protection makes more sense. Let’s address both these concerns.

You may have heard the fantastic news that both the Pfizer and Moderna vaccines under emergency use authorization by the Food and Drug Administration have been shown to be over 94% effective at preventing Covid-19. What you may not have heard is that a recent study indicates being infected with Covid-19 is nearly as effective — over 90%. This is one more piece of data to support the notion that while reinfection is possible, it remains very unlikely.

While it’s obviously better to receive a vaccine than an infection, both are very good at preventing Covid-19 in the short term. Unfortunately, we don’t know how long protection from either natural infection or the vaccine will last, and we probably won’t have this information anytime soon. Let’s say, for example, immunity lasts for an average of 15 years. We wouldn’t be able to determine that at least until the year 2035.

It has become apparent that some areas of the country are indeed approaching herd protection, but what happens next? We have to control what we can control. Some of the factors that make up herd protection are modifiable, such as the degree of mixing and the number becoming vaccinated.

In Montana, we still have a mask mandate and a directive to limit gatherings to fewer than 25 people. If these measures were relaxed now, we could absolutely expect to see a rise in cases, probably not to the level where we peaked in November but an increase nonetheless. This would be a tragedy because it would lead to unnecessary deaths that could be prevented by waiting until a sufficient number of individuals become vaccinated.

What about other regions like the Northeast, where many states are experiencing an increase in cases? After the initial surge in March, the state of New York managed to control the pandemic well for much of the year. While cases and hospitalizations are rising there once again, only an estimated 41% have been infected, and even that might be an overestimate because of New York’s relatively robust testing infrastructure. In places like New York, where so many people remain available for the virus to infect, mitigation efforts are essential to prevent hospitals and health care systems from being overrun.

We have endured nearly a year of Covid-19, and only a few months remain until many of the burdensome precautions we grow weary of following will no longer be necessary to keep us safe. With safe and effective vaccines becoming available, there is a strong incentive for those who have managed to avoid infection to continue to avoid exposure until they have the opportunity to become vaccinated, particularly those at high risk of complications. Moreover, since we don’t how long natural immunity lasts, even those who have recovered from Covid-19 may benefit from a vaccine and the boost in immunity it would likely confer.

Because of a different vaccine, the varicella vaccine, which was developed when I was in elementary school, I never contracted chickenpox. For the same reason, I will most likely never know what it's like to have shingles — a painful rash that can occur in patients who have had chickenpox in the past. I’m okay with that.

Likewise, I’ll probably never know how it feels to suffer from Covid-19. I just received my second dose of the vaccine this week and am feeling well. There’s no FOMO happening here.

But I’ve taken care of plenty who have suffered and even died from Covid-19. Nearly all of them would have chosen to receive the vaccine, were it available, before they became ill. Here’s wishing you a different path to immunity. Here’s to a safe new year and hopes that your shot to receive the shot arrives soon.

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

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