Short version: Here’s a visualization for what’s going on in the northern latitudes that have seen Spring and Fall spikes each followed by drastic reductions:

Seasonality suppressed Covid in the Spring/Summer and we are now seeing suppression from herd effects.

I’ve been trying to figure a way to communicate with those who dismiss seasonality and seem to lean heavily on NPIs (masks, closures, distancing, etc.) as the main driver of Covid behavior. To do this, I started with the remarkably similar curves in much of the northern latitudes

I came up with this graphic: Using a typical northern hemisphere Covid curve, I charted forces acting on Covid numbers.

Covid in this model is a constant upward pressure, meaning if there’s one case out there, Covid will tend to spread in an unprotected population.

This sets a framework for discussing the downward forces.
Covid didn’t spread unchecked. For much of the North, it waned during late Spring and Summer.

I think those who believe NPIs are the main driver of Covid numbers would see the chart like this:

NPI advocates would explain:

When the force of NPIs is greater than the force of Covid, Covid (cases, hospitalizations, deaths) are driven downward. In this scenario, immediate harsh lockdowns quickly turned the curve downward in the Spring.

As summer wore on, restrictions were lifted & NPI effectiveness weakened. In fall, w/return to school, the NPIs were no longer enough to keep Covid in check & cases exploded. Only when people realized the danger, did they start to mitigate seriously again & bend curve downward.

This reasoning was explicitly described by Fauci. https://twitter.com/Dierenbach/status/1343983070656004099?s=20

Frankly, I don’t believe this scenario. NPIs of varying stringency and different timing have resulted in similar outcomes in too many places to declare NPIs strong enough to control the spread of Covid.

Masking for example, has been hailed in Czech Republic and New Mexico as an important tool before cases in both places exploded, seemingly contradicting their effectiveness.

https://rationalground.com/scientific-american-unintentionally-argues-against-lockdowns/ https://twitter.com/snorman1776/status/1349095869593874433?s=20

I haven’t seen convincing real-world data proving effectiveness of NPIs. So what are other possible downward pressures?

Many arguments attributed Spring downturns to NPIs. Before spring surges, there were no NPIs, then the most strict NPIs were enacted quickly & cases waned.

So they assumed NPIs did it. But if that were true, why in Fall did COVID hit harder than in Spring when many places had significant NPIs already in place?

And how did only relatively minor increases in NPIs in the fall (vs change in Spring) cause such a sudden downturn?

Moreover, in the fall, many places saw similar downturns despite radically different NPI schemes.

It’s clear NPIs aren’t the main driver. Something else is going on.

Knowing the flu typically rises and falls within a few weeks, I saw Spring rise & drop as a natural process. I turned to herd immunity as main driver. Many of us felt Covid had run its course & enough people exposed to cause Covid to slow in late Spring.

Fall proved that wrong.

The rise of Covid in the Fall coinciding with the typical timing of rising flu cases made many of us look back at the late Spring and realize it wasn’t herd immunity that slowed Covid, it was likely seasonality.

Again, this was hard to ignore:

Seasonality provides a window (around Oct-Apr) in which certain viruses can operate.

Why? Don’t know. Sunlight, vitamin D, temperature, humidity, pollen? Yet everyone knows of the “cold and flu season”.

Seasonality is a well-established trait of many respiratory viruses.

So I’ve ended up here.

In this graph, I show Seasonality as the main driver of the reduction of Covid over the summer.

I give little credit to NPIs ( Distancing, masks, closures, etc.) as I have seen little real-world data supporting their effectiveness.

Herd effects are generally cumulative in that reinfection is uncommon. When people catch Covid, they are effectively removed from the pool of possible victims. When cases spike quickly, the potential for herd effects to become a factor also increases rapidly.

If me and many others are correct, herd effects, both natural and due to vaccinations, should continue to suppress Covid for the next couple of months and hopefully when the summer rolls around in the North, seasonality will drive it down further.

Considering reduction was happening before vaccines, vaccines should help the effort. I believe we should remove restrictions immediately. Particularly since the suppressing effects of seasonality are just around the corner.

I've provided my theory based on timing of the Spring and Fall surges and downturns and the clear suppression that happened during the summer.

What are your thoughts? What relative role do NPIs, seasonality and herd effects play? Less lethal mutations?

Fill it in, let us know.