COVID-19 Daily Update: Communal Immunity Levels

 

In the last update, I promised that I would talk about immunity levels in the population next. I'm going to do that today, but I want to first show why it matters. 

IHME vs. Youyang Gu, the Battle of the Models:

The IHME model is now predicting 410,451 deaths in the USA from COVID-19 by the end of the year.

They are making this prediction based on the assumption that there will be a large second wave in the fall, that is larger than either the first or the second peak in infections: 

In contrast, Youyang Gu's model predicts no such thing. Unfortunately, Youyang Gu's model only predicts out to November 1st, and the large second wave predicted by IHME happens after that. The two models aren't really all that different up to November, and Youyang Gu refuses to predict anything further out because he claims (with good reason) that no one really knows what is going to happen that far ahead. 

But even so, he's predicting a decline in daily deaths, while IHME is predicting a gradual rise.

Youyang Gu's model has historically outperformed IHME at every turn. He's been right, time and time again, while they have been less accurate.

What will decide which model is correct this time? I think it all comes down to immunity levels in the population. 

Communal Immunity Levels:

I suspect that most models are under estimating how many have already had the disease (and thus, they under-estimate how many are currently immune). 

We know the case counts under estimate by at least a factor of 10, but I think that some of the models that attempt to control for that may also under estimating by quite a bit more. At best this is a "hunch", but it is based on the idea that some of the serology experiments were missing people who had been infected, but hadn't produced sufficient antibody responses to be measured.

But immunity isn't just about how many have already had the disease. We also need to take into account that around 40% of us may be partially immune, before we catch it, due to cross immunity with other corona virus infections from the past year (T-cell reactivity from cross immunity is now well established in multiple studies). 

Why do some middle aged people without any underlying conditions feel really sick, while others end up completely asymptomatic? The full impact of cross immunity on COVID-19 severity and transmission is not yet fully understood, however, I suspect that cross immunity may help to explain some of the variations we see. We know that age and underlying conditions play a role there, but even when that is taken into account, the variability of response seems to me to be too large unless there are other explanations at play.

In terms of estimating the infection fatality ratio (IFR), the under-reporting of deaths which we discussed last time, and the under estimating true infection numbers may roughly cancel... We will need to wait and see exactly how that plays out.

But even if they cancel for the IFR calculation, the realization that more of us may already had the disease than we thought, and that many more may already be partially immune changes what we think will happen in the future... a lot. 

The impact of these assumptions on future modeling can't be understated.

This is everything. It will make all the difference. 

But is it true? Notice that the number of people currently infected is falling (this figure is again taken from Youyang Gu's model estimates):

And notice that it is falling as rapidly now as it was back in April, when we were in our half baked nonsensical and ineffectual lockdown that was never a real lockdown. Why is this happening?

There should be only two things that would make cases fall like this, immunity and physical distancing. 

But the amount of physical distancing in the US has remained stable since mid-June:

The other interesting insight is that the two separate peaks of cases in the US was not actually due to a second wave of cases hitting the same regions. Rather, our half-baked lockdown in April was just enough to keep the virus from spreading into new areas. Once we began relaxing physical distancing, the virus did not return to the regions initially hard hit. Rather, it spread into new regions, then began to decline in those new regions without a second lockdown! 

Physical distancing measures do not appear to be the sole driver of the current decline in cases. 

So what is the reason? 

Herd Immunity? 

One possible explanation might involve the common "herd immunity" claim. The term is badly defined for a natural infection, this idea was designed to explain vaccination induced communal immunity. But the ideas are somewhat similar. 

Many claim that herd immunity is the cause of the current decline in cases. 

The problem with this claim is that the herd immunity threshold depends on how infectious a disease is. The more infectious it is, the more people have to be immune for immunity to play the driving suppressing role.

Simple example: If each person infected normally would infect 2 others, then you need more than 50% of the population to be immune before each infected person naturally infects less than one other. 

Given how rapidly COVID-19 initially spread, estimates are that we need around 60%-85% immunity levels before we would expect COVID-19 to naturally begin to decline. 

But estimates are that only around 8%-20% of the population have been infected so far. 

EITHER those estimates are WAY off... or something else important is going on. 

My Speculation:

If cross immunity plays a major role, then we could imagine that 40% of us were already partially immune. Then, if an additional 20% of us have been infected, we could be reaching the 60% thresholds. 

On top of this, immunity and physical distancing interact in strange ways. 

Physical distancing creates an uneven distribution of immunity between those staying home, and those going out more. Those going out more are more likely to be infected and thus immune, while the fraction of the population that is immune may be low among those staying home. This can cause the infection rates in a population to decline, even if the immunity threshold has not been met for the entire population, but just for the sub-population going out more. However, under this scenario, if all physical distancing were to end tomorrow, then the balance of immunity in the population would shift, and a second wave would begin.

Whether the current decline in cases is largely due to cross immunity, or to a physically distanced temporary immunity threshold being reached by a sub-portion of the population, I think that immunity is playing some role in what is currently going on. 

If that is the case, then a second driving question for future predictions involves now how long immunity lasts. We know of a handful of confirmed reinfections already, but the fact that there are only a handful indicates that for most people, immunity lasts at least four months. 

The question is whether the average immune duration will last until a vaccine is available. If it does, cases will likely continue to decline. If it doesn't, they may begin to go back up at some point between now and December.