Has Israel Achieved Herd Immunity?

This is purely an educated layperson’s half-baked theory, and people much smarter than me with more expertise in epidemiology might instantly spot some obvious reason why what I’m about to write is completely and utterly wrong. But with that disclaimer out of the way…

I think there’s a strong chance that Israel reached COVID herd immunity around the first week of March.

What’s more, if that’s true then there’s some useful insights we can probably gain about the course of the pandemic here in the United States over the next few months.

About three weeks ago I noticed that while the number of new COVID cases in Israel had been fairly flat from mid-February through the first week in March, starting around March 8th the daily case counts suddenly started dropping in an almost perfect exponential decay curve. If you want to see this for yourself, I recommend the excellent COVID-19 data site 91-DIVOC. Displaying the one-week average of new cases (to smooth variations because of case reporting on weekends) and using the log graph (which displays exponential curves as straight lines), you can clearly see that cases dropped by more than a factor of 10 between March 8th and April 8th. This decline didn’t happen anywhere else in the world.

Israel has had the most aggressive vaccine rollout campaign of any country in the world, so it’s not surprising that they would reach herd immunity before anyone else. But it might be surprising that they reached herd immunity so soon, and that the number of cases would start dropping so abruptly. But this seems to make sense in light of an article I recently read in Scientific American about percolation theory, which suggests that networks of connections (such as the connections which transmit virus from person to person) you can get a phase change at a certain level of connectedness. If there’s enough connections in the network, nearly everyone is connected to everyone else. But below the threshold, you only have islands of people connected to each other. Disrupt enough connections—such as with a vaccine—and you abruptly go from global links and growing infections to only local links and pockets of infection which quickly burn out.

What will our summer be like?

Supposing this is true, and Israel’s vaccination campaign really did cross a threshold of herd immunity in early March, this gives us our first real-world indication of just what level of vaccination is needed to quell the pandemic. As of March 8th, 42% of Israel’s population had been fully vaccinated—but this might not be the best way to look at it.

The two-shot mRNA vaccines (which are the bulk of the vaccinations in both the U.S. and Israel) are extremely effective, with well over 90% effectiveness after both doses have been administered. But more recent data suggests that these vaccines provide a fair amount of immunity starting a couple weeks after the first dose. Maybe not 95% immunity, but perhaps 70% or more, which is still enough to stop most disease.

So for trying to predict herd immunity it may be better to ask how many people in Israel had at least one vaccine on February 22nd, two weeks before the March 8th date I’m using as the date herd immunity began. 48% of Israel’s population had at least one dose by that time. But because this is very approximate and I don’t know anything about epidemiology, I’ll round that up to a nice even 50% and predict that

A country will start seeing a rapid drop in COVID cases two weeks after 50% of the population has received at least one vaccine dose.

As of today in the United States, 35% of the country has received at least one shot, and lately we’ve been averaging about 0.725% of the population getting their first dose each day. We will cross the 50% threshold in three weeks, meaning the prediction is that in five weeks—May 15th—we will begin to see an exponential drop in COVID cases. It will likely take a couple weeks for the drop to become apparent, but by mid-June it should be clear to everyone that COVID infections have diminished considerably and are on a steep downward trajectory.

Once again emphasizing that I am not an expert in this field and I’m just playing around with numbers, there’s a lot of factors which might push the prediction a week or two earlier or later. For example, how may people are already immune because they got sick, how good we are about mask wearing, how many places insist on opening up prematurely, etc. But my non-expert intuition is that these are probably all secondary to how many people have been vaccinated, so we’re likely looking at the difference between reaching herd immunity in early May vs. late May. It won’t be May vs. October.

Since it’s always a good idea to test the sensitivity to assumptions, and I did make a couple of important assumptions, here’s the dates I would predict the U.S. reaching herd immunity using different criteria (in each case, these are thresholds Israel reached on March 8th):

• 42% of the population fully vaccinated: The U.S. is at 20% today and finishing vaccine administration on 0.61% of the population each day. We will reach herd immunity around May 16th.

• Two weeks after 34% fully vaccinated: The U.S. will reach 34% fully vaccinated around May 3rd, and reach herd immunity around May 17th.

• 96 vaccine doses administered per 100 people: The U.S. is at 54 per 100 today and administering 0.92 each day. We will reach herd immunity around May 26th.

• Two weeks after 82 vaccine doses have been administered per 100 people: The U.S. will reach 82 doses per 100 people around May 10th and reach herd immunity around May 24th.

• 54% of the population receiving at least one vaccine dose: The U.S. is at 35% today and will reach 54% around May 6th.

The fact that all these different ways to estimate when the U.S. could reach herd immunity are giving results within a few weeks of each other makes me more confident that mid-May is a good guess as to when we will start to see that rapid drop in COVID cases in the U.S. (but, again, it will take a couple weeks for the trend to become apparent).

So Why Aren’t We Hearing About This?

There’s a simple explanation for why most of us in the U.S. aren’t hearing the good news from Israel: the American media is notoriously bad at covering anything outside our borders. But a quick online search found a few articles from the international press tentatively suggesting that Israel might just maybe have reached this important milestone, however even overseas it doesn’t seem to be getting the attention one would imagine it would deserve.

My theory is that after a year of lockdowns, denialism, false hopes, incompetent political leaders, and the general trauma of it all, nobody wants to be the first to stick their neck out and say, “hey, this might actually be ending for real this time.” There’s also the risk that proclaiming victory prematurely might lead people to let their guard down and make things much worse.

And I get it. If I had a readership of millions, or even hundreds, I might be very hesitant to write this article I just wrote. But in this space I’m writing mainly for myself and a few friends/family plus the occasional random person who stumbles by. If I’m wrong, the damage is limited.

But if I’m right, remember that you read it here first.