Herd immunity: How the US could reach a post-pandemic normal with 60% vaccination

As America’s Covid-19 vaccine rollout got underway last December, White House medical adviser Anthony Fauci said the herd immunity threshold for the coronavirus could be 80 to 90 percent. The implication was demoralizing: America would need to get as many as 90 percent of its people vaccinated to truly stop the virus from spreading freely. With roughly 20 percent of the population still resistant to getting a vaccine, near-universal vaccination seems very unlikely — leading a recent New York Times article to suggest that herd immunity may be impossible.

But recent data out of Israel paints a more hopeful picture: With around 60 percent of its population vaccinated, Israel has managed to almost fully reopen its economy and crush the number of coronavirus cases and deaths to nearly zero. The country still has some requirements in place — particularly indoor masking and vaccine passport requirements — but it’s much closer to normal than it could afford to be, with any guarantee of safety, just months ago.

We still don’t know with certainty what the herd immunity threshold is. Some experts said it could be as low as 60 percent. But Israel’s data suggests that we could get our lives much closer to normal without worrying about the risk of deadly infection — at much less than 90 percent vaccination rates.

It’s a sign that there’s a fairly safe gray area — an endgame to Covid-19 of sorts — between lockdowns and the pre-pandemic normal, after a year in which trying to find that balance in the US sparked wave after wave of Covid-19.

The best news for Americans: We’re not too far from getting 60 percent of the country vaccinated. Already, more than 40 percent of the population has gotten at least one dose, and more than 30 percent are fully vaccinated, based on federal data. At current vaccination rates, the US could hit 60 percent partial vaccination as soon as this month or June and 60 percent full vaccination in June or July — all within three months. Already, Covid-19 cases in the US have dropped by around 27 percent in the last two weeks.

There’s still a lot we don’t know. Hitting 60 percent might allow for infection control without being true herd immunity, when the virus can no longer circulate widely. Either way, getting the vaccination rate higher than 60 percent would still help ensure the coronavirus doesn’t come back — especially if variants, changes in the weather and seasons, or state-by-state variation in vaccination rates can let the virus break through our defenses.

So until we know for sure it’s safe, it’s a good idea to continue to be cautious in some ways, including wearing masks and considering limiting some activities to vaccinated people.

Still, Israel shows that we might be able to resume our lives much closer to normal — enjoying everything from restaurants to live music and sports — long before we vaccinate 90 percent of the country (if we can even do that). As University of Florida biostatistician Natalie Dean told me, Israel’s data “bodes well for what we can do in the US. Maybe we can get really far off of 60 percent of people being vaccinated.”

What Israel tells us about the inflection point

Compared to the rest of the world, Israel’s current experience can look almost like another planet. Writing in the New York Times, Isabel Kershner detailed the normalcy Israelis now find themselves in as they get “a taste of a post-pandemic future.” People are dining out, going to packed concerts, and attending sports events — often without masking and with little to no physical distancing.

Part of the story here is that Israel still has some precautions in place, particularly masking and capacity requirements for indoor venues. The country has also embraced widespread use of “Green Passes,” in which proof of vaccination, past Covid-19 infection, or a negative coronavirus test becomes a ticket for some of the riskier activities, although enforcement is reportedly spotty.

But the big element seems to be Israel’s world-leading vaccination campaign. Israel had tried reopening before, only to see some of the world’s biggest surges of the coronavirus last summer and then winter.

When Israel moved to nearly fully reopen in March, it still had a lot of daily new Covid-19 cases — more than twice as many per capita as the US. But this time, nearly 60 percent of the population had gotten at least one vaccine dose (which provides at least some protection), and more than 40 percent were fully vaccinated. With that, Israel’s cases started plummeting — dropping more than 95 percent through today. Israel had nearly fully reopened, but instead of the previous surges it saw in past reopenings, it saw a massive decline in cases.

Today, more than 60 percent of Israel’s population is fully vaccinated, and as the country remains open, Covid-19 deaths reported each day are in the single digits and sometimes zero.

“The Israel data should make us feel optimistic and persuade people to get the shots,” William Hanage, an epidemiologist at Harvard, told me.

None of this means that Israel has definitively defeated Covid-19. The apparent inflection point — 60 percent of the country at least partially vaccinated — could instead reflect the point at which “exponential decay” begins, as cases drop just as quickly as they previously rose. It doesn’t mean Covid-19 is vanquished, and the virus could still come back, even regionally. But it’s the kind of decline in Covid-19 that just about everyone has been waiting for since the vaccine rollout began.

There are some other notable caveats. The biggest: Israel has not completely reopened. It’s very likely that its current restrictions, from mask mandates to vaccine passports, are keeping cases suppressed — and things would be much worse without these interventions. Israel is not evidence that a place can nearly fully reopen by hitting 60 percent vaccination rates; it’s proof that a place can nearly fully reopen with the least intrusive precautions at 60 percent vaccination rates.

It could be that Israel is being too cautious and could fully reopen with none of these precautions at 60 percent vaccination rates, but we just don’t know that yet.

Another important consideration is natural immunity. In places that have seen big waves of Covid-19, including Israel and the US, many people have built up immunity after recovering from infection. We don’t know exactly the total number of people with natural immunity, given the role of asymptomatic spread. We also can’t just add the previous case count to the total number of vaccinated people because some previously infected people have gotten the vaccine (since vaccines provide more protection than natural immunity). But we do know natural immunity is another layer of population protection on top of the vaccine.

Other factors can also play a role. The number of daily new cases is important because more virus out there makes it much more likely that the pathogen will be able to jump from host to host. Seasonality can matter, too: If the weather is nice and people are able to do things outdoors, it’s going to be much harder for the virus to spread as the open air, heat, and humidity weaken it.

What might truly matter is the vaccination of older adults. Israel, notably, has vaccinated 90-plus percent of its population 60 and older. Given that people 65 and up in the US make up 80 percent of Covid-19 deaths, perhaps it’s vaccinating this group, along with populations with comorbidities, that matters most for the coronavirus endgame. That could even bring reported cases down, not just deaths and hospitalizations, since people are less likely to report asymptomatic or mild-symptom cases. (To that end, the US is on track for this goal, with more than 80 percent of people 65 and up now at least partly vaccinated.)

An important consideration is the massive state-by-state variation in vaccination rates. While 61 percent of people in New Hampshire have received at least one shot, only 31 percent of people in Mississippi have. So just hitting 60 percent at a national level likely won’t be enough for every state to swing its doors back open, at least safely.

Even with those caveats, the news from Israel seems pretty good: There seems to be a safe way to get much closer to normal at a 60 percent vaccination rate — and the US is very close to hitting that point.

There are still good reasons to aim higher with vaccines

When I asked experts about all of this, they repeatedly emphasized that we still don’t know what the herd immunity threshold really is. Experts give different estimations: While Fauci estimated the right threshold is 80 to 90 percent, Baylor College of Medicine infectious disease expert Peter Hotez told me his estimate of herd immunity is 60 to 75 percent.

Herd immunity could also be a moving target. “One of the challenges is people hear [of population immunity] and think of it as a very static concept,” Jen Kates, director of global health and HIV policy at the Kaiser Family Foundation, told me. But that’s not true, given the effects that different interventions, the weather, and other variables can have on the rate of infections and, therefore, the level of immunity required to suppress infections. “That makes it hard to say what the percent is for the US. It’s going to vary by place.”

That’s why real-world data matters. “The proof was always in the numbers, in what we actually observe in case incidence,” Dean said. “Everything else was just speculation, estimation.”

At the same time, the uncertainty surrounding the inflection point and herd immunity should make us all a bit cautious about putting too much stock in Israel’s experience. There are also other good reasons to get vaccinated, from the selfish (a vaccine is the best guarantee we have as individuals against the coronavirus) to the population level (more vaccination, above a theoretical herd immunity threshold, stops even more transmission).

“The hope is we’ll start to see declines as we start hitting around 60 percent,” Hotez said. “But I think to really bring it down to true containment mode and really stop transmission, we’ll probably have to go higher than 65 percent.”

One big complication here is children, since there are still no Covid-19 vaccines authorized for kids under 16. If the goal is to vaccinate 60 percent of the population, but there’s no vaccine for kids, the goal is really to vaccinate at least 75 percent of adults, which is a bigger challenge. With 56 percent of adults getting at least one dose, we’re not there yet.

To that end, America still has to do a lot more work to vaccinate people. The number of doses administered daily has declined in recent weeks to around 2.4 million on average from a high of nearly 3.4 million.

Experts attribute this to America’s vaccine rollout problem shifting from supply to demand. Brown University School of Public Health dean Ashish Jha used the analogy of a new iPhone coming out: So far, the US has vaccinated the most enthusiastic, those willing to camp out overnight for the vaccines. Now, the US has to make it less difficult for the less enthusiastic to get the shot — make it easier to get an appointment or remove appointment requirements entirely, and bring vaccines closer to where people are, including their homes, workplaces, doctor’s offices, or even hot spots for socializing and entertainment.

The US also may have to do work with those who are truly resistant. Based on public polls, that’s about 10 to 25 percent of American adults. The country could hit 60 percent of Americans or even 75 percent of adults without them. But converting as many of those people as possible still would help get the US there quicker. That could require extensive messaging efforts, especially from Republican leaders whose constituents are more likely to be hesitant.

If the US does all of this right, there’s genuinely good news: The end not only may be in sight, it’s perhaps even closer than we think.

“After going through this, people have a tendency to go crazy,” Kates said. “[People] are like, ‘Is this real? Are we really okay?’ The trauma is deep, so there’s a bit of anxiety about that. But I think we’re just in a much better place than we were not too long ago.”

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