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What a herd immunity policy would look like, and why it may be premature for Israel

Israeli health officials are reportedly considering aiming for herd immunity via mass coronavirus infection.

What would such a policy look like, and what are the benefits and grounds for caution?

A herd immunity model of the sort being mulled by Jerusalem, according to Channel 12 news, would move away from some anti-transmission measures and tolerate a rise in infection rates, with the assumption that the vast majority of the population will be spared from serious illness.

Once a critical mass of the population has become infected, the logic says, widespread immunity will slam the brakes on infection.

It is supposed that the coronavirus could become endemic.

This is medical shorthand for saying that immune protection would be so strong that there would be less transmission and very little hospitalization and death, though the virus would still circulate to a degree.

Research suggests that a combination of vaccine protection and antibodies from infection has high effectiveness in blocking future infection.

The herd immunity thesis supposes that unleashing Omicron, the variant most capable of breaching the vaccine but seemingly causing only relatively mild illness, could leave a large part of the population well protected. The virus would be all but edged out of Israel, at least for some time.

Such an idea has enthused and outraged different doctors at various points since COVID-19 emerged, and was particularly controversial early in the pandemic when Sweden embraced this approach and it became known as the “Swedish model.”

The influential epidemiologist Michael Edelstein, professor at Bar Ilan University and formerly a key figure in the UK’s public health leadership, said 14 months ago that Israel should adopt a “modified version” of the so-called Swedish model.

In retrospect, he now thinks that he spoke too soon, as Sweden had higher fatalities than he expected.

But this was before the world started rolling up sleeves and inoculating, a layer of protection that has not only reduced infection levels but also made symptoms, on average, much lighter when infection did occur.

“Now, things are different, and we have the tools to give the population a level of protection against the disease that we didn’t have back in the day. There wasn’t and it was much more of a gamble,” Edelstein told The Times of Israel.

He said that it isn’t just the level of “gamble” regarding immediate infection that has been reduced, but quite possibly also that of long COVID, as there is emerging evidence that vaccination reduces the incidence of long COVID.

“In fact, a herd immunity model today would look very different, and we probably shouldn’t still call it the Swedish model,” he argued, saying that a version of such a strategy appropriate for Israel may work for Omicron — but not yet.

“I wouldn’t reject such a plan, but it’s also too early to endorse such an idea,” he told The Times of Israel.

“We need to see much more homegrown Israeli data.”

The premise of a herd immunity approach is that infection can rise significantly with only a limited increase in serious illness and death — and without overwhelming hospitals’ capacity to treat patients.

Edelstein thinks this could make sense, but insists anyone who says they already know for sure isn’t reading the data properly.

Data from the UK and South Africa, which have both faced large-scale Omicron outbreaks before Israel, suggests that this variant is milder than others, and the UK data says it is 50 to 70 percent less likely to cause hospitalization.

But a drop in severity doesn’t necessarily ensure fewer serious cases if infection rates skyrocket, as expected with Omicron, Edelstein observed. “It’s a matter of math,” he said. “If Omicron is four times more transmissible than Delta but two times less severe, you still end with twice the number of people in hospital.”

Yet there is a more fundamental problem with the data that is meant to help us understand the trajectory of Omicron, Edelstein said.

“It’s very tricky to actually make use of data from other countries, because there are so many variables contributing to the epidemiological situation, so it’s hard to generalize the available data.

“It becomes too complex to model the pandemic when there are so many variables that make each country a story of its own: to what extent people are vaccinated, age and behavior profiles of the population, and various other factors.”

Edelstein noted that in South Africa, record-keeping in previous waves of infection was limited, meaning that many people will have unknowingly been coronavirus-positive. Recovered patients are known to experience future infection more lightly.

If a higher-than-documented proportion of Omicron cases in South Africa were recovered patients, this could skew the South African data, exaggerating the lightness of Omicron symptoms, according to Edelstein’s analysis.

The UK is also in “a very different” situation from Israel, he said, commenting: “The data coming from countries with high transmission, such as the UK, seems to show severe cases are low, but we’re on a different schedule for administering vaccines, and Israel gave booster shots earlier which means they will wane here first.

“Every country has its own epidemiological situation now, and in the case of the UK, it’s given different vaccines in many cases, applied different delays between shots, and some other differences. This underscores the fact it’s hard to draw inferences between countries.”

Homegrown Israeli data with the required depth doesn’t exist yet, Edelstein said, adding that the upside of Omicron’s inevitable fast spread is that it means a mass of data will quickly accumulate. He expects that within around a month, there will be reliable data on how the variant conducts itself in Israel, which will allow informed decision-making on the virtues or risks of a herd immunity model.

“It’s important to be open-minded and think creatively, but we first need the Israeli data on the proportion of hospitalizations and fatalities in the specifically Israeli context, because that’s what is needed in order to move forward,” Edelstein said.

If the evidence does stack up, and Israel chooses to aim for herd immunity by letting infection rise, Edelstein said such a move must come in addition to and not as a replacement for vaccines, which should first be given widely because of their power in reducing the intensity of illness.

He said: “It’s not too late for such a plan, but it requires maximum protection to the population beforehand in order to minimize the risk of infection to the absolute minimum, and that means vaccinating those unvaccinated and boosting those not boosted, and possibly re-boosting those who were boosted a while back.”

Source: Nathan Jeffay – TOI