Study: Israel Far From COVID-19 Herd Immunity, 70% of Cases Originate in U.S.

A Tel Aviv University study finds almost no Israelis were infected with the coronavirus by a carrier from China

Ido Efrati
Ido Efrati
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Medical staff member enters the coronavirus ward at the Shamir Medical Center, May 12, 2020.
Medical staff member enters the coronavirus ward at the Shamir Medical Center, May 12, 2020.Credit: Moti Milrod
Ido Efrati
Ido Efrati

The proportion of Israelis infected with the coronavirus probably doesn’t exceed 1 percent, meaning Israel is far from achieving herd immunity, according to a study by researchers at Tel Aviv University.

Meanwhile, most infections in Israel occurred due to “super-spreaders,” the researchers said. In other words, contact with 10 percent of patients or fewer produced 80 percent of Israel’s coronavirus cases.

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The scholars, led by Dr. Adi Stern of the university’s School of Molecular Cell Biology and Biotechnology, said they hope their research can help the government locate and quarantine super-spreaders and make decisions about imposing and removing restrictions. Their findings are slated to be published on the website medRxiv, which is used to immediately share results with the global research community.

Stern’s team sequenced the genome of the coronavirus as it appeared in Israel. This enabled them to identify specific mutations that reveal the chain of infection – both how the virus entered Israel and how it then spread within the country.

Dizengoff Square, Tel Aviv, May 2020
Dizengoff Square, Tel Aviv, May 2020Credit: Tomer Appelbaum

All viruses tend to mutate as they spread, and Stern said the coronavirus accumulates mutations at a fairly regular pace. These mutations don’t affect the characteristics of the virus or its potential for harm, but they do allow the chain of infection from country to country to be traced. When the virus first erupted in China, for instance, it acquired one or two mutations. It then traveled to Europe, where it acquired additional mutations, and from there to the United States, and so forth. Stern’s lab sequenced coronavirus genomes from more than 200 Israeli patients. It also compared the results to sequenced genomes from 4,700 patients from other countries.

“We made major efforts to make the sample representative,” Stern said. “This is 1.5 percent of all Israeli patients – an especially broad sample for research of this type. We also made sure the samples would have a nationwide distribution, be spread out over time and include no relatives or physical proximity between the people sampled.”

The findings underscored one of the major breaches in Israel’s quarantine policy at the height of the crisis – the belated decision to require people returning from the United States to enter quarantine, and the subsequent lack of enforcement of quarantine for thousands of people who returned to Israel directly from hotbeds of the virus in the United States.

The study found that more than 70 percent of Israeli patients were infected by a virus that originated in America, meaning the original carriers caught it in the United States and then brought it to Israel. In most of the remaining patients, the virus originated somewhere in Europe, namely Belgium (8 percent), France (6 percent), Britain (5 percent), Spain (3 percent) or Italy (2 percent).

Only a small number of infections originated from other countries – roughly 2 percent each from Australia, the Philippines and Russia. Almost no Israelis were infected directly by a carrier from China, though both the European and American strains originally came from there.

Cases by country.

The new study shows America to have played an even greater role in Israel’s coronavirus crisis than an epidemiological report published by the Health Ministry on April 11 did. That report stated that in March, around 40 percent of verified coronavirus patients who arrived from abroad came from the United States – at least 573 people.

Stern said the number of new patients arriving from America has declined since then, but that fact says nothing about the role such patients played in infecting others.

“Our study doesn’t necessarily reflect the number of patients who came from the United States, but their role in infecting others,” she said. “Had all the patients who arrived from the United States entered quarantine immediately, their role could have been negligible. However, she noted, they weren’t required to enter quarantine.

“Even though the number of patients arriving from the United States moderated at some point, totaling 27 percent of all patients who returned from abroad, we see that they made a more significant contribution to spreading the infection in Israel,” she stressed. “Looking ahead, perhaps in the future we should close the country’s gates immediately upon the outbreak of a pandemic, and thereby significantly reduce the possibility of the virus entering Israel.”

The statistical model the researchers developed on the basis of their DNA sequencing showed that the transmission coefficient in Israel – meaning the number of people each person infects – fell significantly after the lockdown was imposed.

“The transmission coefficient is an average,” Stern explained. However, when the researchers “dove into the data,” they found that not everybody infected the same number of people. Instead, there were “super-spreaders who created most of the infections,” she said. “We don’t know whether what makes a person a super-spreader is an issue of biology, behavior (like significant contact with big crowds) or a combination of the two.”

If the virus had spread to more than 1 percent of the population, “we would have seen many more mutations,” she added. “It’s like an evolutionary tree that creates more and more branches. Chances are high that the number of people infected in Israel is very similar to the number of verified patients.”

Now, the researchers hope their data will be used by decision makers. “In this study, we developed tools that will enable us to cope better, in real time, with the next outbreak of the coronavirus, or with any other epidemic that might arise,” Stern said.

Five Tel Aviv University students worked on the study – Danielle Miller, Noam Harel, Talia Kustin, Omer Tirosh and Moran Meir. Researchers from Emory University in Atlanta, Israel’s Gertner Institute, the Holon Institute of Technology and the Technion Genome Center also participated, as did researchers from six hospitals – Sheba Medical Center in Tel Hashomer; Assuta Medical Center in Ashdod; Jerusalem’s Hadassah Hospital, Ein Karem; Poriya Hospital in Tiberias; Soroka Medical Center in Be’er Sheva and Barzilai Medical Center in Ashkelon.

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