A leaked scientific report jointly prepared by Israel's health ministry and Pfizer claims that the company’s covid-19 vaccine is stopping nine out of 10 infections and the country could approach herd immunity by next month.

The study, based on the health records of hundreds of thousands of Israelis, finds that the vaccine may sharply curtail transmission of the coronavirus. “High vaccine uptake can meaningfully stem the pandemic and offers hope for eventual control of the pandemic as vaccination programs ramp up across the rest of the world,” according to the authors.

The nationwide study was described by the Israeli news website Ynet on Thursday, and a copy was obtained by MIT Technology Review.

The findings are important because Israel is leading the world in vaccinating its population, turning the country into real-life laboratory to understand if vaccines can end the pandemic.

So far Israel has fully vaccinated 32% of its population, all with the Pfizer vaccine, and now has the world’s highest per capita rate of vaccination against covid-19.

The draft report confirms that the vaccine is able to cut covid-19 illness and deaths by more than 93% and also provides the first large-scale evidence that the vaccine may prevent most infections, including those that don’t cause symptoms.

That could allow Israel to become the first country to achieve so-called herd immunity, or levels of population resistance high enough to check the virus’s spread without lockdowns.

So long as the country continues to vaccinate people quickly and no variant emerges for which the vaccine has lower efficacy, “Israel may approach the SARS-CoV-2d herd immunity threshold by March,” the study claims.

Israel began its a campaign to immunize all 6.4 million citizens over the age of 16 in December and Prime Minister Benjamin Netanyahu has promised that if enough Israelis receive the shots, the country will be able to return to normal. “It’ll not only be the last lockdown, but we’ll be done with covid,’’ he said in a television news interview on February 15. “We’ll be the first to emerge from the coronavirus.”

Out in front

The unpublished, 22-page report was first obtained by Nadav Eyal, a prominent Israeli journalist, who described the findings on Thursday and published screenshots of the text on Twitter.

Pfizer did not confirm the authenticity of the study document. Its lead authors are Sharon Alroy-Preis, head of public health for Israel’s health ministry, and Eric Haas, a ministry researcher. In addition, the study was carried out by a team of eight Pfizer researchers, including epidemiologists Farid Khan and John McLaughlin and the company’s global medical lead for covid vaccines, David Swerdlow, an infectious disease expert previously with the US Centers for Disease Control and Prevention.

The research represents the first joint report by the health ministry and Pfizer since they reached an agreement earlier this year for Israel to share vaccination data in return for a steady supply of doses.

The cooperation is part of a wider effort by Pfizer to track how its vaccine, named Comirnaty, works in large populations. The company told MIT Technology Review earlier this week that it is studying “the vaccine’s real-world effectiveness at several locations worldwide, including Israel,” and “particularly looking at real-world data from Israel to understand any potential impact of the vaccine to protect against covid-19 arising from emerging variants.” Pfizer’s vaccine, like one from Moderna, another mRNA vaccine authorized for use in the US and Europe, uses two injections of messenger RNA carrying information about the virus to train people’s immune system to recognize and combat the infection.

The new findings are broadly consistent with separate announcements in recent days from two of Israel’s large health organizations, Maccabi Healthcare Services and Clalit Health Services, which together care for 80% of Israelis.

On February 14, Ran Balicer, chief of innovation and research at Clalit, the largest Israeli HMO, said that evidence collected on 1.2 million members “shows unequivocally that Pfizer’s coronavirus vaccine is extremely effective in the real world a week after the second dose.”

Other analyses suggest that serious infections and deaths have fallen among older Israelis, who got the vaccine first, but not among those younger than 44 who have not been vaccinated.   

The Israeli report describes observations made during three weeks in January and February when researchers were able to compare health records of unvaccinated people and people who had gotten their second shot more than a week before. They then compared the groups for five covid-19 outcomes: infection, symptoms, hospitalizations, critical hospitalization, and death. The unpublished study says the vaccine was around 93% effective in preventing symptomatic covid-19. Pfizer and its partner, the German biotechnology firm BioNTech, had found 95% effectiveness in their clinical trials carried out in 2020. The country-wide study was also able to show that hospitalizations and deaths dropped by similar amounts in the vaccinated group.

Because Israel tests people fairly comprehensively, the researchers were also able to estimate that the vaccine was 89.4% effective in preventing any detectable infection at all, including asymptomatic infections.

That finding, which is new, suggests that the vaccine could strongly suppress transmission of the virus between people and could help bring the outbreak to an end, a possibility Pfizer and the Israeli researchers say they are closely watching. “Israel provides a unique opportunity to observe the nation-wide impact of an increasing prevalence of immunity on Sars-Cov-2 transmission,” the authors wrote. Eric Topol, a doctor at Scripps Research in California, who reviewed the document, says that “the blocking of infections here speaks to the vaccine’s impact on asymptomatic transmission, which we’ve been unsure about.”

But Topol cautioned that the current study is “not conclusive on its own” and that ruling out asymptomatic transmission will require more frequent testing, a type of study that Pfizer is also undertaking. Another unknown, says Topol, is whether or not protection from the vaccines wanes with time.

Success against the variant

The vaccination campaign in Israel occurred just as the hyper-transmissible “British” variant of the virus, called B.1.1.7, began to predominate. The B.1.1.7 variant was first reported in Israel on December 23 and was the cause of just over 80% of infections there by the middle of February.

Lab research has suggested that vaccines should be just as effective against B.1.1.7 as against the earlier strains, and the real-world experience in Israel is overwhelmingly confirming this.

“B.1.1.7 can be squashed with aggressive vaccination, which is big,” says Topol.

Other variants of the virus remain a concern, however. This week, Pfizer and BioNTech said laboratory tests pointed to weaker protection against a variant spreading in South Africa, known as B.1.351, which other vaccines have struggled against as well.

Although the South African variant is present in Israel, there were not enough cases to weight the vaccine’s success against it. The report cautions that “progress towards herd immunity in Israel could be disrupted” by new variants if the vaccine is less effective against them.

Herd immunity

Herd immunity is the threshold at which cases would start to decline even without measures like masks and distancing, because the virus would run out of susceptible people to infect.

The exact proportion of the population that would have to be immune to reach this threshold is unknown, however—estimates range from 60% to 85%—and it remains to be seen how many Israelis ultimately agree to be vaccinated. Vaccination is optional in the country and is viewed with more skepticism by younger age groups, as well as among ultra-Orthodox Jews and Bedouin Arabs.

Demand for the Pfizer shots in Israel has fallen 37% from a daily peak in January, when the vaccine was still restricted to medical staff and those older than 60. Also, children under 16 are not yet being vaccinated, and they make up about 29% percent of the country’s population.

The Pfizer vaccine requires two shots spaced 21 days apart, although results published in the journal Lancet on Thursday, which were also based on findings in Israel, suggest that even a single shot is about 85% effective two to three weeks after it’s given.

Good news

None of that means the vaccine is foolproof, and Israel is still recording some deaths even among those who’ve been vaccinated.

Still, overall the Pfizer results provide an encouraging picture for many other countries as they race to catch up with Israel’s vaccination efforts.

The US, which is vaccinating more than 1.5 million people a day with both the Pfizer and Moderna shots, holds the record for the most doses given overall—around 56 million. But with its larger population size, that amounts to only about 4.6% fully vaccinated.

On Tuesday, February 16, President Joe Biden outlined a goal of vaccinating the whole country within six months. “By the end of July we will have 600 million doses, enough to vaccinate every single American,” he said during a televised town hall.

Other countries confront bigger hurdles to vaccinating their populations. They may lack access to the messenger RNA vaccines, rely on less effective shots, face shortages, or struggle to access any supplies at all. Most of the developing world may not be fully vaccinated until 2023, according to some estimates, and that may include some of Israel’s neighbors. So far, Israel has not made its supply of vaccines available to most of some 5 million Palestinians in the occupied West Bank and Gaza Strip.