Israeli Study: Pfizer Coronavirus Vaccine Is 91 Percent Effective at Preventing Infection

A joint study with Israeli HMO Maccabi and the Technion, which included data from 1.7 million Israelis, shows protection against coronavirus peaks 35 days after first jab, at 99.3 percent

Ido Efrati
Ido Efrati
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A woman receiving a coronavirus vaccine in Rabin Square, Tel Aviv
A woman receiving a coronavirus vaccine in Rabin Square, Tel AvivCredit: Hadas Parush
Ido Efrati
Ido Efrati

Two weeks after the second dose of the Pfizer-BioNTech coronavirus vaccine is given, it is 99% effective in preventing symptomatic COVID-19 and 91.2% in preventing infection.

The data is among the findings of an analysis of the electronic health records of 1.7 million Israelis. Conducted by Maccabi Health Services and the Technion – Israel Institute of Technology, the study shows the effectiveness of the Pfizer vaccine gradually increasing starting at 12 days after the first dose and reaching maximum effectiveness at around 35 days (about two weeks after the second dose).

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Effectiveness was the same for men and women, but declined “mildly but significantly with age” – particularly after age 80 – and for patients with specific chronic comorbidities, most notably type 2 diabetes” and also conditions including heart or lung disease, the authors wrote.

The study was led by Roy Kishony and Idan Yelin of the biology department and the Lorry I. Lokey Interdisciplinary Center for Life Sciences and Engineering at the Technion, along with KSM, the Maccabi Research and Innovation Center, headed by Tal Patalon. It was posted March 17 to medRxiv, the preprint server for health sciences, before peer review and publication, under the title “Associations of the BNT162b2 COVID-19 vaccine effectiveness with patient age and comorbidities.”

The researchers analyzed the electronic medical records from December 1, 2020 to February 25, 2021 of nearly 1.8 million Maccabi members aged 16 to 90, bringing to bear statistical tools of bioinformatics on broad, deep raw data from the health care network, together with the analysis of Kishony and his colleagues.

Kishony is considered an expert in mapping and forecasting the dynamics of pathogens and the spread of infection using statistical models. He notes that the study used mathematical models that allowed the researchers to control for behavioral effects of the vaccine.

The findings on when the vaccine reaches maximum effectiveness corresponds with those of a study published by the Meuhedet health network earlier this month, of 100 vaccine recipients. It also found that maximal effectiveness is achieved two weeks after the second dose. Currently, inoculated Israelis are eligible for the Green Pass just one week after receiving the second dose.

The Maccabi-Technion study also indicates that the vaccine’s first level of effectiveness is reached 12 days from the first dose and that it remains at this level for more than two weeks. “The study shows that the first dose provides 50% effectiveness. The leap in effectiveness – and significant drop in morbidity and infectiousness – only occurs about a week after the second dose and reaches an optimal level from the 35th day, on average,” says Gabriel Chodick, head of database research and director of the epidemiology unit at Maccabi, who was also an author on the study.

“Comparing effectiveness across age groups, we find that although vaccine effectiveness is relatively similar for age groups between 16-80 years old, a statistically significant lower effectiveness is observed for older patients,” the study says. “We also find that specific chronic comorbidities, including high blood pressure, COPD, immunosuppression and type 2 diabetes, are negatively associated with vaccine efficacy. These results add to previous reports regarding lower vaccine effectiveness for diabetic patients and patients with multiple coexisting conditions,” the authors add.

Another interesting the researchers observed was a rise in illness in the days following inoculation with the first dose. “It’s an epidemiological behavioral phenomenon of a sense of protection and a behavioral change following the first inoculation, even though the vaccine doesn’t give protection yet at this stage. It’s a behavioral change that we notice from the day before the first inoculation, with a drop in coronavirus testing, up to a week after the first inoculation,” Kishony says.

A mobile coronavirus vaccination center at Ben Gurion Airport, this week.Credit: Ofer Vaknin

Over the past few months, and as the vaccination process advanced, a large amount of information has accumulated in Israel on the effectiveness of the vaccines. Many studies, based on an analysis of the HMO databases that include millions of people who have been vaccinated – are much larger than the clinical trials conducted by Pfizer.

The findings and information along with the data on the daily and cumulative infection, illness and mortality show the vaccine is very effective – in terms of preventing symptomatic infections and serious illness. But one critical question, which was not examined during the trials of the vaccine’s developers, has still not received a clear answer: Whether the vaccine prevents infection from the virus, or contributes to a significant reduction in it.

The question of infectiousness is critical because it has a decisive influence on the level of protection for the unvaccinated population – whether these are children, people who are not allowed to be vaccinated for medical reasons, or those who the vaccine is less effective for – and from this is derived the level of risk, and the different instructions concerning wearing masks, social distancing and restrictions on gatherings.

The difficulty in answering this question scientifically and immediately lies in the fact that it requires information on the people who “are outside the system,” in other words healthy people, who are also virus carriers but are asymptomatic – so in many cases they do not interface with the health system.

Certain hints and a certain amount of data on the effectiveness of the vaccine in preventing infection can be found in an investigation and analysis of the outbreak and the chain of infection. This research is being carried out and is now beginning to yield its preliminary results.

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