First Pfizer COVID Vaccine Shot Cuts Risk of Infection by 50 Percent: Study

The Pfizer-BioNtech vaccine reduces the risk of infection by around 50 percent 14 days after the first of two shots is administered, preliminary data from Israel's mass vaccination program suggests.

Sharon Alroy-Preis, a top official in the Israeli health ministry, told the country's Channel 12 News that the conclusions were based on the results of hundreds of thousands of COVID-19 tests.

Israel is currently leading the world in terms of vaccination, vaccinating nearly a fifth of its population.

Around two million people have received vaccinations in the small Middle Eastern nation, with more than five million Israelis set to receive shots by March, if a plan developed by the country's health ministry goes smoothly.

Alroy-Preis said that around one-fifth of more than 1,000 COVID-19 patients in the country with serious disease had already received their first dose of the Pfizer-BioNtech vaccine, highlighting the importance of taking precautions between doses.

"Seventeen percent of the new serious cases today, or 180 cases, are after the first dose," Alroy-Preis told reporters.

The Pfizer-BioNtech vaccine has been shown to reduce an individual's risk of developing COVID-19 by 95 percent. But this level of protection—and long-lasting immunity—is only reached more than a week or so after the second dose. This second dose is supposed to be administered 21 days after the first.

A study published in the BMJ reviewing the results of a Phase III clinical trial, previously found that the vaccine may provide some early protection, but this only begins around 12 days after the first dose.

The BMJ study found that vaccine efficacy between the first and second doses was 52 percent when they were given 21 days apart.

The aforementioned estimates differ slightly from figures released by two of Israel's four main health maintenance organizations (HMOs) on Tuesday, based on data from 400,000 patients in each area.

Clalit, the largest HMO, reported that a person's risk of infection dropped by only 33 percent 14 days after the first shot, while the Maccabi HMO said this figure was around 60 percent. The reason for these discrepancies in the figures is not clear at this time.

The data coming from Israel may provide the best indication yet of the vaccine's efficacy after the first dose given that Pfizer's own Phase III trial involved fewer people than these surveys—around 40,000.

In the U.K., where authorities have decided to administer doses of the Pfizer vaccine 12 weeks apart in order to combat a drastic surge in infections, the country's Joint Committee on Vaccination and Immunisation (JCVI) said that the shot's short-term efficacy rose to 89 percent between days 15 and 21 after the first dose.

But the JCVI said protection from the first dose would likely "wane in the medium term, and the second dose will still be required to provide more durable protection."

Pfizer COVID-19 vaccine in Israel
A health care worker prepares a dose of the COVID-19 vaccine at Clalit Health Services in the Palestinian neighbourhood of Beit Hanina, in the Israeli-annexed east Jerusalem, on January 12, 2021. AHMAD GHARABLI/AFP via Getty Images