Study finds vaccines 89% effective after 2 doses

US study suggests efficacy of 1st Pfizer, Moderna shot increases with time

Mayo Clinic findings show vaccine gives 75% protection 15 days after 1st dose, rising to 83% after 36 days; figure climbs to 89% for people who received both doses

Tyson Foods team members receive COVID-19 vaccines from health officials at the Joslin, Illinois facility, February 19, 2021. (John Konstantaras/AP Images for Tyson Foods)
Tyson Foods team members receive COVID-19 vaccines from health officials at the Joslin, Illinois facility, February 19, 2021. (John Konstantaras/AP Images for Tyson Foods)

Vaccinated people are at far less risk of COVID-19 infection even before receiving their second dose, a new US study has found, backing up Israeli research on one of the most burning international vaccine questions.

The Mayo Clinic figures showed the same efficacy rate as those released by Tel Aviv’s Sheba Medical center, and further show that the ability of the vaccine to protect against infection increases with time, even after just the first dose.

The Minnesota-based clinic, in a study of 31,000 people in four US states who received at least one vaccine shot, found the inoculations were 75 percent effective 15 days after the first shot, and around 83% effective 36 days after the first shot.

The figure, reflecting both symptomatic and asymptomatic illness, climbed to 89% (36 days after the first dose) for people who received both doses.

A vial of the Pfizer-BioNTech vaccine against COVID-19 taken at the Versalles Clinic, in Cali, Colombia, amid the novel coronavirus pandemic on February 19, 2021. (Luis ROBAYO / AFP)

The study, which has not yet been peer-reviewed, included people who had received either the Moderna or Pfizer vaccines.

The Mayo Clinic study backed up Israeli research released last week which found that Pfizer’s vaccine is 75% effective at preventing infection two to four weeks after a single shot.

The Israeli study homed in on stats for symptomatic carriers, i.e., those who felt unwell, and found that a single dose has 85% effectiveness two to four weeks later.

In some countries racing to vaccinate large populations, doctors are arguing over whether they should delay second shots so they can give more people partial protection with one shot. The UK has controversially adopted this approach, despite some concern in the medical profession. 

Sheba Medical staff members receive the second round of the COVID-19 vaccine, at the Sheba Medical Center outside of Tel Aviv, January 10, 2021. (Miriam Alster/Flash90)

Vaccines have been found to be highly effective at reducing symptomatic COVID-19 and mortality, but there is still a dearth of data on whether they prevent transmission and can stop the virus from circulating within a population.

The question is crucial because most of the population in the US and worldwide remains unvaccinated and other groups, mainly children, are ineligible for the shots.

While there is strong data from Phase 3 trials of the Pfizer-BioNTech vaccine, and since, showing that vaccinated people are far less likely to become verified COVID-19 carriers, clinical trials didn’t produce robust results as to whether those who are vaccinated will still spread the virus.

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