Pfizer, Moderna COVID-19 vaccines highly effective after 1st shot in real-world, U.S. study suggests

U.S. government's first look at the real-world use of the mRNA type of COVID-19 vaccines found their effectiveness was nearly as robust as it was in controlled studies.

Findings add to evidence base supporting COVID-19 vaccines

Vials of COVID-19 vaccines from Pfizer-BioNTech, Moderna and AstraZeneca are pictured in Dublin, Ireland in February. A U.S. study assesses how Pfizer and Moderna's vaccines have been working beyond the drugmakers' initial clinical trials. (Clodagh Kilcoyne/Reuters)

COVID-19 vaccines developed by Pfizer with BioNTech and Moderna reduced the risk of infection by 80 per cent two weeks or more after the first of two shots, according to data from a real-world study of vaccinated U.S. health-care personnel and first responders released on Monday.

The findings come as AstraZeneca-Oxford's viral vector faced greater scrutiny in Canada on Monday for those under the age of 55 and as the roll out of first doses of mRNA vaccines from Pfizer and Moderna ramp up across the country. 

The risk of infection fell 90 per cent by two weeks after the second shot, the study of just under 4,000 people found.

The study by the U.S. Centers for Disease Control and Prevention (CDC) evaluated the vaccines' ability to protect against infection, including infections that did not cause symptoms.

Previous clinical trials by the companies evaluated their vaccine's efficacy in preventing illness from COVID-19.

'Reassuring news' 

"This is very reassuring news," said the CDC's Mark Thompson, the study's lead author. "We have a vaccine that's working very well."

The researchers counted 205 infections, with 161 of them in the unvaccinated group. Of the remaining 44, the CDC said 33 of them were in people apparently infected within two weeks of their last shot, the point at which they are considered fully vaccinated. 

No one died, and only two were hospitalized. Thompson did not say whether the people hospitalized were vaccinated or not.

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The findings from the real-world use of these messenger RNA (mRNA) vaccines confirm what was seen in large controlled clinical trials conducted before they received emergency use authorizations from the U.S. Food and Drug Administration.

The study looked at the effectiveness of the mRNA vaccines among 3,950 participants in six states over a 13-week period from Dec. 14, 2020 to March 13, 2021.

"The authorized mRNA COVID-19 vaccines provided early, substantial real-world protection against infection for our nation's health-care personnel, first responders, and other frontline essential workers," CDC Director Rochelle Walensky said in a statement.

Study participants were given nasal swab test kits to use every week to check for signs of infection.

"The evidence base for (currently available) COVID-19 vaccines is already strong, and continues to mount ever higher with studies like this one," said David Holtgrave, dean of the University at Albany's School of Public Health, in an email.

Some real-world dosage gaps differ

The new mRNA technology is a synthetic form of a natural chemical messenger being used to instruct cells to make proteins that mirror part of the novel coronavirus. That teaches the immune system to recognize and attack the actual virus.

In order to alleviate vaccine supply constraints, some countries, including Britain and Canada, are allowing extended gaps between doses that differ from how the vaccines were tested in clinical trials.

In the trials, there was a three-week gap between Pfizer shots and four weeks for the Moderna vaccine.

Different researchers have tried to look at how the vaccines have performed, including work done in Israel and the United Kingdom, and a U.S. study of Mayo Clinic patients.

Unlike the Mayo study, which focused on hospitalization and death, the CDC study looked for any infection — including infections that never resulted in symptoms, or were identified before people started feeling sick. 

About two-thirds of the participants who were vaccinated got Pfizer shots, one-third got Moderna and five got the newest shot from Johnson & Johnson. The study was done in Miami; Duluth, Minnesota; Portland, Oregon; Temple, Texas; Salt Lake City; and Phoenix and other areas in Arizona.

With files from CBC's News and Associated Press


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