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New CDC study confirms mRNA coronavirus vaccines highly effective at preventing infection

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A new study out of the Centers for Disease Control and Prevention provides strong evidence to say that the two mRNA coronavirus vaccines are effective at preventing asymptomatic spread of the virus. "CDC" by Cocoabiscuit is licensed with CC BY-NC-ND 2.0. 

A new study out of the Centers for Disease Control and Prevention provides strong evidence that the mRNA coronavirus vaccines made by Pfizer and Moderna are highly effective at preventing infection — and not only severe disease — by the novel coronavirus, SARS-CoV-2, in real-world conditions.

The study looked at the effectiveness of the two mRNA coronavirus vaccines in preventing SARS-CoV-2 infections among 3,950 health care personnel, first responders and other essential workers in six states over a 13-week period from Dec. 14, 2020, to March 13, 2021.

"Results showed that following the second dose of vaccine (the recommended number of doses), risk of infection was reduced by 90 percent two or more weeks after vaccination. Following a single dose of either vaccine, the participants’ risk of infection with SARS-CoV-2 was reduced by 80 percent two or more weeks after vaccination," the CDC reported.

Participants collected their own nasal swabs each week, which were sent to a central location for PCR testing, also known as the "gold standard" of coronavirus tests. The weekly swabs allowed the researchers to detect asymptomatic infections as well as symptomatic ones.

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Results showed 58% of the infections were detected before people had symptoms. Just 10.2% of infected people never developed symptoms. Among those who were fully vaccinated, there were .04 infections per 1,000 person-days, meaning that among 1,000 persons there would be .04 infections in a day.

There were 0.19 infections per 1,000 person-days among those who had had one dose of the vaccine. In contrast, there were 1.38 infections per 1,000 person-days in unvaccinated people.

The study out of the CDC also noted that after two weeks, one dose of the vaccine was 80% effective against SARS-CoV-2 infection, similar to studies that were done across the globe in the United Kingdom and Israel.

At the beginning of this month, the CDC released new guidance for people who have been fully vaccinated against COVID-19. Being "fully vaccinated," per the CDC, means that you are two weeks out from your second dose if you received either the Pfizer or Moderna vaccine or two weeks out from the single vaccine dose.

“This study shows that our national vaccination efforts are working. 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,” said CDC Director Dr. Rochelle Walensky.

On Monday, however, Walensky said she had a sense of an “impending doom” about a possible fourth surge of the virus after many states across the country, including Arizona, began to lift their mask mandates.


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UA COVID-19 Test Tracker

Daily (5/18)
378 3 0.8%
Total (8/4)
270,694 4,359 1.6%
Includes tests since August 4, 2020
Data from https://covid19.arizona.edu/updates
Updated May 18, 2021