EXPLAINER-What is the science behind the new U.S. mask guidance?

PUBLISHED: Fri, 14 May 2021 19:46:30 GMT
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By Michael Erman

NEW YORK, May 14 (Reuters) – The U.S. Centers for Disease Control and Prevention (CDC) on Thursday advised that fully vaccinated people do not need to wear masks outdoors and can avoid wearing them indoors in most places, updated guidance the agency said will allow life to begin to return to normal.

The following lays out some of recent scientific evidence on which the CDC and Director Rochelle Walensky based their updated guidelines, including vaccine efficacy against virus variants, increased availability and a reduction in cases:

WHAT EVIDENCE IS BEHIND THE CLAIM OF EFFECTIVENESS?

A study https://jamanetwork.com/journals/jama/fullarticle/2779853#:~:text=Findings%20In%20this%20retrospective%20cohort,symptomatic%20infection%20and%200.14%20for published in JAMA found the Pfizer/BioNTech vaccine to be 97% effective in preventing symptomatic COVID-19 infection and 86% effective in preventing asymptomatic cases among a group of Israeli healthcare workers.

Three studies published by the CDC also showed real-world effectiveness of the Pfizer/Biontech and Moderna vaccines. One https://www.cdc.gov/mmwr/volumes/70/wr/mm7013e3.htm found them to be 90% effective against infections regardless of symptom status in a group of healthcare personnel, first responders, and other essential and frontline workers. A second study https://www.cdc.gov/mmwr/volumes/70/wr/mm7018e1.htm showed the vaccines to be 94% effective against COVID-19 hospitalization among fully vaccinated adults over age 65. Another https://www.cdc.gov/mmwr/volumes/70/wr/mm7020e2.htm?s_cid=mm7020 e2_w, published on Friday, showed 94% effectiveness against symptomatic infection among healthcare workers at 33 U.S. sites.

WHAT EVIDENCE DEMONSTRATES VACCINES WORK AGAINST VARIANTS OF CONCERN?

A study https://www.nejm.org/doi/full/10.1056/NEJMc2104974 out of Qatar published in the New England Journal of Medicine showed the Pfizer/BioNTech vaccine was 100% effective in preventing severe COVID-19 from the B.1.1.7 coronavirus variant first identified in the UK and the B.1.351 variant first discovered in South Africa. The vaccine was 89.5% effective in preventing infection from the B.1.1.7 variant and 75% effective in preventing infection from the B.1.351 variant, according to the study.

DO THE VACCINES PREVENT VIRUS TRANSMISSION?

A report https://www.nature.com/articles/s41591-021-01316-7 in Nature showed that people who received at least one shot of the Pfizer/BioNTech vaccine and later became infected by the coronavirus had significantly lower viral loads, or levels of virus, than unvaccinated people. Higher viral loads are associated with being more contagious.

A CDC study https://www.cdc.gov/mmwr/volumes/70/wr/mm7017e1.htm showed that vaccinated nursing facility residents who became infected did not infect others.

(Reporting by Michael Erman; Editing by Caroline Humer and Bill Berkrot)

(c) Copyright Thomson Reuters 2021. Click For Restrictions – https://agency.reuters.com/en/copyright.html

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