Explainer | The science behind new US mask guidance

The US 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 published in JAMA found the Pfizer/BioNTech vaccine to be 97 per cent effective in preventing symptomatic Covid-19 infection and 86 per cent effective in preventing asymptomatic cases among a group of Israeli healthcare workers.

Also read: CDC allows fully vaccinated people in US to shed their masks in most places

Three studies published by the CDC also showed real-world effectiveness of the Pfizer/Biontech and Moderna vaccines. One found them to be 90 per cent effective against infections regardless of symptom status in a group of healthcare personnel, first responders, and other essential and frontline workers. A second study showed the vaccines to be 94 per cent effective against Covid-19 hospitalization among fully vaccinated adults over age 65. Another, published on Friday, showed 94 per cent effectiveness against symptomatic infection among healthcare workers at 33 US sites.

What Evidence Demonstrates Vaccines Work Against Variants of Concern?

A study out of Qatar published in the New England Journal of Medicine showed the Pfizer/BioNTech vaccine was 100 per cent 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 per cent effective in preventing infection from the B.1.1.7 variant and 75 per cent effective in preventing infection from the B.1.351 variant, according to the study.

Do the Vaccines Prevent Virus Transmission?

A report 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 showed that vaccinated nursing facility residents who became infected did not infect others.