The Centers for Disease Control and Prevention (CDC) on Thursday said that fully vaccinated Americans are no longer required to wear masks outdoors and can avoid wearing them indoors in most places. This comes after the health agency, for months, warned that wearing masks and maintaining social distancing were necessary to curb the spread of the COVID-19 pandemic. So what made CDC change its guidelines?

Also read: Though US lifts guidance, not everyone ready to give up masks

CDC Director Rochelle Walensky introduced the new recommendations based on some recent scientific studies. While one of them found that completely vaccinated people transmit the disease rarely, another concluded that the available vaccines appear to be effective against all known strains of coronavirus. 

According to a study published in the Journal of the American Medical Association (JAMA), the Pfizer/BioNTech vaccine was 97% effective in preventing symptomatic COVID-19 infection and 86% effective in preventing asymptomatic cases among a team of Israeli health workers.

In three similar studies published by CDC, the real-world effectiveness of the Pfizer/BioNTech and Moderna vaccines was found to be 90% or more at least against symptomatic COVID infections.

Also read: What if you get doses of 2 different COVID vaccines? A study finds out

One of the main concerns among scientists around the world was that vaccinated people can also carry the virus and infect other people. However, as per a report in Nature, people who contracted the virus after receiving at least one Pfizer/BioNTech vaccine shot showed significantly lower viral loads than unvaccinated people.

A CDC study also showed that vaccinated nursing facility residents, who became COVID infected, did not infect others.

As far as the other variants of coronavirus are concerned, a paper published in the New England Journal of Medicine showed the Pfizer/BioNTech vaccine was 100% effective in preventing severe COVID-19 from the UK and South African strains.