Covid-19 could be potentially a few mutations only away from beating current vaccines, according to the head of the Centers for Disease Control and Prevention.

"These vaccines operate really well in protecting us from severe disease and death, but the big concern is that the next variant that might emerge - just a few mutations potentially away - could potentially evade our vaccines," the centers director Rochelle Walensky says.

However, "right now, fortunately, we are not there," since current vaccines "operate really well in protecting us from severe disease and death."

The centers issued indoor mask guidelines this week to curb the spread of the virus in areas experiencing an increase in infections. It now recommends fully vaccinated adults use masks in public, indoor settings to help limit the spread of the Delta variant and protect others in places with substantial and time transmission.

Viruses evolve over time as they multiply within an infected host. As a result, there are numerous varieties of the coronavirus circulating - each distinguished by a handful of minor alterations in its genetic coding. Many viruses have little actual public-health effects but the more individuals infected the more likely they are to mutate into a variant.

No coronavirus variant spotted so far is more concerning than Delta - which was first identified in India this winter.

Delta has been dubbed the "fittest" variant to date by the World Health Organization because it spreads more easily and may result in more severe cases and an increased risk of hospitalization than other variants such as Alpha, which was discovered in the U.K.

Unvaccinated Americans, according to health officials, are driving the rise in coronavirus cases in the U.S.

According to officials, unvaccinated people are currently responsible for around 99.5% of deaths and 97% of hospitalizations.