Most coronavirus infections spread by people who have no symptoms.
Masks are the only available protection against this. The US Centers for Disease Control and Prevention (CDC) said in newly updated guidance.
The warning follows the second wave of Covid-19 infection. It is spreading at an unprecedented speed.
So far, the World Health Organisation (WHO) has recorded 55.6 million confirmed cases and 1.34 million deaths across the globe.
The United States remains the worst-affected country with 11.8 million confirmed cases and more than 252,000 deaths.
India is the second-worst affected country with more than nine million confirmed cases.
The current Trump administration discourages the use of masks and many US health experts blame this attitude for the devastations that the coronavirus has caused in the United States.
The incoming Biden administration has pledged to promote the use of masks and other preventive measures.
“Asymptomatic or pre-symptomatic infected wearers who feel well and may unaware of their infectiousness to others estimated to account for more than 50 percent of transmissions,” said the new CDC guidelines issued during the weekend.
The CDC noted that studies done by various international agencies show that 40-45 percent of infected people never develop symptoms.
“Among people who do develop the symptomatic illness, transmission risk peaks in the days just before symptom onset (pre-symptomatic infection) and for a few days thereafter,” the US agency added. “The number of infections transmitted peaks when virus levels peak.”
Masks can prevent this spread
Explaining how masks help, the CDC said the face-covers reduce the emission of virus-laden droplets, which called “source control”, and were especially useful for preventing infections from asymptomatic patients.
Masks also help reduce inhalation of these droplets by the wearer, which called “filtration for personal protection”. The community benefit of masking for SARS-CoV-2 control is due to the combination of these effects; individual prevention benefit increases with increasing numbers of people using masks consistently and correctly.