Amid renewed calls for booster doses to prevent the transmission of the new Omicron Covid-19 variant, latest data from the US Centers for Disease Control and Prevention (CDC) indicate that a third of the 43 cases of the super mutant strain in the country had taken the third dose.
According to the CDC, 43 infections with the Omicron variant were identified in 22 states during the first eight days of December.
Of this, 34 were fully vaccinated and 14 took their booster shots, revealed the agency's Morbidity and Mortality Weekly Report (MMWR).
While one person, who was fully vaccinated, required hospitalisation, none of the Omicron patients died.
The most common symptoms were cough, fatigue and congestion or a runny nose. The first cases appeared to be mild, but the report warned that "as with all variants, a lag exists between infection and more severe outcomes".
"Even if most infections are mild, a highly transmissible variant could result in enough cases to overwhelm health systems," the authors warned.
"The clinical severity of infection with the Omicron variant will become better understood as additional cases are identified and investigated."
From August 13 to November 19, 18.7 million older adults have received a booster or additional primary dose of Covid in the US, revealed the CDC report.
Meanwhile the World Health Organization (WHO) said that the variant is spread to 63 countries.
The UN health agency said that based on current limited evidence "Omicron appears to have a growth advantage over Delta".
"It is spreading faster than the Delta variant in South Africa where Delta circulation was low, but also appears to spread more quickly than the Delta variant in other countries where the incidence of Delta is high, such as in the UK," WHO said in a statement.
Whether Omicron's observed rapid growth rate in countries with high levels of population immunity is related to immune evasion, intrinsic increased transmissibility, or a combination of both remains uncertain.
The agency noted that "it is likely that Omicron will outpace the Delta variant where community transmission occurs".
But the data on the clinical severity of Omicron is still limited.
While preliminary findings from South Africa suggest it may be less severe than Delta, "it remains unclear to what extent Omicron may be inherently less virulent".
Preliminary evidence also suggests a reduction in vaccine efficacy against infection and transmission associated with Omicron.
There is some preliminary evidence that the incidence of reinfection has increased in South Africa, which may be associated with humoral (antibody-mediated) immune evasion.
In addition, few studies of limited sample size have shown that sera obtained from vaccinated and previously infected individuals had lower neutralisation activity (the size of the reduction ranges considerably) than with any other circulating VOCs of SARS-CoV-2 and the ancestral strain.
Scientists, including top infectious disease expert Anthony Fauci have urged Americans to get vaccinated, take booster dose and to continue to practice precautions: wearing masks, improving ventilation in closed spaces, getting tested and going into quarantine, or isolation, if needed to slow transmission.