Despite the more than 90% efficacy of the authorized vaccines against the Delta variant, the strain dominates more than 80% of the COVID cases in the US.

The United States Infectious disease expert Dr. Anthony Fauci said that the more infectious strain now dominates more than 80% of the US COVID cases during a hearing at the US Senate.

Republican Senator Rand Paul accused Fauci of lying about the National Institutes of Health providing funding for research at China's Wuhan Institute of Virology. He further alleged that the research might have had a role in developing the novel coronavirus.

Fauci, who has been mostly calm and diplomatic in past responses, this time shot back: "Senator Paul, I have never lied before the Congress... (and) you do not know what you're talking about." 

The exchange between the two heated up and featured raised voices and pointed fingers.

Despite the dismissal of the argument by the WHO, the American administration and the opposition continue to allege that the virus was being made in the Wuhan Institute of virology.

The more contagious delta variant was first found in India this year. It has been the dominant strain across the globe including the US. 

The variant has been found in more than 90 countries of the world.

The CDC is reviewing data from multiple groups of vaccinated people to determine how long protection from COVID-19 shots lasts and will use that information to decide on the potential role of additional booster shots, Fauci said.