Dr Anthony Fauci has been on a media tour defending accusations of another "flip-flop" on the science after the Centres for Disease Control reversed its guidance wearing a mask while vaccinated.
In multiple interviews this week, Mr Fauci said the shifting guidance was consistent with the science despite previous guidance that vaccinated people don’t need masks.
"Something has changed, and what has changed is the virus. The CDC hasn’t changed, and the CDC hasn’t really flip-flopped at all," he said on MSNBC’s Morning Joe.
"What’s happened is when that earlier recommendation was made we were dealing predominantly with the Alpha variant."
He said the level of Alpha virus in vaccinated people was low compared and made them unlikely to transmit the virus compared to the Delta variant, which has become the dominant strain in the country.
The message repeated during interviews with MSNBC anchor Chris Hayes and PBS News Hour host Judy Woodruff comes after a supercut of Mr Fauci’s conflicting advice on not wearing a mask to wearing a mask, and back and forth again, by Twitter user @mazemoore, went viral online.
Pressed by PBS News Hour host Judy Woodruff that the new CDC advice contradicted what people were previously told, that they’re protected once vaccinate, Mr Fauci said, in some respects, it does contract that.
"But what has changed, Judy, is, the virus has changed… when people get breakthrough infections, when they’re vaccinated and they get infected, even when they have a situation where they don’t have an advanced disease, they clearly can transmit it to other people," he said.
"This is not a common event. So I don’t want people to be thinking that all kinds of vaccinated people are transmitting it. No, it’s a very unusual, rare event, but it occurs."
The new CDC guidance, which is based on that rare occurrence, has made the average American more confused, said Dr Scott Gottlieb, who sits on the board of Covid vaccine maker Pfizer and is the former commissioner of the US Food and Drug Administration.
He said that despite a small number of vaccinated that contract "breakthrough" cases of Covid could potentially spread the virus, it shouldn’t translate into general guidance for the entire population.
"I don’t think we’re going to get enough bang for our buck by telling vaccinated people to wear a mask at all times to make it worth our while," he said on CNBC.
When MSNBC host Chris Hayes asked Mr Fauci if he understood why people "might feel a little whipsawed",
"Sure, yeah, it’s thoroughly understandable," he replied. "But there really is a pretty clear explanation of it."
That being: The level of Delta variant virus in the nasopharynx is about 1,000 times higher than with the Alpha variant.