Centers for Disease Control and Prevention Director Rochelle Walensky says ahead of a House Select subcommittee hearing on “Reaching the Light at the End of the Tunnel: A Science-Driven Approach to Ending the Pandemic Quickly and Safely” at the Capitol Hill in New York from Washington, DC, April 15, 2021.

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U.S. health officials have confirmed fewer than 6,000 cases of Covid-19 in fully vaccinated Americans, said Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, on Monday.

That’s only 0.007% of the 84 million Americans with full protection against the virus. Despite the groundbreaking infections, she said the vaccines were working as intended.

“We expect such rare cases with any vaccine, but so far we have received reports of fewer than 6,000 breakthrough cases from more than 84 million people who have been fully vaccinated,” Walensky told reporters at a news conference. Breakthrough cases occur when someone becomes infected with the virus more than 14 days after the second shot, she said.

The CDC chief admitted the number could be underestimated.

“While that number comes from 43 states and territories and is likely underestimated, it’s still very important that these vaccines work. Of the nearly 6,000 cases, about 30% had no symptoms at all,” Walensky said.

“This is really encouraging, encouraging news. It shows what we’ve already discussed about these vaccines. They also help keep you from getting seriously ill,” she said.

Of the roughly 6,000 breakthrough infections, 396 people were hospitalized and 74 people died, according to CDC data released last week.

Half of all American adults have received at least one dose of the coronavirus vaccine. Of those over 65, 81% have received one dose or more, and around two-thirds are fully vaccinated.

US health officials are launching a massive campaign to convince more Americans to take the vaccine. An increasing number of people have become skeptical after the CDC and the Food and Drug Administration last week urged states to temporarily stop distributing Johnson & Johnson vaccines after reports of a rare but potentially fatal bleeding disorder to the CDC .

Some of former President Donald Trump’s supporters are also strongly against taking the vaccine, worrying U.S. health officials who hope enough people will be vaccinated for the country to receive herd immunity to the virus. The Chief Medical Officer of the White House, Dr. Anthony Fauci previously said 75% to 85% of the US population would need to be vaccinated to create an “umbrella” of immunity that will prevent the virus from spreading.

“It is very worrying that people are politically unwilling to be vaccinated,” Fauci said Monday on CBS This Morning. “I find this really extraordinary because they say you are encroaching on our freedoms by asking us to wear masks and doing restrictions that affect public health problems. The easiest way to overcome this is to yourself get vaccinated. ”

The US reports 723 Covid deaths per day, based on a seven-day average based on data compiled by Johns Hopkins University.

At Biden’s urging, all 50 US states opened vaccination appointments for people aged 16 and over by Monday.

– CNBC’s Nate Rattner contributed to this report.

Correction: This story has been updated to correct death dates. Of the roughly 6,000 breakthrough infections, at least 74 patients died on April 13.