"What we did in the spring is not going to work in the fall," Birx said.
A pandemic of the novel coronavirus has now killed more than 1 million people worldwide.
Over 36.7 million people across the globe have been diagnosed with COVID-19, the disease caused by the new respiratory virus, according to data compiled by the Center for Systems Science and Engineering at Johns Hopkins University. The criteria for diagnosis -- through clinical means or a lab test -- has varied from country-to-country. Still, the actual numbers are believed to be much higher due to testing shortages, many unreported cases and suspicions that some national governments are hiding or downplaying the scope of their outbreaks.
Since the first cases were detected in China in December, the virus has rapidly spread to every continent except Antarctica.
The United States is the worst-affected country, with more than 7.6 million diagnosed cases and at least 213,570 deaths.
California has the most cases of any U.S. state, with more than 847,000 people diagnosed, according to Johns Hopkins data. California is followed by Texas and Florida, with over 811,000 cases and over 728,000 cases, respectively.
More than 190 vaccine candidates for COVID-19 are being tracked by the World Health Organization, at least 10 of which are in crucial phase three studies. Of those 10 potential vaccines in late-stage trials, there are currently five that will be available in the United States if approved.
The World Health Organization (WHO) on Friday announced a new record high of daily COVID-19 cases worldwide, according to The Associated Press.
WHO officials said 350,766 new cases were reported, which is a jump of almost 12,000 cases from the record set earlier in the week, according to the AP.
Two more members of the White House residence staff tested positive for COVID-19 nearly three weeks ago, according to Melania Trump's office and two other sources.
With these two additional staffers, the total number of residential staff who tested positive at that time is at least four. One is an assistant to White House chief usher Timothy Harleth, while the three others are members of the housekeeping staff and work on the residence’s third floor, according to the first lady’s office and sources.
The first lady’s office didn’t reply to a question about whether these two new people have had contact with the president or first lady. On Sunday, sources told ABC News the other two had not come into direct contact with the president or first lady.
ABC News’ Ben Gittleson and John Santucci contributed to this report.
Spain's national government on Friday declared a state of emergency in the capital so that it can resume partial restrictions on movement there, which had been rejected by a local court.
Madrid's regional government opposes the new restrictions, introduced last week, which ban 4.8 million residents from leaving the capital and nine suburbs.
A Madrid court on Thursday sided with the regional government president, Isabel Diaz Ayuso, who had challenged the measures, saying they are draconian and would ravage the economy.
The move comes as the Madrid region battles one of Europe's worst clusters of COVID-19 cases. The region's 14-day infection rate of 563 cases per 100,000 population is more than twice Spain's national average of 256, and five times Europe's average rate of 113 for the week ending Sept. 27.
The U.S. Centers for Disease Control and Prevention said Friday it's not planning to deploy a large team of staff members to the White House to conduct contact tracing.
CDC spokesman Benjamin Haynes confirmed to ABC News that there are now two contact tracers assigned to the White House. The first CDC staff member assigned to the operation has been at the White House since March, while the second person joined the effort recently.
"There are no plans to send anyone else at this point," Haynes told ABC News on Friday.
ABC News reported on Tuesday that the agency had assembled a large team of experts to trace the explosion of COVID-19 cases at the White House but the unit has largely been put on standby as President Donald Trump opted to run his own operation through the White House medical unit.
COVID-19 cases among White House staff and their close contacts continue to climb after Trump revealed last week that he and his wife tested positive for the disease.
CDC guidelines recommend tracing cases as soon as possible because a single infected person can begin to spread COVID-19 two days before the individual has any symptoms or tests positive.
ABC News' Anne Flaherty contributed to this report.
