(CNN)The United States is seeing a large decline in new Covid-19 cases -- but there's a major caveat.
According to a CNN analysis of data from Johns Hopkins University, the US is seeing a 29% decline in new Covid-19 cases compared to this time last week, the steepest one-week decline the country has seen during the pandemic.
Improvements have been made; in a White House briefing Friday, US Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said the US continues to see a five-week decline, with the seven-day average of cases declining 69% since peaking on January 11.
But it's unclear exactly how much of the decline can be attributed to the winter weather that's gripped much of the country, forcing officials to close testing sites, which affects the collection of crucial data.
Several states impacted by winter storms are seeing large declines in new cases this week, per Johns Hopkins data, including Texas, where cases are down 56% compared to last week.
The COVID Tracking Project said Thursday it had been unable to update the daily number of tests performed in Texas for four days because of the winter weather. Overall, the US has seen nearly a 13.5% decrease in testing compared to this time last week.
Again, that's not to say the United States hasn't made progress in driving down case numbers. Early this week, Dr. Ashish Jha, dean of the Brown University School of Public Health, and Dr. Jonathan Reiner, professor of medicine and surgery at George Washington University, both pointed to increased mask-wearing as one reason for declining numbers.
Whatever progress has been made, however, is also threatened by the rise of Covid-19 variants, some of which appear to be more transmissible. And while they could spell trouble, pharmaceutical companies and scientists are confident vaccines will evolve with them, according to senior White House adviser Andy Slavitt.
"I spoke to all the pharmaceutical companies and scientists, and they all say the same thing: Even if these vaccines diminish a little bit, they will be able to continually update them," Slavitt, who is responsible for the Covid-19 response, told CNN's Chris Cuomo Thursday.With more than 1,500 cases of variants reported in the US, officials have been racing to administer vaccines quickly to get ahead of further mutations, with about 59.5 million doses administered so far across the country, according to the CDC.
"Even though the numbers look promising in terms of new cases going down, that's misleading," said Dr. Peter Hotez, dean of tropical medicine at Baylor College in Houston, pointing to the UK Covid-19 variant. "I think we're about to get hit very hard, so we have to race ahead of the variant."
New research out of Israel and Canada has found that only a single dose of the Pfizer/BioNTech's vaccine offers significant protection against the virus, but Slavitt stressed that does not mean people should skip the second dose.
"We don't know how long or how durable that benefit is without the booster," he said. "We don't know how effective it is against variants."
Dr. Anthony Fauci echoed that in Friday's White House briefing, saying he had not changed his mind after reading the Israeli report. He said the US would "stick with the scientifically documented efficacy and optimal response" of two doses.
He added he worried that if a large number of people received just one dose and had less optimal immune response, they could be exposed to the virus and start incubating viral mutations.
"You might theoretically be inducing new variants," he said.