The US hit a positive milestone in the fight against the coronavirus pandemic Monday, with the lowest number of new COVID-19 cases reported since October 25, according to data out of Johns Hopkins University.

On Monday, 64,938 new cases were reported; back in October, the low was 62,020, Forbes wrote.

Experts say the dips in case rates are no license to ease up on mitigation measures such as mask-wearing and social distancing, even if you're vaccinated.

"Now is the time to not let up our guard," Dr. Rochelle Walensky, the director of the Centers for Disease Control and Prevention (CDC), said on "Face the Nation" Sunday.

"Now is the time to double down."

There are several potential explanations for the drop

The current low comes after the seven-day average of daily new coronavirus cases in the US dropped below 100,000 on Friday for the first time since November.

In December, by contrast, the average daily infection rate was 200,000, and at its peak in January, it was close to 250,000. Since January 12, new case rates have steadily declined, the Washington Post reported.

Experts say the rollout of COVID-19 vaccines does not explain the drop. "I don't think the vaccine is having much of an impact at all on case rates," former CDC Director Tom Frieden said on CNN Sunday. "It's what we're doing right: staying apart, wearing masks, not traveling, not mixing with others indoors."

Other experts say seasonal patterns in the virus may also play a role, and some have wondered if declining rates are merely a reflection of fewer people getting tested.

No matter the cause, experts say that remaining diligent is critical to keep case rates dropping, especially as more contagious coronavirus variants continue to spread. "We're nowhere out of the woods," Frieden said.