Experts have said the U.S. is in a race against time to vaccinate as many people as possible before more troublesome variants emerge, but there are ways to mitigate outbreaks even as people wait to become eligible for the shots, said Carl Bergstrom, a professor of biology at the University of Washington.

"Throughout the course of the pandemic, one of the most important drivers has been changes in our collective behavior," Bergstrom said. "That's what makes modeling the long-term trajectory of the pandemic so extraordinarily difficult."

Indeed, behavioral changes have at times been associated both with spikes in cases, as people adopted more lax attitudes and states rolled back restrictions, and with valleys, as new measures were put in place and people became more vigilant.

The dynamic was largely to blame for the alarming surge in cases, hospitalizations and deaths in Los Angeles County, California, in December and January.

To avoid another spike in the spring, the U.S. has to stay aggressive with vaccinations and mitigation tactics, such as mask-wearing and social distancing, Bergstrom said. These strategies are even more crucial to combat more transmissible variants of the coronavirus, he said. The U.K. strain has already been reported in more than half of the states, but as in the U.K., the numbers could increase rapidly. 

"That's just how exponential growth works," Bergstrom said. "It comes in at a low frequency, and the first few doublings you don't really notice because it's overwhelmed by what you're seeing with the regular strain. But when it shifts, it shifts quite suddenly, and then you really see it take off."

That prospect is why many experts approach the recent declines with cautious optimism. Also of concern is the possibility that progress could be wiped out if a strain emerges that evades the current vaccines. Some early research has already shown that the Moderna and the Pfizer-BioNTech vaccines are less effective against the South African variant, although levels of neutralizing antibodies are expected to still be protective.

It's also possible, Bergstrom said, that other problematic variants are already circulating in the U.S. undetected.

All of those factors combine to make it difficult to assess where the U.S. stands and to predict how the coming months could play out, he said.

"There are so many moving parts, and it's so complicated," Bergstrom said, "but whatever we can do to stay aggressive, the more people we can get vaccinated and the longer we can hang in there, the more we can stave off a huge spring spike."

Read the full article in NBC News.