Halfway through a bleak winter, Dr. Diane Griffin started to feel something in recent weeks that had been missing for much of the past year: optimism. Griffin, an infectious disease expert at the Johns Hopkins University School of Medicine in Baltimore, said recent declines in hospitalizations, daily deaths and confirmed new infections were fueling hope that the U.S. has arrived at a turning point in the trajectory of the coronavirus pandemic. "What we're seeing is incremental and encouraging," Griffin said, "but we're not out of the woods yet." More than a year into the pandemic, developments that seem at odds with one another have made it difficult to determine just where the U.S. and the world stand. Griffin's optimism is tempered by the fact that while case numbers have been decreasing steadily, they are still much higher than they were during the first wave, which overwhelmed parts of the country last spring and summer. And looming over all the recent progress is the threat of more contagious coronavirus variants that are already spreading rapidly in the U.S. — along with concerns that variants could pose problems for the current round of vaccines. Scientists say that while it's heartening to see declines after some of the deadliest and most challenging months of the pandemic, it may be too soon to know whether the U.S. is experiencing a temporary reprieve or whether it is finally starting to get control of the pandemic. And while some optimism is warranted, the psychological challenges of the pandemic — in which human behavior and collective responsibility remain crucial to keep outbreaks under control — mean communities must remain vigilant. "We are seeing a dip right now, but I don't think that changes the thought that we have to be very careful in the weeks ahead," said Dr. Bruce Y. Lee, a professor of health policy and management at the City University of New York. "Whether this is a momentary dip or whether we reached the peak and now things are coming down — that's not completely clear." Further clouding the forecasts are questions about how the known coronavirus variants, including a more transmissible strain that is thought to have emerged in the U.K., might alter the pandemic's course. Some research suggests there's already cause for concern. A recent study that was published to the preprint server medRxiv but hasn't yet been peer-reviewed found that cases of the U.K. variant are doubling in the U.S. every 10 days. The findings seem to confirm a report the Centers for Disease Control and Prevention released last month that projected that the U.K. variant could become the predominant strain in the country by March. Lee said case numbers are likely to rise again as the U.K. variant becomes more widespread and as other strains that were first reported in South Africa and Brazil circulate in the U.S. But it's possible that other forces — including increased rates of vaccination across the country, the anticipation that people will spend more time outdoors in the spring and other shifts in behavior — will help counteract a new surge and prevent another wave of runaway outbreaks. "It was expected that January and February were going to be tough months, because of winter and because of the holidays, but by March or April, we may start to see things improving," Lee said. For one, spring will bring warmer temperatures to much of the country, making it easier to spend time outdoors, where the risk of infection is thought to be lower. And improvements are likely to continue as more Americans are vaccinated, Griffin said. Early results from Israel, where more than a third of the population has received at least the first of two vaccine doses, provide the first real-time glimpses that efforts to administer shots rapidly and widely appear to be working. Preliminary analyses show that the vaccines appear to be contributing to sharp declines in infections and illness, particularly in people over age 60, who were among the first to get the shots. Israel has administered more than 60 doses of either the Moderna or the Pfizer-BioNTech vaccine for every 100 residents, far outpacing every other country. (Israel's vaccine strategy has come under fire for excluding Palestinians.) Figures released last week by Israel's Health Ministry indicated that out of nearly 750,000 people over 60 who were fully vaccinated, only 0.07 percent, or 531 people, tested positive for the virus. Of that group, 38 people were hospitalized for moderate or severe illness. Griffin said the early results from Israel's vaccine program were "very encouraging," adding that vaccines could at least stave off serious disease and deaths in the U.S., even if the variants cause an overall rise in cases. The rollout of vaccines in the U.S. was hampered early on by supply constraints and a lack of overall strategy during the Trump administration. Since then, the federal government under President Joe Biden has been trying to ease bottlenecks in manufacturing and distribution, and it is expanding how and where vaccinations can be administered. The efforts are part of Biden's goal of giving at least 100 million shots in his first 100 days in office. As of Sunday, the U.S. had administered 40 million doses, according to the CDC. 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."