While infections, hospitalizations, and deaths from Covid-19 have been steadily declining in the United States in recent months, experts warn that rising cases in Europe may be "a harbinger for what's about to happen in the United States," Rob Stein writes for NPR's "Shots."

Will the US see a 'winter resurgence' of Covid-19?

Currently, several models project that U.S. Covid-19 infections will continue to decline at least until the end of 2022. However, researchers caution that there are multiple variables that could change current projections, including whether more infectious strains start circulating around the nation.

According to Stein, "[t]he first hint of what could be in store is what's happening in Europe." Recently, many European countries, including the U.K., France, and Italy, have seen an increase in Covid-19 infections.

"In the past, what's happened in Europe often has been a harbinger for what's about to happen in the United States," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. "So I think the bottom line message for us in this country is: We have to be prepared for what they are beginning to see in Europe."

"We look around the world and see countries such as Germany and France are seeing increases as we speak," said Lauren Ancel Meyers, director of the UT COVID-19 Modeling Consortium at the University of Texas at Austin. "That gives me pause. It adds uncertainty about what we can expect in the coming weeks and the coming months."

However, Justin Lessler, an epidemiologist at the University of North Carolina who helps run the COVID-19 Scenario Modeling Hub, noted that the United States may not have the same experience as Europe, largely because it is unclear whether Europe's increase is related to individuals' vulnerability to new strains.

"If it is mostly just behavioral changes and climate, we might be able to avoid similar upticks if there is broad uptake of the bivalent vaccine," Lessler added. "If it is immune escape across several variants with convergent evolution, the outlook for the U.S. may be more concerning."

Some researchers believe the United States is already experiencing early signs of this. "For example, the levels of virus being detected in wastewater is up in some parts of the country, such in Pennsylvania, Connecticut, Vermont and other parts of Northeast," Stein writes. "That could an early-warning sign of what's coming, though overall the virus is declining nationally."

"It's really too early to say something big is happening, but it's something that we're keeping an eye on," said Amy Kirby, national wastewater surveillance program lead at CDC.

According to David Rubin, the director of the PolicyLab at Children's Hospital of Philadelphia, which tracks the pandemic, Covid-19 infections and hospitalizations are already rising in some parts of New England, and other northern regions, including the Pacific Northwest.

"We're seeing the northern rim of the country beginning to show some evidence of increasing transmission," Rubin said. "The winter resurgence is beginning."

How likely is a severe Covid-19 surge?

Unless a "dramatically different new variant emerges," it is "highly unlikely this year's surge would get as severe as the last two years in terms of severe disease and deaths," Stein writes.

"We have a lot more immunity in the population than we did last winter," said Jennifer Nuzzo, who leads the Pandemic Center at the Brown University School of Public Health.

"Not only have people gotten vaccinated, but a lot of people have now gotten this virus. In fact, some people have gotten it multiple times. And that does build up [immunity] in the population and reduce overall over risk of severe illness," Nuzzo said.

Another factor that could affect the severity of the impact of rising infections is the number of people who receive updated Covid-19 vaccines, which help boost waning immunity from previous infections or shots.

However, the United States' booster uptake has been slow. "Nearly 50% of people who are eligible for a booster have not gotten one," said William Hanage, an associate professor of epidemiology at the Harvard T.H. Chan School of Public Health. "It's wild. It's really crazy."

Since updated boosters became available in September, less than 8 million of the over 200 million people who are eligible have received one.

According to Nuzzo, it is critical for people to stay up to date on their vaccines, especially with the high likelihood of another Covid-19 surge. "The most important thing that we could do is to take off the table that this virus can cause severe illness and death," Nuzzo said.

"There are a lot of people who could really benefit from getting boosted but have not done so," she added. (Stein, "Shots," NPR, 10/7)