When the coronavirus receded across much of the globe last month and the omicron surge declined, many Americans were hopeful that was perhaps the signal that the United States was entering a new phase of the pandemic.
However, new data indicators, domestically and internationally, suggest that the virus continues to spread.
Although official counts of COVID-19 infections and hospitalizations are still declining, new wastewater data updated this week from the Centers for Disease Control and Prevention shows the United States may be seeing the beginnings of an uptick in COVID-19 infections.
Between Feb. 24 and March 10, 37% of wastewater sites that are monitored by the CDC have seen an increase of 100% or more in the presence of the COVID-19 virus in their wastewater. Approximately 30% of these sites have seen an increase of 1,000% or more.
"It is likely we will see a new rise in cases across the United States as our wastewater data is showing a concerning signal," said Rebecca Weintraub, assistant professor of global health and social medicine at Harvard Medical School. "Now is a key moment to communicate why we need to accelerate the uptake of the COVID-19 vaccine, remind communities why boosters are needed, secure an ongoing supply of tests and N95 to communities -- especially the red zones."
Throughout the pandemic, wastewater surveillance has been a tool used as a preliminary indicator of COVID-19 trends in the U.S.
Because asymptomatic patients can shed the virus, wastewater surveillance can capture infections that may not have been identified in official counts. In addition, many Americans are taking at-home COVID-19 tests and are not reporting their results to officials, and thus, experts say, infection totals are likely undercounted.
Wastewater data is sparse across the country, but indicators show some sites in the Northeast, including in New York, Connecticut and Pennsylvania, as well as across Ohio, have seen notable increases in the presence of COVID-19 in local wastewater.
In New York City, some sites saw a 50% increase in the presence of COVID-19 in the city's wastewater.
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COVID-19 trouble brewing overseas
The uptick in the presence of COVID-19 in U.S. wastewater sites comes as other countries in Europe and in Asia are seeing significant viral resurgences.
Across some parts of Asia, COVID-19 has been surging to unprecedented levels. In Hong Kong, the number of virus-positive residents requiring hospitalization has been pushing health care facilities to the edge.
In China, more than 50 million people in the northeastern province of Jilin and the southern cities of Shenzhen and Dongguan, are heading into lockdown after a viral resurgence.
In Europe, COVID-19 cases have steadily been rising after many countries have moved to end COVID-19 restrictions.
Since the beginning of the month, new cases per capita in the United Kingdom have grown by 32% and hospitalizations are also up by 5% in the last week. In Germany, infections are up by 45%, while in Italy, daily cases have increased by 26%.
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"Across Europe and in the U.K., we are seeing COVID-19 cases go up in countries just exiting from an Omicron BA.1 surge," Dr. Sam Scarpino, managing director of pathogen surveillance at the Rockefeller Foundation and a member of its Pandemic Prevention Institute, told ABC News. "Since the beginning of the SARS-CoV-2 pandemic, what's happened in Europe has happened around the globe. ... We can't afford to sit around and let this early warning from Europe again go unheeded."
Many health experts have been raising the alarm about the global increase in infections and hospitalizations, suggesting that Americans should be prepared for the U.S. to follow a similar viral trend.
"The next wave in Europe has begun," Dr. Eric Topol, professor of molecular medicine at Scripps Research, said in a tweet and a blog post on Saturday. "Any proclamation that the pandemic is over ignores the potential recrudescence of a new variant with high transmission and immune escape."
Presence of omicron subvariant BA.2 steadily growing
What is behind this latest COVID-19 resurgence is still unclear. However, experts say it is likely a confluence of factors.
"While we know from genome sequences that the BA.2 omicron subvariant is what's infecting people, we still don't know what's causing the resurgence," Scarpino said. "Is it the increased transmissibility of BA.2, more vaccine breakthroughs, relaxing of non-pharmaceutical interventions, waning immunity, or all of the above?"
Last month, U.S. officials from the CDC unveiled a new plan for determining COVID-19 risk in communities and updated its recommendations for use of face coverings, allowing nearly all of the country to go mask-free under the new guidelines.
Across the pond, in the U.K., Prime Minister Boris Johnson recently declared an end to the country's COVID-19 mitigation measures. Similarly, countries such as Denmark and the Netherlands have already ended restrictions, while in France, most COVID-19 limitations were lifted on Monday, just weeks before the presidential elections.
The presence of BA.2, a subvariant of omicron, has also been growing rapidly across the globe.
"BA.2 is itself highly transmissible, and both BA.1 and BA.2 appear to generate comparatively short-lived protection against reinfection. So it is likely that the combination of higher inherent transmissibility and higher rates of interaction as restrictions ease are combining to generate this resurgence," Matthew Ferrari, director of the Center for Infectious Disease Dynamics at Pennsylvania State University, told ABC News.
In the U.S., the presence of BA.2 has been nearly doubling every week, according to federal data. Estimates indicate that the omicron subvariant now comprises an estimated nearly 11% of new cases in the U.S. as of March 5.
"We've been watching it closely, of course," White House press secretary Jen Psaki said during a White House press briefing on Monday, pointing to the fact that BA.2 appears to be more transmissible. "We currently have about 35,000 cases in this country. We expect some fluctuation, especially at this relatively low level, and certainly that to increase."
Experts say how significant a COVID-19 resurgence could be is still unclear, given how many Americans were infected in the nation's omicron surge.
"I am hopeful that the large U.S. omicron wave will dampen a new surge, but I am concerned that we will see a resurgence as restrictions are eased," Ferrari said. "Dropping masks and other restrictions will necessarily result in an increase in risk. How big that increase will be remains to be seen."
ABC News' Ben Gittleson contributed to this report.