The daily number of newly reported coronavirus cases in the United States has declined precipitously over the past two weeks, likely thanks to four key factors, experts say. But experts also fear that another surge in new cases could be on the horizon.

When will the Covid-19 epidemic end? Here are the good, bad, and ugly scenarios.

The 4 reasons coronavirus cases are declining

Experts have cited a mix of four different factors that have likely led to America's recent decline in newly reported coronavirus cases.

1. Increasing Covid-19 vaccination rates

Researchers at the University of Washington's Institute for Health Metrics and Evaluation (IHME) in a recent briefing noted that one reason for the United States' declining coronavirus case rates is the increasing number of Americans getting vaccinated against Covid-19. "[T]he continued scale-up of vaccination helped by the fraction of adults willing to accept the vaccine reaching 71%" has contributed to falling rates of newly reported coronavirus cases in America, the researchers wrote.

According to the Washington Post, the United States administered 1.62 million doses of the country's two authorized Covid-19 vaccines per day last week. Each of those vaccines require that patients receive two doses a few weeks apart.

CDC data shows that, as of Tuesday morning, the federal government had distributed about 71.7 million doses of the vaccines. Of those, about 55.2 million doses had been administered in the United States. That total includes about 39.7 million people who have received "one or more doses" of a vaccine and about 15 million who've received two doses, the data shows.

However, former CDC Director Tom Frieden said he doesn't believe that Covid-19 vaccines at this time are "having much of an impact at all on case rates." Instead, Frieden had another theory.

2. Adherence to coronavirus countermeasures

Frieden said he believes the United States' rate of newly reported coronavirus cases is declining because Americans are adhering more stringently to public health measure that can mitigate the virus's spread. He said the recent decline is due to "what we're doing right: staying apart, wearing masks, not traveling, not mixing with other indoors."

There is some evidence to back Frieden's claims, as the United States saw its worst peak in newly reported coronavirus cases, hospitalizations, and deaths after recent holidays—including Thanksgiving, Christmas, and New Year's Eve—when people were more likely to gather with others who don't live in their households.

3. The changing seasons

The researchers at IHME in their recent briefing also cited another potential factor driving contributing to the United States' recent drop in newly reported coronavirus cases: the possibility that transmission of the virus could ebb and flow along with certain seasons in the country. "Declining seasonality" of the virus "will contribute to declining transmission potential from now until August," the researchers wrote.

Last year, some research suggested that transmission of the novel coronavirus might be more likely in cooler climates, which could bode well for the United States as in enters into its warmer spring and summer months. In addition, research has shown that coronavirus infection is more likely to occur indoors or in other poorly ventilated areas, and the amount of time that Americans spend indoors with others could decline as temperatures warm throughout the country.

4. Undertesting for the novel coronavirus

A fourth, less optimistic reason why newly reported coronavirus cases may be declining in the United States is that the country could be undertesting for the virus, some experts say, as testing rates have decreased in recent weeks—and not necessarily because of a drop in demand.

For instance, a poll by conducted by STAT News and The Harris Poll between Feb. 5 and Feb. 7 found that 24% of the 2,043 respondents said they were unable to get a coronavirus test when they wanted one. The reasons respondents cited for not being able to access a test included not having a testing site near them and having to wait too long for a test.

Separately, a recent report by the health care consulting company Premier found that supplies for coronavirus tests, including pipette tips and micro pipettes, have seen significantly increased demand over the last year, leading to shortages of supplies necessary to complete the tests. According to the report, average daily usage of pipette tips in hospitals has increased by more than 50% since May 2020, and the number of outstanding orders for more pipette tips has tripled. Meanwhile, the average amount of time between an order for pipette tips being placed and the order being received has increased from a few days in fall 2020 to over 25 days in January.

In addition, Eleanor Murray, a professor of epidemiology at Boston University School of Public Health, in a tweet posted earlier this month wrote that she was concerned the U.S. health care system's increased focused on administering Covid-19 vaccines might be straining resources and making it more difficult for people to get tested for the novel coronavirus. In reference to the country's recent decline in newly reported coronavirus cases, she wrote, "I worry that it's at least partly an artifact of resources being moved from testing to vaccination."

According to The Atlantic's Covid Tracking Project, coronavirus testing rates in the United States have declined steadily from more than two million a day in mid-January to about 1.6 million a month later. "Demand for testing may have dropped because fewer people are sick or have been exposed to infected individuals, but also perhaps because testing isn't being promoted as heavily," the authors of the latest update to the project wrote.

Experts worry America's coronavirus epidemic could soon see another surge

Nonetheless, experts largely agree that the recent drops in America's numbers of newly reported coronavirus cases, hospitalizations, and deaths show progress in mitigating the country's coronavirus epidemic. However, many worry that progress may be short lived.

Frieden noted that while the United States' daily reported number of new coronavirus cases is declining, new case rates are still higher than they were during surges the country had seen last spring and summer—and things could worsen once again.

"We've had three surges," Frieden said. "Whether or not we have a fourth surge is up to us, and the stakes couldn't be higher."

Many states are starting to roll back certain restrictions intended to stem the novel coronavirus's spread, which has some experts concerned that we could see another surge in new cases as early as this spring.

For example, in Iowa, Gov. Kim Reynolds (R) has rescinded a mask mandate and limits on gatherings, a move which Lina Tucker Reinders, executive director of the Iowa Public Health Association, criticized.

"We know that masks and social distancing and limiting gatherings have been effective in helping to curtail the spread," Tucker Reinders said. "When something is working, it should be reinforced, not abandoned."

In Wisconsin, Gov. Tony Evers (D) and state legislators have had a battle over a statewide mask mandate implemented by Evers. Bud Chumbley, executive director of the Wisconsin Medical Society, said legislators are sending the "wrong message at the wrong time" by trying to rescind Evers' mask mandate.

"We had 600,000 people get Covid in this state and 6,000 died" since the beginning of America's coronavirus epidemic, Chumbley said. "I'm not sure all those people would think this is the right time to bring up a constitutional question [about the mask mandate] in the middle of a[n] [epidemic], but that's their rationale."

The emergence and spread of new coronavirus variants in the United States could also lead to another surge in new cases, experts say. Already, officials have identified the more contagious B.1.1.7 variant, which was first identified in the United Kingdom, in at least 37 U.S. states—and the prevalence of identified cases of the variant is doubling among positive coronavirus tests each week and a half, according to a recent study.

Officials also have identified cases of other more transmissible variants of the novel coronavirus in the United States, including the B.1.351 variant first identified in South Africa and the P.1 variant first identified in Brazil. Research indicates that all three of those variants may be less susceptible to current Covid-19 vaccines than earlier variants of the virus, and new research suggests that the B.1.1.7 variant may also be more deadly.

And although Covid-19 vaccines may still help to protect Americans against the new variants to some degree, the United States has been struggling to quickly roll out its Covid-19 vaccines, with supply running far short of demand in many states. That's left experts concerned that the vaccination effort ultimately may be too slow to get ahead of the new variants' spread, the Post reports.

To avoid another possible surge, experts say Americans must continue practicing measures intended to curb the novel coronavirus's spread and get vaccinated against Covid-19 when they can.

"Masks, distancing, ventilation, avoiding gatherings, getting vaccinated when eligible. These are the tools we have to continue the long trip down the tall mountain," Caitlin Rivers, an epidemiologist at Johns Hopkins University, wrote in a tweet posted this month. "The variants may throw us a curve ball, but if we keep driving down transmission we can get to a better place" (Thebault, Washington Post, 2/14; Silverman, STAT News, 2/15; Bowden, The Hill, 2/14; Achenbach et. al., Washington Post, 2/12; CDC vaccine data, updated 2/16).