After another brutal spike in coronavirus cases and deaths this summer — fueled by the Delta variant — infections are declining in the United States, down 50 percent from their peak in September.
Experts say what comes next is hard to predict, and we often do not know why the virus spreads the way it does. But looking back at the outbreak so far can provide some clues about how the virus may spread in the future.
Average cases during phases of the pandemic
Average cases per 100,000 people
June – August
September – November
December – February
March – May
Summer and Fall 2021
June – Oct. 20
Note: Most Nebraska counties did not report data during the summer of 2021.
The country has suffered through five waves of the pandemic now, depending on how you count. “Each of these waves has a different complexity and pattern,” said Alessandro Vespignani, the director of the Network Science Institute at Northeastern University in Boston.
During the first wave, for instance, strict stay-at-home measures and drastic changes in behavior may have stalled the virus for a time. Last fall, with those measures and behavior comparatively relaxed, record-breaking surges in the Midwest rippled outward to the South and both coasts. By the time the highly contagious Delta variant fueled a wave across the country this summer, vaccines were widely available, shifting the pattern once again.
“Vaccines have clearly changed which places have been hit and how much they’ve been hit,” said Jennifer Nuzzo, an epidemiologist at Johns Hopkins University.
Below is a look at five times that the U.S. case curve hit a peak, and the lessons and insights experts have gleaned from each wave.
The first outbreaks
In the spring of 2020, the first wave hit a few areas particularly hard, including New York City, New Orleans and Albany, Ga. A lot came down to random chance insofar as where the virus struck first, experts said, though population density and transportation hubs may have played a role.
Tests were hard to come by during this period, so cases were drastically underreported. But death data indicates the Northeast’s outbreak was one of the worst of the whole pandemic — one in about 400 New York City residents died within the span of two months.
Early stay-at-home orders and widespread, drastic behavioral changes flattened the curve in those outbreaks, however, preventing the coronavirus from rippling across the country in waves, the way it would in later surges.
While hospitals overflowed in the Northeast corridor, nearby areas like Maine did not see large outbreaks. Isolated hot spots broke out largely in places where people were unable to socially distance, like nursing homes, prisons and meatpacking plants.
“I think it’s easy to miss how bad things could have gotten and how much better we did than we could have largely because of the lockdowns,” said Justin Lessler, a professor of epidemiology at the University of North Carolina.
Hot spots in the Sun Belt
Cases surged again in the summer of 2020, but this time Sun Belt states suffered the worst outbreaks. Many states that set new records for cases and deaths were also those that reopened first, including South Carolina, Alabama, Georgia and Mississippi. Experts say seasonality — perhaps the Sun Belt’s summer heat driving people indoors — may also have been a factor.
The summer surge slammed many metropolitan areas of the South and Southwest, including Houston, Miami and Phoenix. Without tight virus restrictions in place, the virus spread outward into suburbs and exurbs. By the end of the summer, most of the worst outbreaks were occurring in rural areas.
“If you think of the spring wave in 2020, it was more pointlike around urban areas. In the other waves, you see more of a general flow,” Dr. Vespignani said, “Like when you throw a stone in a pond.”
The winter wave
The flow of cases is clearer in the surge that began in the Upper Midwest in September 2020. North and South Dakota had few virus restrictions in place to contain an outbreak, and both states had particularly bad spikes. One in 10 residents tested positive for the virus in the fall in North Dakota, and experts think many more cases went undetected.
From there, the outbreak expanded beyond the Midwest, reaching both coasts and stretching down to the South in a devastating wave. The country saw more daily cases and deaths in January than any other time before or since.
“You do see this movement, almost like it’s moving from county to county,” said Jeffrey Shaman, an infectious disease epidemiologist at Columbia University, who said researchers found community-to-community transmission played an important role in virus spread during the 2009 H1N1 pandemic. But Dr. Shaman said factors other than proximity could have also played an important role with Covid.
Disparate communities may have similar school opening dates, for instance, experience the same cold fronts, or share similar behavior patterns, all of which could lead to independent outbreaks at the same time.
“When you’re looking at anything after October of last year, the virus is everywhere. It didn’t need to be reintroduced,” Dr. Shaman said.
Then, in one community after another, cases fell often as quickly as they had risen. A sharp fall after a peak is not uncommon during epidemics, experts said. When a virus rapidly spreads through a community, it eventually runs out of people to infect.
A mystery in Michigan
By Spring 2021, U.S. cases had retreated far from their winter peak. At the same time, a more-contagious variant that had fueled an enormous surge in the United Kingdom, called Alpha, was quickly becoming dominant in the United States.
Michigan saw a large surge in cases and deaths, worrying experts that the variant would cause a similar nationwide outbreak. Instead, the virus seemed to stop at the Michigan border in May.
Epidemiologists still do not know why Michigan was unlucky — or why the outbreak did not spread to neighboring states. But some noted that it took place right around when all adults first became eligible for the vaccine, and before social distancing behavior loosened significantly.
It’s possible that people became more cautious during the resurgence, slowing the spread, said Dr. Lessler, the University of North Carolina epidemiologist. Then vaccines helped stamp it out.
Case and death records
broken across the South
Case and death records
broken across the South
In June, U.S. coronavirus cases were at a low point not seen since the beginning of the pandemic, and nearly half the population had received at least one shot. States lifted virtually all virus restrictions and people relaxed their behavior in celebration.
The timing proved disastrous, especially for areas with lower vaccination rates. Another variant, this time Delta, took hold and quickly grew to account for a majority of U.S. cases. Missouri saw the first big surge of the Delta wave.
“That’s where the fire was ignited; then the fire started to spread to other places,” Dr. Vespignani said.
Soon, that outbreak moved across Arkansas, then Louisiana, both states with low vaccination rates. Florida became another early Delta hot spot. By the end of August, most states in the South had hit new records for daily cases or deaths and the virus turned northward, causing surges in the upper Midwest and Mountain West.
While the Delta wave rolled across much of the country, some places were relatively spared.
“That fire was never able to get, for instance, into the Northeast corridor,” Dr. Vespignani said. “It’s where there’s one of the highest vaccination rates. It’s like there’s a wall.”
Some experts say that the vaccination campaign and much of the country having already experienced several waves of outbreaks — which have conferred some immunity to those who were infected and recovered — have made them cautiously optimistic for the winter.
Dr. Lessler, who helps run the Covid-19 Scenario Modeling Hub, a consortium of research groups that model the future of the outbreak, said none of the groups forecast a substantial winter peak in the United States this year.
“We might see a little bump in cases, and of course people could radically change behavior or we could see a variant,” Dr. Lessler said, but he added that he did not think a substantial peak was likely.
All the same, there are bound to remain places where the virus can spread, as each new wave has shown. And questions still remain about how long immunity will last.
“The difference between the Michigan Alpha wave in Spring 2021 and the Delta wave is really telling you that the wall that you’ve built might work for one variant, but it might not be enough for the next one,” Mr. Vespignani said. “There might be another variant that is more transmissible and with more immune evasion. That’s why we need to build the wall as high as possible.”