With COVID-19 cases increasing in 46 states, 10 of which broke single-day records for new cases on Thursday, America has officially entered what experts refer to as the “exponential” phase of spread — a rapid multiplying of cases that can’t be contained through traditional measures. On Thursday alone, the U.S. saw 160,000 new cases of the virus, more than any day since the pandemic began.
In California, the second state to surpass 1 million cases, thousands of people in cars lined up at Dodger Stadium on Thursday to get tested; in parts of Washington state, individuals waited four to five hours.
The current spike in cases — which some states are calling a third wave — has brought more than 100,000 new cases a day since Nov. 4. But that’s not counting those that may be going undetected. “We have widespread, uncontrolled COVID-19 in many parts of the country,” says Dr. Amesh Adalja, senior scholar at the Johns Hopkins Center for Health Security. “And we know the cases reported are an underestimate of what is out there ... we’re missing many cases because people aren’t getting tested. So the true number is much higher than what we’re actually seeing.”
Adalja says that exponential spread means a virus is no longer increasing on a linear scale but is instead spiking at a rapid rate. Dr. Oguzhan Alagoz, an engineering professor at the University of Wisconsin-Madison who models the spread of infectious diseases, agrees, saying exponential spread refers to an “unbelievably high growth rate.” In this case, that means new COVID-19 cases in many parts of the country are doubling or tripling each week. Part of this, he says, is due to the fact that COVID-19 seems more contagious than other viruses.
With influenza, for example, the R0 (basic reproductive number) — or the average number of people to whom someone spreads a virus — is one to two. With COVID-19, the R0 number has hovered around three. “So say if I infect three people, those three people are going to each infect another three people, and those three are going to infect another three,” Alagoz explains. “This is why it’s multiplying in such a short time.”
Looking closely at the numbers in the U.S., it’s not hard to find examples of exponential spread, both on the national level and among individual states.
According to the COVID Tracking Project, the U.S. surpassed 5 million cases of COVID-19 on Aug. 9, but by Nov. 9 had nearly eclipsed 10 million. In Wisconsin — one of the hardest-hit states — the average number of new cases a day went from 2,800 in early October to nearly triple that this week. In Michigan, another Midwestern state recently affected by the pandemic, active hospitalizations went from 669 in early October to more than 2,000 this week, filling up intensive care units and leading Gov. Gretchen Whitmer to declare it a “dire” situation.
In North Dakota, where a motorcycle rally of nearly 500,000 people in neighboring South Dakota is believed to have set off an outbreak, average daily cases have gone from 400 in early October to 1,800 a day this week. Adalja says spikes like this are dangerous. “When you see an outbreak, it may not necessarily begin with exponential spread, but once you start to get a critical number of cases, then the spread really just takes off in a way that’s really explosive,” he says. “It is a particularly worrisome type of spread because it becomes very hard to control when you’re in that type of a phase of growth.”
Dr. William Schaffner, an infectious disease expert at Vanderbilt University School of Medicine, agrees that the U.S. has reached a new level in the pandemic. “The virus is spreading rampantly. I’ve heard someone refer to it as ‘volcanic’ and I think that’s accurate.” Schaffner says that COVID fatigue is likely playing a role, with individuals becoming more lax with social distancing and mask wearing.