The COVID-19 Pandemic: When will the epidemic curve bend?Nov 8, 2021
Colorado’s epidemic curve continued its inexorable surge last week with statewide COVID-19 hospitalizations rising to 1,296 on Friday. While still far short of the 2020 peak of 1,847, the threat to healthcare capacity is more severe than during the December/January surge a year ago because of greater utilization and strained staffing. How long will the surge continue and how high will the epidemic curve rise?
These questions were addressed in the most recent report from the Colorado COVID-19 Modeling Group, released last week. As a reminder, the modeling team uses a Susceptible, Exposed, Infected, Recovered, Vaccinated (SEIRV) model to track the pandemic in Colorado and to project what could happen under different scenarios (e.g., with increased use of masks or vaccinating 5- to 11-year-olds). Fitting the SEIRV model to the hospitalization data, the team estimates the effective reproductive number (Re), similar to R0 but dynamic across the pandemic, to describe the trajectory of the epidemic. The most recent estimate is 1.2, above one and hence indicating that the curve will continue to rise. Over the last several weeks, the epidemic curve has tracked steadily upward along a slope corresponding to Re at 1.2.
Where does this trajectory take the state’s epidemic? If we continue with Re at 1.2, the curve will peak in late November or early December, reaching around 1,500 to 1,600 COVID-19 hospitalizations. A peak and subsequent fall are projected as the number of susceptible individuals declines because of naturally acquired immunity. The peaks under some scenarios could be higher, but compensatory behaviors that may kick-in in the face of the rise are likely to help Colorado stay below the December 2020 peak.
The report systematically addresses the “why question” — why is Colorado’s epidemic curve rising so steeply? We are among the minority of states with rising cases and hospitalizations and, in fact, have one of the fastest rising epidemic curves at present. The modeling report systematically explores reasons for the rise, finding no single factor that explains the state’s current surge. Of course, the current epidemic is an epidemic among the unvaccinated, but it spreads to the vaccinated as well. The vaccinated account for 22% of those hospitalized with COVID-19 at present. What the modeling team could not find was one or more specific factors that differentiated Colorado from the majority of states where epidemic curves are declining.
Twenty months into the pandemic and 10 months since vaccination was implemented, we do face waning immunity, both natural and acquired. A just-published article in Science describes time trends of vaccine effectiveness against infection and death in a cohort of 780,000 Veterans. For each of the three vaccines used in the United States, effectiveness against infection waned, although effectiveness in preventing death remained high. The decline in effectiveness against infection was greatest for the Janssen (J&J) vaccine. During the Delta surge, the infection rate went up while protection against death remained high. The SEIRV model incorporates a waning of immunity, documented to occur for SARS-CoV-2 and other coronaviruses, but there is uncertainty as to the degree and the time course of the decline.
Members of the Modeling Group spoke to the media last week about Colorado’s situation. One often asked question: What should Coloradans and state and local governments do to end the surge? My answer starts with what we can do as individuals to protect ourselves and others. The list includes increasing our use of masks in indoor settings; after all, we don’t need a mask mandate to wear a mask. We can avoid large gatherings, particularly if attendees are likely to not be masked. And above all, following recommendations for vaccination for ourselves and our families is critical for protecting everyone. At present, two Colorado counties—Boulder and Larimer—have mask mandates. More should, and a statewide mandate would make a difference.
Nothing new to say about Governor DeSantis this week, but there is Aaron Rogers, quarterback for the NFL’s Green Bay Packers. Too many headlines have been directed at his vaccination, actually non-vaccination, status. The NFL has described protocols for those who are vaccinated and for those who are not vaccinated. Rogers, who has not followed the protocol for the unvaccinated, has tested positive for SARS-CoV-2 and did not play in his team’s game over the weekend. Cagily, he has described himself as immunized, referring to a homeopathic treatment to increase antibody levels. His retort to the controversy concerning his non-vaccination raises now stereotyped tropes around autonomy of decision-making. Rogers has his own paragraph today because he caricatures some of the challenges of vaccinating the unvaccinated and, like others, he says nothing about protecting his own community — the team that he plays on. Rogers’ news is rivaled by critics of the recent announcement of the vaccination of Sesame Street’s Big Bird.
Jonathan Samet, MD, MS
Dean, Colorado School of Public Health