As we all do our best to stay connected with colleagues, friends, and family through social media, I often wonder how many people’s profile pictures are surgically cropped to remove the other half. I am of the age that folded physical photographs in half or took scissors to film. In the case of COVID-19, the familiar “flatten the curve” illustration is the ultimate example of a cropped photo. This criticism does not diminish the importance of people staying home to flatten the curve; it is a reminder that when you unfold the other half of that photo, there’s a resurgence curve.
A new modeling study from researchers at Harvard University’s T.H. Chan School of Public Health, published Tuesday in the journal Science, cautions that intermittent periods of social distancing may be necessary for the United States to prevent overwhelming the health care system. The study lays out a range of scenarios for how SARS-CoV-2 may spread over the next five years while considering important variables like seasonal transmission of the virus and whether people who are infected develop short-term or longer-term immunity.
The models highlight the profound tension between flattening the curve and just pushing the curve forward in time. When we strike the right balance, we carefully build up herd immunity without ever surging beyond the critical care capacity of our healthcare system. To do so, we have to feather the throttle. If we push too hard off the line, the cases will surge and overburden the health care system. If we haven’t yet solved the medical and personal protective equipment (PPE) shortages or severe lack of testing materials, we end up in the same situation as we are currently experiencing. However, if we coast in this physically-distanced state for too long, there will be too many susceptible people in the population, and we end up just pushing the curve out into the fall or winter when our healthcare system will be burdened by influenza and other seasonal infections. Flattening the curve is vital because it buys us time to learn more about the virus, to strengthen the health care capacity, and to spare lives.
The authors carefully acknowledge the emotional, economic, and educational hardships involved with the current interventions and caution that their study is not an endorsement of a particular scenario, but rather, it is intended to spur discussion and innovation around controlling this pandemic long-term. So, how do we safely release ourselves from the current binds of social distancing?
“I think the key limitation is that we’re going to need a lot of viral tests to really understand where our prevalence is. So, the idea would be to pull off one of the social distancing measures that we’ve taken up and then to wait a week or two and see what happens to incidence. If it doesn’t go up by too much, then you can think about releasing another one. That kind of incremental release over time has a lot of promise and it will probably vary from place to place because not all interventions will have the same effect in all places,” says Stephen Kissler, a postdoctoral fellow in Yonatan Grad’s research group and co-first author of the study. “As long as we’re doing enough testing to stay on top of the true incidence, this incremental release will have success. My biggest concern is that we won’t have enough testing, and instead, we’ll have a three-week lag before we see people starting to show up in the hospital again. By then, it’ll be too late to reverse our actions to prevent the same thing from happening.”
The United States has failed to adequately test on a scale that provides accurate information on what proportion of our population has been infected with SARS-CoV-2. As many states struggle with supply shortages of test kits and associated materials, much of the focus on testing shifts to serology.
Initial serology tests will be able to answer the critical question of whether or not someone has immunity to SARS-CoV-2. Beyond this binary level of information, further expanded serological tests can answer lingering questions about whether there is cross-immunity to other coronaviruses (gaining immunity from being infected with other coronaviruses like those that can cause the common cold) and what level of immune response a person has mounted to SARS-CoV-2.
“There are serological assays that allow us to determine how strong an immune response is and whether that response is strong enough to fight off infection in the future. If we do these studies in the same people over time, then we should be able to determine the length of immunity that we can expect to see from SARS-CoV-2,” says Kissler.
“We know that immunity wanes to the other coronaviruses and probably will to [SARS-CoV-2] too, but we don’t yet know at what rate. That information will help us to predict with what frequency we’re going to see recurrent outbreaks of SARS-CoV-2 in the future. We will be able to see whether there’s any cross-immunity between other coronaviruses and SARS-CoV-2, and all of this information helps us understand which of these long-term scenarios we might be in.”
The duration of immunity directly impacts what the future may hold. The path forward looks drastically different depending on whether people are only immune for a matter of weeks versus maintaining a more intermediate or longer-term immunity. If SARS-CoV-2 elicits an immune response similar to the two coronaviruses in this study, we can expect annual or biennial waves of infections. If the protection lasts longer, the resurgence may not come for a few years. The consensus is that it is unlikely that people will mount lifelong immunity to SARS-CoV-2.
The question of seasonality remains for SARS-CoV-2 and whether or not the summer will bring relief. “From the related coronaviruses in this study, we observed a seasonality that’s typical for a respiratory virus where there is increased transmissibility in the late fall and winter time and then we see a decline in the spring,” says Christine Tedijanto, a graduate student in Marc Lipsitch’s research group and co-first author of the study. “I think one of the things to keep in mind is that as we approach warmer months, it’s going to help, but it probably won’t get rid of this completely. Because there are so many susceptible people in the population, even with a moderate decline in the summertime, it’s likely that SARS-CoV-2 will still spread.”
Everyone wants to peer into the crystal ball for an answer to how we get out of this situation, but the answer is not a magical date out in the near or far future per se. These models provide a critical glimpse into the future and the likely outcomes of various non-pharmaceutical interventions like social distancing. Researchers remain hopeful that the various ongoing trials of potential therapeutics and vaccines will be fruitful in providing safe and effective solutions. Their arrival will drastically alter these models.
For now, as we feather the throttle and prepare to step into the new normal, there are other aspects of this pandemic that need resolution. We have to unfold that cropped photo and acknowledge the resurgence. The greatest gift of these models is that it gives us time, albeit limited time, to prepare for what lies ahead. We have to ensure that we have adequate levels of PPE and medical equipment, increased capacity for testing, the ability to track the disease and conduct contact tracing, and a plan for isolating and quarantining people who may be sick.
The summer may not spare us from future SARS-CoV-2 seasonality. Still, the potential relief in caseload is an opportunity to carefully repair the damaged industries and economies while preparing for the next wave. These models will evolve as we learn more about SARS-CoV-2 and the duration of human immunity. For now, we have to thoughtfully build herd immunity across the population to spare lives and prevent the catastrophic burden on our health care system.
