COVID-19: A Basic Primer on Respiratory Virus Epidemiology

Eventually, we will all become infected with SARS-CoV-2. That is not quite true, but may apply to more than half of us. The initial appearances of hotspots across the global map have now been replicated on the U.S. map, and more recently on our state maps. SARS-CoV-2 is spreading exponentially and is everywhere. Let us think about that for a moment and consider some very basic epidemiological modeling.

When unchecked by immunity, illnesses due to respiratory viruses can increase exponentially with the rate governed by what we call the basic reproduction number. This number can best be described as the number of new cases derived—on average—from each existing case. When this number is high—such as with measles—new cases can explode. When this number is close to 1.0, we get a very slow burn. Moreover, should the reproductive number fall below 1.0, an outbreak ends. Primary care clinicians are all familiar with influenza, which is in the low-moderate range of about 1.6. SARS-CoV-2 is higher than influenza with estimates around 2.4.¹

Epidemiological math then gets more complicated. The realized reproduction number is modifiable. Public health approaches, such as nonpharmaceutical interventions (NPI: social distancing, handwashing, respiratory hygiene, and use of PPE) and community mitigation efforts (quarantining, isolation, school closures, bans on mass gatherings, and travel bans), can effectively lower the transmissibility, thus lowering the realized reproductive number. For example, estimates from Wuhan, China suggest that public health measures lowered the reproductive number to 1.05.[1] In addition, immunity acquired by infection and recovery, or through an effective vaccine, will also lower it. This is the quintessence of “flattening the curve” that we hear bantered around so much these days (see figure 1 in CDC. Community mitigation guidelines to prevent pandemic influenza — United States, 2017).²

So, where does that leave us? Assuming very generously that there are 100 additional cases in the U.S. for each confirmed case, less than 1% of us have experienced SARS-CoV-2. Without a vaccine and/or very sustained NPI and community mitigation efforts, another 170,000,000 of us may need to become infected and recover to make SARS-Cov-2 go away...  and a case burden of this level is unfathomable in terms of COVID-19 morbidity and mortality. I am hoping that my basic epidemiological musings are wildly incorrect. In the meantime, keep up with NPI, support your public health agencies in their efforts for community mitigation, and hope for a safe and effective vaccine.

References

1. Kucharski AJ, Russell, Diamond C, Liu Y, Edmunds J, Funk S, et al. Early dynamics of transmission and control of COVID-19: a mathematical modelling study. Lancet Infect Dis. Published online March 11, 2020 at DOI:https://doi.org/10.1016/S1473-3099(20)30144-4
2. CDC. Community mitigation guidelines to prevent pandemic influenza — United States, 2017. Accessed 3/20/2020 at: https://stacks.cdc.gov/view/cdc/45220