SARS-CoV-2 in people and the environment: What’s coming?
The novel coronavirus (SARS-CoV-2) that emerged out of China in December 2019 is a highly infectious pathogen with the potential to rapidly overload health care systems and inflict substantial mortality around the world.
An effective vaccine for this virus is under development and although moving quickly, will not be available to protect the most vulnerable for another year. The average case fatality rate for this virus has been estimated to be at least 1 percent, but this estimate is expected to vary geographically as a function of health care capacity, baseline population health and other socio-demographic factors.
Indeed, during the 2009 A/H1N1 influenza pandemic some countries experienced several fold higher mortality rates compared to well-developed countries. In particular, older adults and those with underlying health issues are at highest risk of developing severe disease and death. At the time of writing, this pandemic virus is now straining or overwhelming health systems around the world.
As of March 15, more than 5,762 individuals have succumbed to COVID-19, the disease caused by the novel coronavirus. One of the largest transmission hotspots is now located in Italy, where the number of deaths has surpassed 1,440. As the virus continues its march around the world, public health authorities are ramping up the intervention strategies to slow the spread of the virus.
An increasing number of nations are implementing sweeping social distancing strategies ranging from school closures and cancellation of large public gatherings, to ordering quarantines on entire cities. While it is difficult to predict the course of a pandemic virus, it is worth discussing the potential future trajectory of the virus, at least before a vaccine becomes available, and draw from lessons from past pandemic events in light of the ability of the virus to survive under different environmental conditions.
Social distancing strategies can have a profound impact on the transmission rate of the virus. However, the net effect of these interventions on the transmission rate will depend on scope of the social distancing interventions as well as on how long such interventions can be sustained. For instance, an 18-day nationwide school closure and cancellation of large public gatherings during the spring wave of the 2009 A/H1N1 influenza pandemic in Mexico was associated with a 30 percent reduction in the transmission rate. However, the transmission rate increased in southeast states after the mandatory school suspension resumed and before summer vacation started.
In the context of a pandemic virus with potential to generate explosive outbreaks in confined settings, we may need to sustain social distancing interventions for longer periods through remote learning and telework efforts in order to significantly slow down the spread of the virus. This is particularly relevant for countries where the true magnitude of the epidemic is yet to be fully elucidated as mass testing strategies are ramped up.
The survival of the novel coronavirus in the environment is expected to respond to seasonal variation around the world. In particular, higher temperatures cause the virus to die off faster, but dry conditions of low humidity can help it survive long term. We know from other coronaviruses that cold temperatures result in longer survival; as temperature rises (around room temp) the virus dies off more quickly . It is important to note that the relationship between humidity and survival is not linear; low humidity (about 20 percent) results in longer survival than higher humidity. Inactivation is fastest at humidity levels around where we like our indoor environments (about 50 percent), and it slows down again as humidity gets higher (about 80 percent). The same relationship between survival and temperature is found in water using surrogate animal coronaviruses; cold temperatures can result in long term survival, while ambient temperatures virus can survive for days.
As social distancing measures and border screening processes are ramped up and temperature rises in the Northern Hemisphere, we may observe a decline in the transmission rate of the virus during the next few months. However, it is unclear whether community transmission could be halted in the context of highly susceptible populations.
In the best scenario, transmission will slow substantially with only sporadic transmission events perhaps driven by undetected case importations. During the 2009 A/H1N1 influenza pandemic, the rate of spread slowed during the summer vacation period, but transmission surged in the fall when schools reopened, suggesting the need for sustained social distancing strategies before a vaccine becomes available to protect the most vulnerable. Meanwhile, as temperatures drop in the Southern Hemisphere, nations there should be learning the lessons in real-time and be ready to detect transmission chains early and slow the transmission rate.
Gerardo Chowell is a professor of Epidemiology and Biostatistics at Georgia State University. Lisa Casanova is an associate Professor of Environmental Epidemiology at Georgia State University.