The “How” of Infection
We still have much to learn about the transmission of Covid-19.
The United States was clearly unprepared for the strain that Covid-19 would put on its health-care system, including our limited supply of personal protective equipment (PPE). The scarcity of antigen tests for Covid meant that we could not identify who was infected and, even worse, how they were infected. As a result, the nation adopted a stay-at-home approach to mitigate the spread of the virus.
Since then, we have accumulated some information about the high-risk places where the virus is likely to spread—cruise ships, health-care facilities, nursing homes, prisons, factories, warehouses, and meat-processing plants. We also know, not surprisingly, that Covid can easily spread within a household—in closed indoor spaces, where individuals are constantly mixing at close distances for prolonged periods. We have also learned about super-spreader events such as dinner parties, family picnics, and funerals, where large groups of people congregate temporarily, get infected, and then go on to spread the infection more widely.
Reports of specific outbreaks in high-risk places are helpful in designing containment measures for these settings but unhelpful in understanding how the virus is transmitted person-to-person elsewhere—and as we relax stay-at-home orders and reopen social and commercial life across the country, that’s the understanding we need. We rely on epidemiologic models based on imperfect assumptions and laboratory-based experiments to make real-world recommendations about touching surfaces and establishing distance from others. This imprecision is especially problematic as the public looks for guidance on such activities as airplane travel, use of public restrooms, outdoor dining, outdoor sports, and sitting in offices, breathing recirculated air. We need confidence that we can participate in social and commercial life safely.
Nowhere is our limited understanding more evident than when trying to understand racial and ethnic differences in Covid cases at the individual and neighborhood level. As is well known, African-Americans and Latinos are overrepresented among individuals diagnosed with Covid-19, and heavily minority residential areas—especially when densely populated and poor—have higher-than-average rates of infection. Various explanations have been offered for this pattern of infection: population density; high-risk employment (e.g., in factories, warehouses, or nursing homes); and the need to use public transportation. We must learn the relative impact of each of these to reduce disparities in infection and to guide public policy.
The process for ascertaining how the disease is transmitted outside of the vectors of highest risk is known as “disease investigation.” Contact tracing is a process for controlling the spread of an infection by notifying people who may have been exposed and testing them; this information may also help improve our broader understanding of the disease. Public-health professionals involved in the investigation of an infectious-disease outbreak combine surveillance data, contact-tracing information, environmental conditions, biological knowledge, and epidemiological analysis to create a picture for each local outbreak of the persons who became infected, the time when they got infected, and the place where they got infected. From this information, they compile an understanding of how the infection is transmitted.
The accumulation of data from disease investigations of Covid-19 will help us move forward. In the meantime, it would be helpful to divide new cases into those associated with high-risk settings and those not. By some estimates, high-risk places may account for more than half of new infections, which suggests that while community-level spread is growing with increased social and commercial life, it’s not likely to reach sufficient magnitude to justify going back to quarantines, unless we have evidence that we’re overwhelming the health-care system. If we can bolster and support containment strategies at high-risk places and in high-risk neighborhoods, and of course protect those most vulnerable for severe disease, we should be able to continue moving forward, while expanding our understanding of the transmission of this new disease and refining our containment approach.
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